Acceleration of sub-relativistic electrons with an evanescent optical wave at a planar interface - Koz{\'{a}}k, M.; Beck, P.; Deng, H.; McNeur, J.; Sch{\"{o}}nenberger, N.; Gaida, C.; Stutzki, F.; Gebhardt, M.; Limpert, J.; Ruehl, A.; Hartl, I.; Solgaard, O.; et al. (Optics Express, 2017)
We report on a theoretical and experimental study of the energy transfer between an optical evanescent wave, propagating in vacuum along the planar boundary of a dielectric material, and a beam of sub-relativistic electrons. The evanescent wave is excited via total internal reflection in the dielectric by an infrared (λ = 2 μm) femtosecond laser pulse. By matching the electron propagation velocity to the phase velocity of the evanescent wave, energy modulation of the electron beam is achieved. A maximum energy gain of 800 eV is observed, corresponding to the absorption of more than 1000 photons by one electron. The maximum observed acceleration gradient is 19 ± 2 MeV/m. The striking advantage of this scheme is that a structuring of the acceleration element’s surface is not required, enabling the use of materials with high laser damage thresholds that are difficult to nano-structure, such as SiC, Al2O3 or CaF2.
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Dynamics of electron injection in a laser-wakefield accelerator - Xu, J.; Buck, A.; Chou, S.-W.; Schmid, K.; Shen, B.; Tajima, T.; Kaluza, M.C.; Veisz, L. (Physics of Plasmas, 2017)
The detailed temporal evolution of the laser-wakefield acceleration process with controlled injection, producing reproducible high-quality electron bunches, has been investigated. The localized injection of electrons into the wakefield has been realized in a simple way - called shock-front injection - utilizing a sharp drop in plasma density. Both experimental and numerical results reveal the electron injection and acceleration process as well as the electron bunch's temporal properties. The possibility to visualize the plasma wave gives invaluable spatially resolved information about the local background electron density, which in turn allows for an efficient suppression of electron self-injection before the controlled process of injection at the sharp density jump. Upper limits for the electron bunch duration of 6.6 fs FWHM, or 2.8 fs (r.m.s.) were found. These results indicate that shock-front injection not only provides stable and tunable, but also few-femtosecond short electron pulses for applications such as ultrashort radiation sources, time-resolved electron diffraction or for the seeding of further acceleration stages.
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Ground-state ionization energies of boronlike ions - Malyshev, A. V.; Glazov, D. A.; Volotka, A. V.; Tupitsyn, I. I.; Shabaev, V. M.; Plunien, G.; St{\"{o}}hlker, T. (Physical Review A, 2017)
High-precision QED calculations of ground-state ionization energies are performed for all boronlike ions with nuclear charge numbers in the range 16≤Z≤96. Rigorous QED calculations are performed within the extended Furry picture and include all many-electron QED effects up to the second order of the perturbation theory. The contributions of third- and higher-order electron-correlation effects are accounted for within the Breit approximation. Nuclear recoil and nuclear polarization effects are taken into account as well. In comparison with previous evaluations of the ground-state ionization energies of boronlike ions the accuracy of the theoretical predictions is improved significantly.
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Magetostatic amplifier with tunable maximum by twisted-light plasma interactions - Wu, D.; Wang, J. W. (Plasma Physics and Controlled Fusion, 2017)
Laser beams with Laguerre–Gaussian (LG) mode carry orbital angular momentum (OAM); however, when interacting with plasmas, the net angular momentum acquired by plasmas is basically zero after interaction. Here, we find when there exists a small magetostatic seed along the laser propagation direction, the barrier would be broken, giving rise to dramatic angular momentum transfer from LG-lasers to plasmas. Hence, the net OAM remaining in the plasmas system would continuously enhance the magetostatic field, until the corresponding Larmor frequency of electrons is comparable to the laser frequency in vacuum. Three-dimensional particle-in-cell simulations are performed to confirm our theory, producing spatial-uniform, temporal-stable and extremely-intense magetostatic fields.
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Optical coherence tomography with nanoscale axial resolution using a laser-driven high-harmonic source - Fuchs, S.; W{\"{u}}nsche, M.; Nathanael, J.; Abel, J. J.; R{\"{o}}del, C.; Biedermann, J.; Reinhard, J.; H{\"{u}}bner, U.; Paulus, G.G. (Optica, 2017)
Extreme ultraviolet microscopy is technologically demanding and thus largely confined to synchrotron radiation facilities. However, specific benefits like high resolution and exceptional material contrast provide strong motivation for the development of table-top alternatives. We report on the first demonstration of coherence tomography, i.e., noninvasive cross-sectional imaging, with high harmonics. A depth resolution of 24 nm and very good material contrast are achieved. Excessively demanding optics for extreme ultraviolet radiation are avoided and artifacts due to the elementary geometry are suppressed with a novel three-step one-dimensional phase-retrieval algorithm. The images are recorded in reflection geometry, facilitating the analysis of, e.g., operating semiconductor samples.
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Photoexcitation of atoms by Laguerre-Gaussian beams - Peshkov, A. A.; Seipt, D.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2017)
In a recent experiment, Schmiegelow et al. [Nat. Commun. 7, 12998 (2016)] investigated the magnetic sublevel population of Ca^+ ions in a Laguerre-Gaussian light beam if the target atoms were just centered along the beam axis. They demonstrated in this experiment that the sublevel population of the excited atoms is uniquely defined by the projection of the orbital angular momentum of the incident light. However, little attention has been paid so far to the question of how the magnetic sublevels are populated when atoms are displaced from the beam axis by some impact parameter b. Here, we analyze this sublevel population for different atomic impact parameters in first-order perturbation theory and by making use of the density-matrix formalism. Detailed calculations are performed especially for the 4s ^2S_1/2 -> 3d ^2D_5/2 transition in Ca^+ ions and for the vector potential of a Laguerre-Gaussian beam in Coulomb gauge. It is shown that the magnetic sublevel population of the excited ^2D_5/2 level varies significantly with the impact parameter and is sensitive to the polarization, the radial index, as well as the orbital angular momentum of the incident light beam.
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Polarization Dependence of Bulk Ion Acceleration from Ultrathin Foils Irradiated by High-Intensity Ultrashort Laser Pulses - Scullion, C.; Doria, D.; Romagnani, L.; Sgattoni, A.; Naughton, K.; Symes, D. R.; McKenna, P.; Macchi, A.; Zepf, M.; Kar, S.; Borghesi, M. (Physical Review Letters, 2017)
The acceleration of ions from ultrathin (10–100 nm) carbon foils has been investigated using intense (∼6×10²⁰ W cm⁻²) ultrashort (45 fs) laser pulses, highlighting a strong dependence of the ion beam parameters on the laser polarization, with circularly polarized (CP) pulses producing the highest energies for both protons and carbons (25−30 MeV/nucleon); in particular, carbon ion energies obtained employing CP pulses were significantly higher (∼2.5 times) than for irradiations employing linearly polarized pulses. Particle-in-cell simulations indicate that radiation pressure acceleration becomes the dominant mechanism for the thinnest targets and CP pulses.
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Angle-resolved x-ray spectroscopic scheme to determine overlapping hyperfine splittings in highly charged heliumlike ions - Wu, Z. W.; Volotka, A. V.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2017)
An angle-resolved x-ray spectroscopic scheme is presented for determining the hyperfine splitting of highly charged ions. For heliumlike ions, in particular, we propose to measure either the angular distribution or polarization of the 1s2p ^3P_1,F -> 1s^2 ^1S_0,F_f emission following the stimulated decay of the initial 1s 2s ^1S_0,F_i level. It is found that both the angular and polarization characteristics of the emitted x-ray photons strongly depend on the (relative) splitting of the partially overlapping hyperfine 1s 2p ^3P_1,F resonances and may thus help resolve their hyperfine structure. The proposed scheme is feasible with present-day photon detectors and allows a measurement of the hyperfine splitting of heliumlike ions with a relative accuracy of about 10^-4.
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Auger decay of 4d inner-shell holes in atomic Hg leading to triple ionization - Andersson, J.; Beerwerth, R.; Roos, A.; Squibb, R. J.; Singh, R.; Zagorodskikh, S.; Talaee, O.; Koulentianos, D.; Eland, J. H. D.; Fritzsche, S.; Feifel, R. (Physical Review A, 2017)
Formation of triply ionized states upon the creation of 4d inner-shell holes in atomic Hg is investigated by using synchrotron radiation of 730 eV photon energy and a versatile multielectron coincidence detection technique in combination with multiconfiguration Dirac-Fock calculations. By carefully selecting Coster-Kronig electrons detected only in coincidence with a 4d photoelectron, the Coster-Kronig spectrum has been extracted and the corresponding branching ratios of the 4d hole have been determined. The results are found to differ from previously established experimental ratios based on electron impact ionization but to agree now better with theory. We also present an Auger cascade analysis of pathways leading to triply ionized states of atomic Hg upon removal of a 4d inner-shell electron.
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High Average Power Near-Infrared Few-Cycle Lasers - Rothhardt, J.; H{\"{a}drich, S. and Delagnes, J. C. and Cormier, E. and Limpert, J. (Laser and Photonics Reviews, 2017)
Ultra-short laser pulses with only a few optical cycles duration have gained increasing importance during the recent decade and are currently employed in many laboratories worldwide. In addition, modern laser technology nowadays can provide few-cycle pulses at very high average power which advances established studies and opens exciting novel research opportunities. In this paper, the two complementary approaches for providing few-cycle pulses at high average power, namely optical parametric amplification and nonlinear pulse compression, are reviewed and compared. In addition, their limitations and future scaling potential are discussed. Furthermore, selected applications particularly taking advantage of the high average power and high repetition rate are presented.
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High-average-power femtosecond laser at 258 nm - M{\"{u}}ller, M.; Klenke, A.; Gottschall, T.; Klas, R.; Rothhardt, C.; Demmler, S.; Rothhardt, J.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2017)
We present an ultrafast fiber laser system delivering 4.6 W average power at 258 nm based on two-stage fourth-harmonic generation in beta barium borate (BBO). The beam quality is close to being diffraction limited with an M^2 value of 1.3×1.6. The pulse duration is 150 fs, which, potentially, is compressible down to 40 fs. A plain BBO and a sapphire-BBO compound are compared with respect to the achievable beam quality in the conversion process. This laser is applicable in scientific and industrial fields. Further scaling to higher average power is discussed.
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Non-Abelian Higgs models: Paving the way for asymptotic freedom - Gies, H.; Zambelli, L. (Physical Review D, 2017)
Asymptotically free renormalization group trajectories can be constructed in non-Abelian Higgs models with the aid of generalized boundary conditions imposed on the renormalized action. We detail this construction within the languages of simple low-order perturbation theory, effective field theory, as well as modern functional renormalization group equations. We construct a family of explicit scaling solutions using a controlled weak-coupling expansion in the ultraviolet, and obtain a standard Wilsonian renormalization group relevance classification of perturbations about scaling solutions. We obtain global information about the quasifixed function for the scalar potential by means of analytic asymptotic expansions and numerical shooting methods. Further analytical evidence for such asymptotically free theories is provided in the large-N limit. We estimate the long-range properties of these theories and identify initial/boundary conditions giving rise to a conventional Higgs phase.
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Photoelectron distribution of nonresonant two-photon ionization of neutral atoms - Hofbrucker, J.; Volotka, A. V.; Fritzsche, S. (Physical Review A, 2017)
Photoelectron angular distributions following the nonresonant two-photon K-shell ionization of neutral atoms are studied theoretically. Using the independent particle approximation and relativistic second-order perturbation theory, the contributions of screening and relativistic effects to the photoelectron angular distribution are evaluated. A simple nonrelativistic expression is presented for the angle-differential cross section in dipole approximation for two-photon ionization by elliptically polarized photons, and its limitations are analyzed numerically. Moreover, we show that screening effects of the inactive electrons can significantly affect the photoelectron distributions and can also lead to a strong elliptical dichroism. Numerical results are presented for the case of two-photon K-shell ionization of neutral Ne, Ge, Xe, and U atoms.
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Transverse Coherence Limited Coherent Diffraction Imaging using a Molybdenum Soft X-ray Laser Pumped at Moderate Pump Energies - Z{\"{u}}rch, M.; Jung, R.; Sp{\"{a}th, C. and T{\"{u}}mmler, J. and Guggenmos, A. and Attwood, D. and Kleineberg, U. and Stiel, H. and Spielmann, C. (Scientific Reports, 2017)
Coherent diffraction imaging (CDI) in the extreme ultraviolet has become an important tool for nanoscale investigations. Laser-driven high harmonic generation (HHG) sources allow for lab scale applications such as cancer cell classification and phase-resolved surface studies. HHG sources exhibit excellent coherence but limited photon flux due poor conversion efficiency. In contrast, table-top soft X-ray lasers (SXRL) feature excellent temporal coherence and extraordinary high flux at limited transverse coherence. Here, the performance of a SXRL pumped at moderate pump energies is evaluated for CDI and compared to a HHG source. For CDI, a lower bound for the required mutual coherence factor of |μ12| ≥ 0.75 is found by comparing a reconstruction with fixed support to a conventional characterization using double slits. A comparison of the captured diffraction signals suggests that SXRLs have the potential for imaging micron scale objects with sub-20 nm resolution in orders of magnitude shorter integration time compared to a conventional HHG source. Here, the low transverse coherence diameter limits the resolution to approximately 180 nm. The extraordinary high photon flux per laser shot, scalability towards higher repetition rate and capability of seeding with a high harmonic source opens a route for higher performance nanoscale imaging systems based on SXRLs.
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Electro-optically controlled divided-pulse amplification - Stark, H.; M{\"{u}}ller, M.; Kienel, M.; Klenke, A.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2017)
A novel technique for divided-pulse amplification is presented in a proof-of-principle experiment. A pulse burst, cut out of the pulse train of a mode-locked oscillator, is amplified and temporally combined into a single pulse. High combination efficiency and excellent pulse contrast are demonstrated. The system is mostly fiber-coupled, enabling a high interferometric stability. This approach provides access to the amplitude and phase of the individual pulses in the burst to be amplified, potentially allowing the compensation of gain saturation and nonlinear phase mismatches within the burst. Therefore, this technique enables the scaling of the peak power and pulse energy of pulsed laser systems beyond currently prevailing limitations.
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Experimental discrimination of ion stopping models near the Bragg peak in highly ionized matter - Cayzac, W.; Frank, A.; Ortner, A.; Bagnoud, V.; Basko, M. M.; Bedacht, S.; Bl{\"{a}ser, C. and Blazevic, A. and Busold, S. and Deppert, O. and Ding, J. and Ehret, M. and Fiala, P. and Frydrych, S. and Gericke, D. O. and Hallo, L. and Helfrich, J. and Jahn, D. and Kjartansson, E. and Knetsch, A. and Kraus, D. and Malka, G. and Neumann, N. W. and P{\'{e}}pitone, K. and Pepler, D. and Sander, S. and Schaumann, G. and Schlegel, T. and Schroeter, N. and Schumacher, D. and Seibert, M. (Nature Communications, 2017)
The energy deposition of ions in dense plasmas is a key process in inertial confinement fusion that determines the α-particle heating expected to trigger a burn wave in the hydrogen pellet and resulting in high thermonuclear gain. However, measurements of ion stopping in plasmas are scarce and mostly restricted to high ion velocities where theory agrees with the data. Here, we report experimental data at low projectile velocities near the Bragg peak, where the stopping force reaches its maximum. This parameter range features the largest theoretical uncertainties and conclusive data are missing until today. The precision of our measurements, combined with a reliable knowledge of the plasma parameters, allows to disprove several standard models for the stopping power for beam velocities typically encountered in inertial fusion. On the other hand, our data support theories that include a detailed treatment of strong ion-electron collisions.
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High flux, beamed neutron sources employing deuteron-rich ion beams from D2O-ice layered targets - Alejo, A.; Krygier, A. G.; Ahmed, H.; Morrison, J. T.; Clarke, R. J.; Fuchs, J.; Green, A.; Green, J. S.; Jung, D.; Kleinschmidt, A.; Najmudin, Z.; Nakamura, H.; et al. (Plasma Physics and Controlled Fusion, 2017)
A forwardly-peaked bright neutron source was produced using a laser-driven, deuteron-rich ion beam in a pitcher-catcher scenario. A proton-free ion source was produced via target normal sheath acceleration from Au foils having a thin layer of D₂O ice at the rear side, irradiated by sub-petawatt laser pulses ( 200 J, 750 fs) at peak intensity ∼2×10²⁰ W cm⁻². The neutrons were preferentially produced in a beam of 70° FWHM cone along the ion beam forward direction, with maximum energy up to 40 MeV and a peak flux along the axis ∼2×10⁹ n sr⁻¹ for neutron energy above 2.5 MeV. The experimental data is in good agreement with the simulations carried out for the d(d,n)³He reaction using the deuteron beam produced by the ice-layered target.
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Hybrid soliton dynamics in liquid-core fibres - Chemnitz, M.; Gebhardt, M.; Gaida, C.; Stutzki, F.; Kobelke, J.; Limpert, J.; T{\"{u}}nnermann, A.; Schmidt, M. (Nature Communications, 2017)
The discovery of optical solitons being understood as temporally and spectrally stationary optical states has enabled numerous innovations among which, most notably, supercontinuum light sources have become widely used in both fundamental and applied sciences. Here, we report on experimental evidence for dynamics of hybrid solitons{–}a new type of solitary wave, which emerges as a result of a strong non-instantaneous nonlinear response in CS2-filled liquid-core optical fibres. Octave-spanning supercontinua in the mid-infrared region are observed when pumping the hybrid waveguide with a 460 fs laser (1.95 μm) in the anomalous dispersion regime at nanojoule-level pulse energies. A detailed numerical analysis well correlated with the experiment uncovers clear indicators of emerging hybrid solitons, revealing their impact on the bandwidth, onset energy and noise characteristics of the supercontinua. Our study highlights liquid-core fibres as a promising platform for fundamental optics and applications towards novel coherent and reconfigurable light sources.
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Magnetic brane solutions in Gauss–Bonnet–Maxwell massive gravity - Hendi, S. H.; Panah, B. E.; Panahiyan, S.; Momennia, M. (Physics Letters B, 2017)
Magnetic branes of Gauss–Bonnet–Maxwell theory in the context of massive gravity is studied in detail. Exact solutions are obtained and their interesting geometrical properties are investigated. It is argued that although these horizonless solutions are free of curvature singularity, they enjoy a cone-like geometry with a conic singularity. In order to investigate the effects of various parameters on the geometry of conic singularity, its corresponding deficit angle is studied. It will be shown that despite the effects of Gauss–Bonnet gravity on the solutions, deficit angle is free of Gauss–Bonnet parameter. On the other hand, the effects of massive gravity, cosmological constant and electrical charge on the deficit angle will be explored. Also, a brief discussion related to possible geometrical phase transition of these topological objects is given.
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Pulse Duration of Seeded Free-Electron Lasers - Finetti, P.; H{\"{o}}ppner, H.; Allaria, E.; Callegari, C.; Capotondi, F.; Cinquegrana, P.; Coreno, M.; Cucini, R.; Danailov, M.; Demidovich, A.; De Ninno, G.; Di Fraia, M.; et al. (Physical Review X, 2017)
The pulse duration, and, more generally, the temporal intensity profile of free-electron laser (FEL) pulses, is of utmost importance for exploring the new perspectives offered by FELs; it is a nontrivial experimental parameter that needs to be characterized. We measured the pulse shape of an extreme ultraviolet externally seeded FEL operating in high-gain harmonic generation mode. Two different methods based on the cross-correlation of the FEL pulses with an external optical laser were used. The two methods, one capable of single-shot performance, may both be implemented as online diagnostics in FEL facilities. The measurements were carried out at the seeded FEL facility FERMI. The FEL temporal pulse characteristics were measured and studied in a range of FEL wavelengths and machine settings, and they were compared to the predictions of a theoretical model. The measurements allowed a direct observation of the pulse lengthening and splitting at saturation, in agreement with the proposed theory.
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Soft x-ray induced femtosecond solid-to-solid phase transition - Tavella, F.; H{\"{o}}ppner, H.; Tkachenko, V.; Medvedev, N.; Capotondi, F.; Golz, T.; Kai, Y.; Manfredda, M.; Pedersoli, E.; Prandolini, M. J.; Stojanovic, N.; Tanikawa, T.; et al. (High Energy Density Physics, 2017)
Abstract Soft x-rays were applied to induce graphitization of diamond through a non-thermal solid-to-solid phase transition. This process was observed within poly-crystalline diamond with a time-resolved experiment using ultrashort soft x-ray pulses of duration 52.5 fs and cross correlated by an optical pulse of duration 32.8 fs. This scheme enabled for the first time the measurement of a phase transition on a timescale of ∼150 fs. Excellent agreement between experiment and theoretical predictions was found, using a dedicated code that followed the non-equilibrium evolution of the irradiated diamond including all transient electronic and structural changes. These observations confirm that soft x-rays can induce a non-thermal ultrafast solid-to-solid phase transition on a hundred femtosecond timescale.
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Spin-dependent quantum theory of high-order above-threshold ionization - Zille, D.; Seipt, D.; M{\"{o}}ller, M.; Fritzsche, S.; Paulus, G.G.; Milo{{s}}evi{}, D. B. (Physical Review A, 2017)
The strong-field-approximation theory of high-order above-threshold ionization of atoms is generalized to include the electron spin. The obtained rescattering amplitude consists of a direct and exchange part. On the examples of excited He atoms as well as Li^+ and Be^2+ ions, it is shown that the interference of these two amplitudes leads to an observable difference between the photoelectron momentum distributions corresponding to different initial spin states: Pronounced minima appear for singlet states, which are absent for triplet states.
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Studying the Dynamics of Relativistic Laser-Plasma Interaction on Thin Foils by Means of Fourier-Transform Spectral Interferometry - Bagnoud, V.; Hornung, J.; Schlegel, T.; Zielbauer, B.; Brabetz, C.; Roth, M.; Hilz, P.; Haug, M.; Schreiber, J.; Wagner, F. (Physical Review Letters, 2017)
We apply Fourier-transform spectral interferometry (FTSI) to study the interaction of intense laser pulses with ultrathin targets. Ultrathin submicrometer-thick solid CH targets were shot at the PHELIX laser facility with an intensity in the mid to upper 10^19 W/cm2 range using an innovative double-pulse structure. The transmitted pulse structure was analyzed by FTSI and shows a transition from a relativistic transparency-dominated regime for targets thinner than 500 nm to a hole-boring-dominated laser-plasma interaction for thicker targets. The results also confirm that the inevitable preplasma expansion happening during the rising slope of the pulse, a few picoseconds before the maximum of the pulse is reached, cannot be neglected and plays a dominant role in laser-plasma interaction with ultrathin solid targets.
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Commissioning of a Si(Li) Compton polarimeter with improved energy resolution - Vockert, M.; Weber, G.; Spillmann, U.; Krings, T.; Herdrich, M.; St{\"{o}}hlker, T. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
Abstract On the basis of a double-side segmented Si(Li) crystal a new Compton polarimeter was developed within the SPARC collaboration. The new detector is equipped with a cryogenic first stage of the preamplifiers to improve the energy resolution compared to previous detectors with preamplifiers operating at room temperature. We present first results from a commissioning measurement of the new instrument at the ESR storage ring of GSI in Darmstadt, Germany and contrast it with the performance of an precursor polarimeter system.
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Electron impact scattering and calculated ionization cross sections for SFx (x=1–5) radicals - Goswami, B.; Antony, B.; Fritzsche, S. (International Journal of Mass Spectrometry, 2017)
Abstract The spherical complex optical potential (SCOP) formalism is employed to solve the e−-SFx (x=1–5) scattering system. In this article, the total cross sections by electron impact from 50 to 5000eV are calculated. The complex scattering potential ionization contribution (CSP-ic) method is used to compute the electron-induced total ionization cross sections from the inelastic cross section in the energy range from ionization threshold to 5000eV. For most of the reported radicals, the magnitude and shape of cross section compares well with previous measurements and calculations, wherever available. However, for many targets results are predicted for the first time in this work. From the electron-impact scattering cross sections for the SFx (x=1–5) radicals, we also estimate the gas-kinetic radius and the van der Waals coefficient.
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Fast calculator for X-ray emission due to Radiative Recombination and Radiative Electron Capture in relativistic heavy-ion atom collisions - Herdrich, M.; Weber, G.; Gumberidze, A.; Wu, Z.; St{\"{o}}hlker, T. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
Abstract In experiments with highly charged, fast heavy ions the Radiative Recombination (RR) and Radiative Electron Capture (REC) processes have significant cross sections in an energy range of up to a few GeV / u . They are some of the most important charge changing processes in collisions of heavy ions with atoms and electrons, leading to the emission of a photon along with the formation of the ground and excited atomic states. Hence, for the understanding and planning of experiments, in particular for X-ray spectroscopy studies, at accelerator ring facilities, such as FAIR, it is crucial to have a good knowledge of these cross sections and the associated radiation characteristics. In the frame of this work a fast calculator, named RECAL, for the RR and REC process is presented and its capabilities are demonstrated with the analysis of a recently conducted experiment at the Experimental Storage Ring (ESR) at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany. A method is presented to determine unknown X-ray emission cross sections via normalization of the recorded spectra to REC cross sections calculated by RECAL.
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High precision hyperfine measurements in Bismuth challenge bound-state strong-field QED - Ullmann, J.; Andelkovic, Z.; Brandau, C.; Dax, A.; Geithner, W.; Geppert, C.; Gorges, C.; Hammen, M.; Hannen, V.; Kaufmann, S.; K{\"{o}}nig, K.; Litvinov, Y.; et al. (Nature Communications, 2017)
Electrons bound in highly charged heavy ions such as hydrogen-like bismuth 209^Bi^82+ experience electromagnetic fields that are a million times stronger than in light atoms. Measuring the wavelength of light emitted and absorbed by these ions is therefore a sensitive testing ground for quantum electrodynamical (QED) effects and especially the electron–nucleus interaction under such extreme conditions. However, insufficient knowledge of the nuclear structure has prevented a rigorous test of strong-field QED. Here we present a measurement of the so-called specific difference between the hyperfine splittings in hydrogen-like and lithium-like bismuth 209^Bi^82+,80+ with a precision that is improved by more than an order of magnitude. Even though this quantity is believed to be largely insensitive to nuclear structure and therefore the most decisive test of QED in the strong magnetic field regime, we find a 7-σ discrepancy compared with the theoretical prediction.
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Refractive-index measurement of Si at γ-ray energies up to 2 MeV - G{\"{u}}nther, M. M.; Jentschel, M.; Pollitt, A. J.; Thirolf, P. G.; Zepf, M. (Physical Review A, 2017)
The refractive index of silicon at γ-ray energies from 181 to 1959 keV was investigated using the GAMS6 double crystal spectrometer and found to follow the predictions of the classical scattering model. This is in contrast to earlier measurements on the GAMS5 spectrometer, which suggested a sign change in the refractive index for photon energies above 500 keV. We present a reevaluation of the original data from 2011 as well as data from a 2013 campaign in which we show that systematic errors due to diffraction effects of the prism can explain the earlier data.
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Relativistic calculations of differential ionization cross sections: Application to antiproton-hydrogen collisions - Bondarev, A. I.; Kozhedub, Y. S.; Tupitsyn, I. I.; Shabaev, V. M.; Plunien, G.; St{\"{o}}hlker, T. (Physical Review A, 2017)
A relativistic method based on the Dirac equation for calculating fully differential cross sections for ionization in ion-atom collisions is developed. The method is applied to ionization of the atomic hydrogen by antiproton impact, as a nonrelativistic benchmark. The fully differential, as well as various doubly and singly differential, cross sections for ionization are presented. The role of the interaction between the projectile and the target nucleus is discussed. Several discrepancies in available theoretical predictions are resolved. The relativistic effects are studied for ionization of hydrogenlike xenon ion under the impact of carbon nuclei.
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Depletion of Intense Fields - Seipt, D.; Heinzl, T.; Marklund, M.; Bulanov, S. S. (Physical Review Letters, 2017)
The interaction of charged particles and photons with intense electromagnetic fields gives rise to multiphoton Compton and Breit-Wheeler processes. These are usually described in the framework of the external field approximation, where the electromagnetic field is assumed to have infinite energy. However, the multiphoton nature of these processes implies the absorption of a significant number of photons, which scales as the external field amplitude cubed. As a result, the interaction of a highly charged electron bunch with an intense laser pulse can lead to significant depletion of the laser pulse energy, thus rendering the external field approximation invalid. We provide relevant estimates for this depletion and find it to become important in the interaction between fields of amplitude a0∼103 and electron bunches with charges of the order of 10 nC.
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Heisenberg-Euler effective action in slowly varying electric field inhomogeneities of Lorentzian shape - Karbstein, F. (Physical Review D, 2017)
We use a locally constant field approximation (LCFA) to study the one-loop Heisenberg-Euler effective action in a particular class of slowly varying inhomogeneous electric fields of Lorentzian shape with 0≤d<4 inhomogeneous directions. We show that, for these fields, the LCFA of the Heisenberg-Euler effective action can be represented in terms of a single parameter integral, with the constant field effective Lagrangian with rescaled argument as integration kernel. The imaginary part of the Heisenberg-Euler effective action contains information about the instability of the quantum vacuum towards the formation of a state with real electrons and positrons. Here, in particular, we focus on the dependence of the instantaneous vacuum decay rate on the dimension d of the field inhomogeneity. Specifically, for weak fields, we find an overall parametric suppression of the effect with (E0/Ecr)^(d/2), where E0 is the peak field strength of the inhomogeneity and Ecr the critical electric field strength.
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Noise reduction in third order cross-correlation by angle optimization of the interacting beams - Schanz, V. A.; Wagner, F.; Roth, M.; Bagnoud, V. (Optics Express, 2017)
We report on a novel technique to reduce the noise level in scanning third order cross-correlation. Large angles between the interacting beams combined with adapted crystal parameters lead to a significant decrease of noise photon generation while maintaining efficient generation of the third order signal. An enhanced scanning cross-correlator was developed based on the new technique proposed. In tests at the PHELIX laser facility this novel correlator performed within a dynamic range of 12.5 orders of magnitude.
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An addendum to the Heisenberg-Euler effective action beyond one loop - Gies, H.; Karbstein, F. (Journal of High Energy Physics, 2017)
We study the effective interactions of external electromagnetic fields induced by fluctuations of virtual particles in the vacuum of quantum electrodynamics. Our main focus is on these interactions at two-loop order. We discuss in detail the emergence of the renowned Heisenberg-Euler effective action from the underlying microscopic theory of quantum electrodynamics, emphasizing its distinction from a standard one-particle irreducible effective action. In our explicit calculations we limit ourselves to constant and slowly varying external fields, allowing us to adopt a locally constant field approximation. One of our main findings is that at two-loop order there is a finite one-particle reducible contribution to the Heisenberg-Euler effective action in constant fields, which was previously assumed to vanish. In addition to their conceptual significance, our results are relevant for high-precision probes of quantum vacuum nonlinearity in strong electromagnetic fields.
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Brilliant petawatt gamma-ray pulse generation in quantum electrodynamic laser-plasma interaction - Chang, H. X.; Qiao, B.; Huang, T. W.; Xu, Z.; Zhou, C. T.; Gu, Y. Q.; Yan, X. Q.; Zepf, M.; He, X. T. (Scientific Reports, 2017)
We show a new resonance acceleration scheme for generating ultradense relativistic electron bunches in helical motions and hence emitting brilliant vortical γ-ray pulses in the quantum electrodynamic (QED) regime of circularly-polarized (CP) laser-plasma interactions. Here the combined effects of the radiation reaction recoil force and the self-generated magnetic fields result in not only trapping of a great amount of electrons in laser-produced plasma channel, but also significant broadening of the resonance bandwidth between laser frequency and that of electron betatron oscillation in the channel, which eventually leads to formation of the ultradense electron bunch under resonant helical motion in CP laser fields. Three-dimensional PIC simulations show that a brilliant γ-ray pulse with unprecedented power of 6.7 PW and peak brightness of 10²⁵ photons/s/mm²/mrad²/0.1% BW (at 15 MeV) is emitted at laser intensity of 1.9 × 10²³ W/cm².
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Experimental investigation of picosecond dynamics following interactions between laser accelerated protons and water - Senje, L.; Coughlan, M.; Jung, D.; Taylor, M.; Nersisyan, G.; Riley, D.; Lewis, C. L. S.; Lundh, O.; Wahlstr{\"{o}}m, C.-G.; Zepf, M.; Dromey, B. (Applied Physics Letters, 2017)
We report direct experimental measurements with picosecond time resolution of how high energy protons interact with water at extreme dose levels (kGy), delivered in a single pulse with the duration of less than 80 ps. The unique synchronisation possibilities of laser accelerated protons with an optical probe pulse were utilized to investigate the energy deposition of fast protons in water on a time scale down to only a few picoseconds. This was measured using absorbance changes in the water, induced by a population of solvated electrons created in the tracks of the high energy protons. Our results indicate that for sufficiently high doses delivered in short pulses, intertrack effects will affect the yield of solvated electrons. The experimental scheme allows for investigation of the ultrafast mechanisms occurring in proton water radiolysis, an area of physics especially important due to its relevance in biology and for proton therapy.
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Laser spectroscopy measurement of the 2s-hyperfine splitting in lithium-like bismuth - Sanchez, R.; Lochmann, M.; J{\"{o}}hren, R.; Andelkovic, Z.; Anielski, D.; Botermann, B.; Bussmann, M.; Dax, A.; Fr{\"{o}}mmgen, N.; Geppert, C.; Hammen, M.; Hannen, V.; et al. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2017)
We have recently reported on the first direct measurement of the 2s hyperfine transition in lithium-like bismuth (209Bi^80+) at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany. Combined with a new measurement of the 1s hyperfine splitting (HFS) in hydrogen-like (209Bi^82+) the so-called specific difference Δ'E=-61.37(36) meV could be determined and was found to be in good agreement with its prediction from strong-field bound-state quantum electrodynamics. Here we report on additional investigations performed to estimate systematic uncertainties of these results and on details of the experimental setup. We show that the dominating uncertainty arises from insufficient knowledge of the ion beam velocity which is determined by the electron-cooler voltage. Two routes to obtain a cooler-voltage calibration are discussed and it is shown that agreement can be reached either between the experimental Δ'E and the theoretical result, or between the two measurements of the HFS in hydrogen-like bismuth, but not both at the same time.
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Photon Propagation in Slowly Varying Electromagnetic Fields - Karbstein, F. (Russian Physics Journal, 2017)
Effective theory of soft photons in slowly varying electromagnetic background fields is studied at one-loop order in QED. This is of relevance for the study of all-optical signatures of quantum vacuum nonlinearity in realistic electromagnetic background fields as provided by high-intensity lasers. The central result derived in this article is a new analytical expression for the photon polarization tensor in two linearly polarized counterpropagating pulsed Gaussian laser beams. Treating the peak field strengths of both laser beams as free parameters, this field configuration can be considered as interpolating between the limiting cases of a purely right- or left-moving laser beam (if one of the peak field strengths is set equal to zero) and the standing-wave type scenario with two counter-propagating beams of equal strength.
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Plasma effects on atomic data for the K-vacancy states of highly charged iron ions - Deprince, J.; Fritzsche, S.; Kallman, T.; Palmeri, P.; Quinet, P. (AIP Conference Proceedings, 2017)
The main goal of the present work is to estimate the effects of plasma environment on the atomic parameters associated with the K-vacancy states in highly charged iron ions within the astrophysical context of accretion disks around black holes. In order to do this, multiconfiguration Dirac-Fock computations have been carried out by considering a time averaged Debye-H{\"{u}}ckel potential for both the electron-nucleus and electron-electron interactions. In the present paper, a first sample of results related to the ionization potentials, the K-thresholds, the transition energies and the radiative emission rates is reported for the ions Fe23+ and Fe24+.
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Quasi-supercontinuum source in the extreme ultraviolet using multiple frequency combs from high-harmonic generation - W{\"{u}}nsche, M.; Fuchs, S.; Aull, S.; Nathanael, J.; M{\"{o}}ller, M.; R{\"{o}}del, C.; Paulus, G.G. (Optics Express, 2017)
A quasi-supercontinuum source in the extreme ultraviolet (XUV) is demonstrated using a table-top femtosecond laser and a tunable optical parametric amplifier (OPA) as a driver for high-harmonic generation (HHG). The harmonic radiation, which is usually a comb of odd multiples of the fundamental frequency, is generated by near-infrared (NIR) laser pulses from the OPA. A quasi-continuous XUV spectrum in the range of 30 to 100 eV is realized by averaging over multiple harmonic comb spectra with slightly different fundamental frequencies and thus different spectral spacing between the individual harmonics. The driving laser wavelength is swept automatically during an averaging time period. With a total photon flux of 4×10^9 photons/s in the range of 30 eV to 100 eV and 1×10^7photons/s in the range of 100 eV to 200 eV, the resulting quasi-supercontinuum XUV source is suited for applications such as XUV coherence tomography (XCT) or near-edge absorption fine structure spectroscopy (NEXAFS).
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Transition radiation at the boundary of a chiral isotropic medium - Galyamin, S.; Tyukhtin, A.; Peshkov, A. (Physical Review E, 2017)
This study analyzes the radiation produced by a point charge intersecting the interface between a vacuum and a chiral isotropic medium. We deduce analytical expressions for the Fourier components of an electromagnetic field in both vacuum and medium for arbitrary charge velocity. The main focus is on investigating the far field in a vacuum. The distinguishing feature of the interface with a chiral isotropic medium is that the field in the vacuum area contains both copolarization (coinciding with the polarization of the self-field of a charge) and cross-polarization (orthogonal to the polarization of the self-field). Using a saddle-point approach, we obtain asymptotic representations for the field components in the far-field zone for typical frequency ranges of the Condon model of the chiral medium. We note that a so-called lateral wave is generated in a vacuum for certain parameters. The main contribution to the radiation at large distances is presented by two (co- and cross-) spherical waves of transition radiation. These waves are coherent and result in a total spherical wave with elliptical polarization, with the polarization coefficient being determined by the chirality of the medium. We present typical radiation patterns and ellipses of polarization.
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X-ray emission generated by laser-produced plasmas from dielectric nanostructured targets - Samsonova, Z.; H{\"{o}}fer, S.; Hoffmann, A.; Landgraf, B.; Z{\"{u}}rch, M.; Uschmann, I.; Khaghani, D.; Rosmej, O.; Neumayer, P.; R{\"{o}}der, R.; Trefflich, L.; Ronning, C.; et al. (AIP Conference Proceedings, 2017)
We present an experimental study of X-ray generation from nanostructured ZnO targets. Samples of different morphology ranging from nanowires to polished surfaces are irradiated by relativistically intense femtosecond laser pulses. X-ray emission of plasma is generated by 45 fs 130 mJ laser pulses at 400 nm with picosecond temporal contrast better than 1E−9 interacting with an array of ZnO nanowires. The measured spectra indicate the existence of highly ionized states of Zn (up to He-like Zn). The obtained flux of ∼1E10 photons per laser shot at the neutral Zn Kα energies around 8.65 keV and at the Zn Heα energies around 9 keV is almost 3 times higher for nanostructured targets compared to the reference polished sample and implies 1E−4 conversion efficiency from the laser energy to the total energy of the emitted X-ray photons.
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Backreflection diagnostics for ultra-intense laser plasma experiments based on frequency resolved optical gating - Wagner, F.; Hornung, J.; Schmidt, C.; Eckhardt, M.; Roth, M.; St{\"{o}}hlker, T.; Bagnoud, V. (Review of Scientific Instruments, 2017)
We report on the development and implementation of a time resolved backscatter diagnostics for high power laser plasma experiments at the petawatt-class laser facility PHELIX. Pulses that are backscattered or reflected from overcritical plasmas are characterized spectrally and temporally resolved using a specially designed second harmonic generation frequency resolved optical gating system. The diagnostics meets the requirements made by typical experiments, i.e., a spectral bandwidth of more than 30nm with sub-nanometer resolution and a temporal window of 10ps with 50fs temporal resolution. The diagnostics is permanently installed at the PHELIX target area and can be used to study effects such as laser-hole boring or relativistic self-phase-modulation which are important features of laser-driven particle acceleration experiments.
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BREIT code: Analytical solution of the balance rate equations for charge-state evolutions of heavy-ion beams in matter - Winckler, N.; Rybalchenko, A.; Shevelko, V.; Al-Turany, M.; Kollegger, T.; St{\"{o}}hlker, T. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2017)
A detailed description of a recently developed BREIT computer code (Balance Rate Equations of Ion Transportation) for calculating charge-state fractions of ion beams passing through matter is presented. The code is based on the analytical solutions of the differential balance equations for the charge-state fractions as a function of the target thickness and can be used for calculating the ion evolutions in gaseous, solid and plasma targets. The BREIT code is available on-line and requires the charge-changing cross sections and initial conditions in the input file. The eigenvalue decomposition method, applied to obtain the analytical solutions of the rate equations, is described in the paper. Calculations of non-equilibrium and equilibrium charge-state fractions, performed by the BREIT code, are compared with experimental data and results of other codes for ion beams in gaseous and solid targets. Ability and limitations of the BREIT code are discussed in detail.
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High average power nonlinear compression to 4 GW, sub-50 fs pulses at 2 μm wavelength - Gebhardt, M.; Gaida, C.; Stutzki, F.; H{\"{a}drich, S. and Jauregui, C. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2017)
The combination of high-repetition-rate ultrafast thulium-doped fiber laser systems and gas-based nonlinear pulse compression in waveguides offers promising opportunities for the development of high-performance few-cycle laser sources at 2 μm wavelength. In this Letter, we report on a nonlinear pulse compression stage delivering 252 μJ, sub-50 fs-pulses at 15.4 W of average power. This performance level was enabled by actively mitigating ultrashort pulse propagation effects induced by the presence of water vapor absorptions.
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Monte Carlo approach to calculate ionization dynamics of hot solid-density plasmas within particle-in-cell simulations - Wu, D.; He, X. T.; Yu, W.; Fritzsche, S. (Physical Review E, 2017)
A physical model based on a Monte Carlo approach is proposed to calculate the ionization dynamics of hot-solid-density plasmas within particle-in-cell (PIC) simulations, and where the impact (collision) ionization (CI), electron-ion recombination (RE), and ionization potential depression (IPD) by surrounding plasmas are taken into consideration self-consistently. When compared with other models, which are applied in the literature for plasmas near thermal equilibrium, the temporal relaxation of ionization dynamics can also be simulated by the proposed model. Besides, this model is general and can be applied for both single elements and alloys with quite different compositions. The proposed model is implemented into a PIC code, with (final) ionization equilibriums sustained by competitions between CI and its inverse process (i.e., RE). Comparisons between the full model and model without IPD or RE are performed. Our results indicate that for bulk aluminium at temperature of 1 to 1000 eV, (i) the averaged ionization degree increases by including IPD; while (ii) the averaged ionization degree is significantly over estimated when the RE is neglected. A direct comparison from the PIC code is made with the existing models for the dependence of averaged ionization degree on thermal equilibrium temperatures and shows good agreements with that generated from Saha-Boltzmann model and/or FLYCHK code.
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Monte Carlo approach to calculate proton stopping in warm dense matter within particle-in-cell simulations - Wu, D.; He, X. T.; Yu, W.; Fritzsche, S. (Physical Review E, 2017)
A Monte Carlo approach to proton stopping in warm dense matter is implemented into an existing particle-in-cell code. This approach is based on multiple electron-electron, electron-ion, and ion-ion binary collision and accounts for both the free and the bound electrons in the plasmas. This approach enables one to calculate the stopping of particles in a more natural manner than existing theoretical treatment. In the low-temperature limit, when “all” electrons are bound to the nucleus, the stopping power coincides with the predictions from the Bethe-Bloch formula and is consistent with the data from the National Institute of Standard and Technology database. At higher temperatures, some of the bound electrons are ionized, and this increases the stopping power in the plasmas, as demonstrated by A. B. Zylstra et al. [Phys. Rev. Lett. 114, 215002 (2015)]. At even higher temperatures, the degree of ionization reaches a maximum and thus decreases the stopping power due to the suppression of collision frequency between projected proton beam and hot plasmas in the target.
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Numerical investigation of the sequential-double-ionization dynamics of helium in different few-cycle-laser-field shapes - Wustelt, P.; M{\"{o}}ller, M.; Sch{\"{o}}ffler, M.; Xie, X.; Hanus, V.; Sayler, A.; Baltuska, A.; Paulus, G.; Kitzler, M. (Physical Review A, 2017)
We investigate sequential double ionization of helium by intense near-circularly polarized few-cycle laser pulses using a semiclassical ionization model with two independent electrons. Simulated He^2+ ion momentum distributions are compared to those obtained in recent benchmark experiments. We study the inﬂuence of a number of pulse parameters such as peak intensity, carrier-envelope phase, pulse duration, and second- and third-order spectral phase on the shape of the ion momentum distributions. Good agreement is found in the main features of these distributions and of their dependence on the laser pulse duration, peak intensity, and carrier-envelope phase. Furthermore, we ﬁnd that for explaining certain ﬁne-scale features observed in the experiments, it becomes important to consider subtle timing variations in the two-electron emissions introduced by small values of chirp. This result highlights the possibility of measuring and controlling multielectron dynamics on the attosecond time scale by ﬁne tuning the ﬁeld evolution of intense close-to-single-cycle laser pulses.
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Quantitative shadowgraphy and proton radiography for large intensity modulations - Kasim, M.; Ceurvorst, L.; Ratan, N.; Sadler, J.; Chen, N.; S{\"{a}vert, A. and Trines, R. and Bingham, R. and Burrows, P. and Kaluza, M. and Norreys, P. (Physical Review E, 2017)
Shadowgraphy is a technique widely used to diagnose objects or systems in various fields in physics and engineering. In shadowgraphy, an optical beam is deflected by the object and then the intensity modulation is captured on a screen placed some distance away. However, retrieving quantitative information from the shadowgrams themselves is a challenging task because of the nonlinear nature of the process. Here, we present a method to retrieve quantitative information from shadowgrams, based on computational geometry. This process can also be applied to proton radiography for electric and magnetic field diagnosis in high-energy-density plasmas and has been benchmarked using a toroidal magnetic field as the object, among others. It is shown that the method can accurately retrieve quantitative parameters with error bars less than 10%, even when caustics are present. The method is also shown to be robust enough to process real experimental results with simple pre- and postprocessing techniques. This adds a powerful tool for research in various fields in engineering and physics for both techniques.
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Real-time, single-shot, carrier-envelope-phase measurement of a multi-terawatt laser - Adolph, D.; M{\"{o}}ller, M.; Bierbach, J.; Schwab, M.; S{\"{a}vert, A. and Yeung, M. and Sayler, A. M. and Zepf, M. and Kaluza, M.C. and Paulus, G.G. (Applied Physics Letters, 2017)
We present the single-shot carrier-envelope phase (CEP) determination of a 1 Hz, multi-terawatt (TW) laser system with a setup based on spectral broadening in a hollow-core fiber and a stereographic measurement of the energy-dependent above-threshold ionization plateau. The latter is extremely sensitive to variations in CEP. As compared to the f-2f interferometers, this technique reduces the uncertainties due to the shot-to-shot intensity fluctuations, which are prevalent in the TW laser systems. The experimental results pave the way towards the investigation and control over CEP-sensitive processes at ultra-high intensities.
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Realizing total reciprocity violation in the phase for photon scattering - De{\'{a}}k, L.; Botty{\'{a}}n, L.; F{\"{u}}l{\"{o}}p, T.; Merkel, D. G.; Nagy, D. L.; Sajti, S.; Schulze, K. S.; Spiering, H.; Uschmann, I.; Wille, H.-C. (Scientific Reports, 2017)
Reciprocity is when wave or quantum scattering satisfies a symmetry property, connecting a scattering process with the reversed one. While reciprocity involves the interchange of source and detector, it is fundamentally different from rotational invariance, and is a generalization of time reversal invariance, occurring in absorptive media as well. Due to its presence at diverse areas of physics, it admits a wide variety of applications. For polarization dependent scatterings, reciprocity is often violated, but violation in the phase of the scattering amplitude is much harder to experimentally observe than violation in magnitude. Enabled by the advantageous properties of nuclear resonance scattering of synchrotron radiation, we have measured maximal, i.e., 180-degree, reciprocity violation in the phase. For accessing phase information, we introduced a new version of stroboscopic detection. The scattering setting was devised based on a generalized reciprocity theorem that opens the way to construct new types of reciprocity related devices.
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Refractive index dispersion measurement using carrier-envelope phasemeters - Hansinger, P.; T{\"{o}}pfer, P.; Dimitrov, N.; Adolph, D.; Hoff, D.; Rathje, T.; Sayler, A. M.; Dreischuh, A.; Paulus, G.G. (New Journal of Physics, 2017)
We introduce a novel method for direct and accurate measurement of refractive index dispersion based on carrier-envelope phase detection of few-cycle laser pulses, exploiting the difference between phase and group velocity in a dispersive medium. In a layout similar to an interferometer, two carrier-envelope phasemeters are capable of measuring the dispersion of a transparent or reflective sample, where one phasemeter serves as the reference and the other records the influence of the sample. Here we report on proof-of-principle measurements that already reach relative uncertainties of a few 10^−4 . Further development is expected to allow for unprecedented precision.
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Spin-dependent rescattering in strong-field ionization of helium - Zille, D.; Seipt, D.; M{\"{o}}ller, M.; Fritzsche, S.; Gr{\"{a}fe, S. and M{\"{u}}ller, C. and Paulus, G.G. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2017)
We investigate the influence of singlet and triplet spin states on rescattered photoelectrons in strong-field ionization of excited helium. Choosing either a symmetric or antisymmetric spatial wave function as the initial state results in different scattering cross sections for the 1s2s¹S and ³S states. These cross sections are used in the semi-classical model of strong-field ionization. Our investigations show that the photoelectron momentum distributions of rescattered electrons exhibit a significant dependence on the relative spin state of the projectile and the bound electron which should be observable in experiments. The proposed experimental approach can be understood as a testbed for probing the spin dynamics of electrons during strong-field ionization and the presented results as a baseline for their identification.
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Towards high-resolution laser ionization spectroscopy of the heaviest elements in supersonic gas jet expansion - Ferrer, R.; Barzakh, A.; Bastin, B.; Beerwerth, R.; Block, M.; Creemers, P.; Grawe, H.; Groote, R.; Delahaye, P.; Fl{\'{e}}chard, X.; Franchoo, S.; Fritzsche, S.; et al. (Nature Communications, 2017)
Resonant laser ionization and spectroscopy are widely used techniques at radioactive ion beam facilities to produce pure beams of exotic nuclei and measure the shape, size, spin and electromagnetic multipole moments of these nuclei. However, in such measurements it is difficult to combine a high efficiency with a high spectral resolution. Here we demonstrate the on-line application of atomic laser ionization spectroscopy in a supersonic gas jet, a technique suited for high-precision studies of the ground- and isomeric-state properties of nuclei located at the extremes of stability. The technique is characterized in a measurement on actinium isotopes around the N=126 neutron shell closure. A significant improvement in the spectral resolution by more than one order of magnitude is achieved in these experiments without loss in efficiency.
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Beam load structures in a basic relativistic interaction model - Thomas, J.; G{\"{u}}nther, M. M.; Pukhov, A. (Physics of Plasmas, 2017)
Some recent experiments have shown that the beam load in bubble and blow-out experiments is located in a volume as small as a few μm^3. Now, we show what kinds of inner structures are possible in such a high dense electron ensemble. Our analysis starts from a first principles model for relativistically corrected mutual electron interaction in a phenomenological bubble model. Discussing 2D and 3D beam load configurations, we show that, depending on the bunch emittance, the beam load might be in a highly ordered and dense configuration, a less ordered but still dense state, or a configuration where each electron performs an individual random motion.
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Critical Schwinger pair production. II. Universality in the deeply critical regime - Gies, H.; Torgrimsson, G. (Physical Review D, 2017)
We study electron-positron pair production by spatially inhomogeneous electric fields. Depending on the localization of the field, a critical point (critical surface) exists in the space of field configurations where the pair production probability vanishes. Near criticality, pair production exhibits universal properties similar to those of continuous phase transitions. We extend results previously obtained in the semiclassical (weak-field) critical regime to the deeply critical regime for arbitrary peak field strength. In this regime, we find an enhanced universality, featuring a unique critical exponent β=3 for all sufficiently localized fields. For a large class of field profiles, we also compute the nonuniversal amplitudes.
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Horizon 2020 EuPRAXIA design study - Walker, P. A.; Alesini, P. D.; Alexandrova, A. S.; Anania, M. P.; Andreev, N. E.; Andriyash, I.; Aschikhin, A.; Assmann, R. W.; Audet, T.; Bacci, A.; Barna, I. F.; Beaton, A.; et al. (Journal of Physics: Conference Series, 2017)
The Horizon 2020 Project EuPRAXIA (“European Plasma Research Accelerator with eXcellence In Applications”) is preparing a conceptual design report of a highly compact and cost-effective European facility with multi-GeV electron beams using plasma as the acceleration medium. The accelerator facility will be based on a laser and/or a beam driven plasma acceleration approach and will be used for photon science, high-energy physics (HEP) detector tests, and other applications such as compact X-ray sources for medical imaging or material processing. EuPRAXIA started in November 2015 and will deliver the design report in October 2019. EuPRAXIA aims to be included on the ESFRI roadmap in 2020.
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Intracavity stretcher for chirped-pulse amplification in high-power laser systems - Liebetrau, H.; Hornung, M.; Keppler, S.; Hellwing, M.; Kessler, A.; Schorcht, F.; Hein, J.; Kaluza, M.C. (Optics Letters, 2017)
We present pulse stretching with an intracavity Offner-type pulse stretcher applied to a high-energy, short-pulse laser system. The compact intracavity design, offering a tunable stretching factor, allows the pulses to be stretched to several nanoseconds and, at the same time, to be amplified to 100 μJ. The stretched pulses have been further amplified with the high-power laser system Polaris and have been recompressed to durations as short as 102 fs, reaching peak powers of 100 TW. Furthermore, the temporal intensity contrast is investigated and compared to the formerly used stretcher setup.
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Online Monitoring of Laser-Generated XUV Radiation Spectra by Surface Reflectivity Measurements with Particle Detectors - Hoffmann, A.; Egelkamp, C.; Winters, D.; K{\"{u}}hl, T.; Spielmann, C. (Applied Sciences, 2017)
In this contribution, we present a wavelength-sensitive method for the detection of extreme ultraviolet (XUV) photon energies between 30 eV and 120 eV. The method is based on 45° reflectivity from either a cesium iodide-coated or an uncoated metal surface, which directs the XUV beam onto an electron or ion detector and its signal is used to monitor the XUV beam. The benefits of our approach are a spectrally sensitive diagnosis of the XUV radiation at the interaction place of time-resolved XUV experiments and the detection of infrared leak light though metal filters in high-harmonic generation (HHG) experiments. Both features were tested using spectrally shaped XUV pulses from HHG in a capillary, and we have achieved excellent agreement with XUV spectrometer measurements and reflectivity calculations. Our obtained results are of interest for time-resolved XUV experiments presenting an additional diagnostic directly in the interaction region and for small footprint XUV beamline diagnostics.
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Performance of a quantum defect minimized disk laser based on cryogenically cooled Yb:CaF2 - Kahle, M.; K{\"{o}}rner, J.; Hein, J.; Kaluza, M.C. (Optics { &} Laser Technology, 2017)
Abstract A low quantum defect is the fundamental key to a high efficiency of any laser. To study the anticipated performance boost for a 980 nm-diode pumped cryogenically cooled Yb:CaF2 disk laser we compared its operation at output wavelengths of 991 nm, 996 nm, and 1032 nm. Despite the higher quantum defect a maximum efficiency of 74% (output versus incident power) with an output power of 15.8 W was achieved at the 1032 nm output wavelength. This observation led to a detailed analysis of remaining loss mechanisms we are reporting on in this paper.
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Charge Radii of Neutron Deficient ⁵²,⁵³Fe Produced by Projectile Fragmentation - Minamisono, K.; Rossi, D. M.; Beerwerth, R.; Fritzsche, S.; Garand, D.; Klose, A.; Liu, Y.; Maass, B.; Mantica, P. F.; Miller, A. J.; M{\"{u}}ller, P.; Nazarewicz, W.; et al. (Physical Review Letters, 2016)
Bunched-beam collinear laser spectroscopy is performed on neutron deficient Fe52,53 prepared through in-flight separation followed by a gas stopping. This novel scheme is a major step to reach nuclides far from the stability line in laser spectroscopy. Differential mean-square charge radii δ⟨r2⟩ of Fe52,53 are determined relative to stable Fe56 as δ⟨r2⟩56,52=−0.034(13) fm2 and δ⟨r2⟩56,53=−0.218(13) fm2, respectively, from the isotope shift of atomic hyperfine structures. The multiconfiguration Dirac-Fock method is used to calculate atomic factors to deduce δ⟨r2⟩. The values of δ⟨r2⟩ exhibit a minimum at the N=28 neutron shell closure. The nuclear density functional theory with Fayans and Skyrme energy density functionals is used to interpret the data. The trend of δ⟨r2⟩ along the Fe isotopic chain results from an interplay between single-particle shell structure, pairing, and polarization effects and provides important data for understanding the intricate trend in the δ⟨r2⟩ of closed-shell Ca isotopes.
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Experimental observation of attosecond control over relativistic electron bunches with two-colour fields - Yeung, M.; Rykovanov, S.; Bierbach, J.; Li, L.; Eckner, E.; Kuschel, S.; Woldegeorgis, A.; R{\"{o}}del, C.; S{\"{a}vert, A. and Paulus, G.G. and Coughlan, M. and Dromey, B. and Zepf, M. (Nature Photonics, 2016)
Energy coupling during relativistically intense laser–matter interactions is encoded in the attosecond motion of strongly driven electrons at the pre-formed plasma–vacuum boundary. Studying and controlling this motion can reveal details about the microscopic processes that govern a vast array of light–matter interaction phenomena, including those at the forefront of extreme laser–plasma science such as laser-driven ion acceleration, bright attosecond pulse generation and efficient energy coupling for the generation and study of warm dense matter. Here we experimentally demonstrate that by precisely adjusting the relative phase of an additional laser beam operating at the second harmonic of the driving laser it is possible to control the trajectories of relativistic electron bunches formed during the interaction with a solid target at the attosecond scale. We observe significant enhancements in the resulting high-harmonic yield, suggesting potential applications for sources of ultra-bright, extreme ultraviolet attosecond radiation to be used in atomic and molecular pump–probe experiments.
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Extraction of enhanced, ultrashort laser pulses from a passive 10-MHz stack-and-dump cavity - Breitkopf, S.; Wunderlich, S.; Eidam, T.; Shestaev, E.; Holzberger, S.; Gottschall, T.; Carstens, H.; T{\"{u}}nnermann, A.; Pupeza, I.; Limpert, J. (Applied Physics B, 2016)
Periodic dumping of ultrashort laser pulses from a passive multi-MHz repetition-rate enhancement cavity is a promising route towards multi-kHz repetition-rate pulses with Joule-level energies at an unparalleled average power. Here, we demonstrate this so-called stack-and-dump scheme with a 30-m-long cavity. Using an acousto-optic modulator, we extract pulses of 0.16 mJ at 30-kHz repetition rate, corresponding to 65 stacked input pulses, representing an improvement in three orders of magnitude over previously extracted pulse energies. The ten times longer cavity affords three essential benefits over former approaches. First, the time between subsequent pulses is increased to 100 ns, relaxing the requirements on the switch. Second, it allows for the stacking of strongly stretched pulses (here from 800 fs to 1.5 ns), thus mitigating nonlinear effects in the cavity optics. Third, the choice of a long cavity offers increased design flexibility with regard to thermal robustness, which will be crucial for future power scaling. The herein presented results constitute a necessary step towards stack-and-dump systems providing access to unprecedented laser parameter regimes.
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Monte Carlo study of the effective Sherman function for electron polarimetry - Dr{{a}}gowski, M.; Wlodarczyk, M.; Weber, G.; Ciborowski, J.; Enders, J.; Fritzsche, Y.; Poliszczuk, A. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2016)
The PEBSI Monte Carlo simulation was upgraded towards usefulness for electron Mott polarimetry. The description of Mott scattering was improved and polarisation transfer in Moller scattering was included in the code. An improved agreement was achieved between the simulation and available experimental data for a 100 keV polarised electron beam scattering off gold foils of various thicknesses. The dependence of the effective Sherman function on scattering angle and target thickness, as well as the method of finding optimal conditions for Mott polarimetry measurements were analysed.
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Multiconfiguration calculations of electronic isotope shift factors in Al I - Filippin, L.; Beerwerth, R.; Ekman, J.; Fritzsche, S.; Godefroid, M.; J{\"{o}}nsson, P. (Physical Review A, 2016)
The present work reports results from systematic multiconfiguration Dirac–Hartree–Fock calculations of electronic isotope shift factors for a set of transitions between low-lying levels of neutral aluminium. These electronic quantities together with observed isotope shifts between different pairs of isotopes provide the changes in mean-square charge radii of the atomic nuclei. Two computational approaches are adopted for the estimation of the mass- and field-shift factors. Within these approaches, different models for electron correlation are explored in a systematic way to determine a reliable computational strategy and to estimate theoretical error bars of the isotope shift factors.
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Nuclear Excitation by Two-Photon Electron Transition - Volotka, A. V.; Surzhykov, A.; Trotsenko, S.; Plunien, G.; St{\"{o}}hlker, T.; Fritzsche, S. (Physical Review Letters, 2016)
A new mechanism of nuclear excitation via two-photon electron transitions (NETP) is proposed and studied theoretically. As a generic example, detailed calculations are performed for the E1E1 1s2sS01→1s2S01 two-photon decay of a He-like Ac87+225 ion with a resonant excitation of the 3/2+ nuclear state with an energy of 40.09(5) keV. The probability for such a two-photon decay via the nuclear excitation is found to be PNETP=3.5×10−9 and, thus, is comparable with other mechanisms, such as nuclear excitation by electron transition and by electron capture. The possibility for the experimental observation of the proposed mechanism is thoroughly discussed.
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Phase-stable, multi-textmuJ femtosecond pulses from a repetition-rate tunable Ti:Sa-oscillator-seeded Yb-fiber amplifier - Saule, T.; Holzberger, S.; de Vries, O.; Pl{\"{o}}tner, M.; Limpert, J.; T{\"{u}}nnermann, A.; Pupeza, I. (Applied Physics B, 2016)
We present a high-power, MHz-repetition-rate, phase-stable femtosecond laser system based on a phase-stabilized Ti:Sa oscillator and a multi-stage Yb-fiber chirped-pulse power amplifier. A 10-nm band around 1030 nm is split from the 7-fs oscillator output and serves as the seed for subsequent amplification by 54 dB to 80 W of average power. The textmuJ-level output is spectrally broadened in a solid-core fiber and compressed to textasciitilde30 fs with chirped mirrors. A pulse picker prior to power amplification allows for decreasing the repetition rate from 74 MHz by a factor of up to 4 without affecting the pulse parameters. To compensate for phase jitter added by the amplifier to the feed-forward phase-stabilized seeding pulses, a self-referencing feed-back loop is implemented at the system output. An integrated out-of-loop phase noise of less than 100 mrad was measured in the band from 0.4 Hz to 400 kHz, which to the best of our knowledge corresponds to the highest phase stability ever demonstrated for high-power, multi-MHz-repetition-rate ultrafast lasers. This system will enable experiments in attosecond physics at unprecedented repetition rates, it offers ideal prerequisites for the generation and field-resolved electro-optical sampling of high-power, broadband infrared pulses, and it is suitable for phase-stable white light generation.
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Relativistic calculations of the nonresonant two-photon ionization of neutral atoms - Hofbrucker, J.; Volotka, A. V.; Fritzsche, S. (Physical Review A, 2016)
The nonresonant, two-photon, one-electron ionization of neutral atoms is studied theoretically in the framework of relativistic second-order perturbation theory and independent particle approximation. In particular, the importance of relativistic and screening effects in the total two-photon ionization cross section is investigated. Detailed computations have been carried out for the K-shell ionization of neutral Ne, Ge, Xe, and U atoms. The relativistic effects significantly decrease the total cross section; for the case of U, for example, they reduce the total cross section by a factor of two. Moreover, we have found that the account for the screening effects of the remaining electrons leads to occurrence of an unexpected minimum in the total cross section at the total photon energies equal to the ionization threshold; for the case of Ne, for example, the cross section drops there by a factor of three.
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54 J pulses with 18 nm bandwidth from a diode-pumped chirped-pulse amplification laser system - Hornung, M.; Liebetrau, H.; Keppler, S.; Kessler, A.; Hellwing, M.; Schorcht, F.; Becker, G. A.; Reuter, M.; Polz, J.; K{\"{o}}rner, J.; Hein, J.; Kaluza, M.C. (Optics Letters, 2016)
We report on results from the fully diode-pumped chirped-pulse amplification laser system Polaris. Pulses were amplified to a maximum energy of 54.2 J before compression. These pulses have a full width at half-maximum spectral bandwidth of 18 nm centered at 1033 nm and are generated at a repetition rate of 0.02 Hz. To the best of our knowledge, these are the most energetic broadband laser pulses generated by a diode-pumped laser system so far. Due to the limited size of our vacuum compressor, only attenuated pulses could be compressed to a duration of 98 fs containing an energy of 16.7 J, which leads to a peak power of 170 TW. These pulses could be focused to a peak intensity of 1.3×1021 W/cm2. Having an ultra-high temporal contrast of 1012 with respect to amplified spontaneous emission these laser pulses are well suited for high-intensity laser–matter experiments.
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Changes in nuclear structure along the Mn isotopic chain studied via charge radii - Heylen, H.; Babcock, C.; Beerwerth, R.; Billowes, J.; Bissell, M. L.; Blaum, K.; Bonnard, J.; Campbell, P.; Cheal, B.; Day Goodacre, T.; Fedorov, D.; Fritzsche, S.; et al. (Physical Review C, 2016)
The hyperfine spectra of Mn51,53−64 were measured in two experimental runs using collinear laser spectroscopy at ISOLDE, CERN. Laser spectroscopy was performed on the atomic 3d54s2S5/26→3d54s4pP3/26 and ionic 3d54s5S2→3d54p5P3 transitions, yielding two sets of isotope shifts. The mass and field shift factors for both transitions have been calculated in the multiconfiguration Dirac-Fock framework and were combined with a King plot analysis in order to obtain a consistent set of mean-square charge radii which, together with earlier work on neutron-deficient Mn, allow the study of nuclear structure changes from N=25 across N=28 up to N=39. A clear development of deformation is observed towards N=40, confirming the conclusions of the nuclear moments studies. From a Monte Carlo shell-model study of the shape in the Mn isotopic chain, it is suggested that the observed development of deformation is not only due to an increase in static prolate deformation but also due to shape fluctuations and triaxiality. The changes in mean-square charge radii are well reproduced using the Duflo-Zuker formula except in the case of large deformation.
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Compact Aberration-Free Relay-Imaging Multi-Pass Layouts for High-Energy Laser Amplifiers - K{\"{o}}rner, J.; Hein, J.; Kaluza, M. (Applied Sciences, 2016)
We present the results from a theoretical investigation of laser beam propagation in relay imaging multi-pass layouts, which recently found application in high-energy laser amplifiers. Using a method based on the well-known ABCD-matrix formalism and proven by ray tracing, it was possible to derive a categorization of such systems. Furthermore, basic rules for the setup of such systems and the compensation for low order aberrations are derived. Due to the introduced generalization and parametrization, the presented results can immediately be applied to any system of the investigated kinds for a wide range of parameters, such as number of round-trips, focal lengths and optics sizes. It is shown that appropriate setups allow a close-to-perfect compensation of defocus caused by a thermal lens and astigmatism caused by non-normal incidence on the imaging optics, as well. Both are important to avoid intensity spikes leading to damages of optics in multi-pass laser amplifiers.
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High speed and high resolution table-top nanoscale imaging - Tadesse, G. K.; Klas, R.; Demmler, S.; H{\"{a}drich, S. and Wahyutama, I. and Steinert, M. and Spielmann, C. and Z{\"{u}}rch, M. and Pertsch, T. and T{\"{u}}nnermann, A. and Limpert, J. and Rothhardt, J. (Optics Letters, 2016)
We present a table-top coherent diffractive imaging (CDI) experiment based on high-order harmonics generated at 18 nm by a high average power femtosecond fiber laser system. The high photon flux, narrow spectral bandwidth, and high degree of spatial coherence allow for ultrahigh subwavelength resolution imaging at a high numerical aperture. Our experiments demonstrate a half-pitch resolution of 15 nm, close to the actual Abbe limit of 12 nm, which is the highest resolution achieved from any table-top extreme ultraviolet (XUV) or x-ray microscope. In addition, sub-30 nm resolution was achieved with only 3 s of integration time, bringing live diffractive imaging and three-dimensional tomography on the nanoscale one step closer to reality. The current resolution is solely limited by the wavelength and the detector size. Thus, table-top nanoscopes with only a few-nanometer resolutions are in reach and will find applications in many areas of science and technology.
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Intensity dependence of the dissociative ionization of DCl in few-cycle laser fields - Li, H.; Tong, X. M.; Schirmel, N.; Urbasch, G.; Betsch, K. J.; Zherebtsov, S.; S{\"{u}}ssmann, F.; Kessel, A.; Trushin, S. A.; Paulus, G.G.; Weitzel, K.-M.; Kling, M. F. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2016)
We have studied the dissociative ionization of DCl in 4 fs laser fields at 720 nm central wavelength using intensities in the range (1.3–3.1) × 10^14 W cm−2 . By employing the phase-tagged velocity-map imaging technique, information about the angular distribution of deuterium ions as a function of their kinetic energy and the carrier-envelope phase is obtained. On the basis of the experimental data and semi-classical simulations, three regions are distinguished for the resulting D+ ions with different kinetic energies. The one with the lowest kinetic energy, around 5–7 eV, is from dissociation involving the X-state of DCl+ , populated through direct ionization with the laser field. The second region, around 7–11 eV, originates from rescattering induced dissociative ionization. Above 2 × 10^14 W cm−2 D+ ions with kinetic energies exceeding 15 eV are obtained, which we ascribe to double ionization induced by rescattered electrons.
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Non-perturbative measurement of low-intensity charged particle beams - Fernandes, M.; Geithner, R.; Golm, J.; Neubert, R.; Schwickert, M.; St{\"{o}}hlker, T.; Tan, J.; Welsch, C. P. (Superconductor Science and Technology, 2016)
Non-perturbative measurements of low-intensity charged particle beams are particularly challenging to beam diagnostics due to the low amplitude of the induced electromagnetic fields. In the low-energy antiproton decelerator (AD) and the future extra low energy antiproton rings at CERN, an absolute measurement of the beam intensity is essential to monitor the operation efficiency. Superconducting quantum interference device (SQUID) based cryogenic current comparators (CCC) have been used for measuring slow charged beams in the nA range, showing a very good current resolution. But these were unable to measure fast bunched beams, due to the slew-rate limitation of SQUID devices and presented a strong susceptibility to external perturbations. Here, we present a CCC system developed for the AD machine, which was optimised in terms of its current resolution, system stability, ability to cope with short bunched beams, and immunity to mechanical vibrations. This paper presents the monitor design and the first results from measurements with a low energy antiproton beam obtained in the AD in 2015. These are the first CCC beam current measurements ever performed in a synchrotron machine with both coasting and short bunched beams. It is shown that the system is able to stably measure the AD beam throughout the entire cycle, with a current resolution of 30 nA .
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Polarization transfer in Rayleigh scattering of hard x-rays - Blumenhagen, K.-H.; Fritzsche, S.; Gassner, T.; Gumberidze, A.; M{\"{a}rtin, R. and Schell, N. and Seipt, D. and Spillmann, U. and Surzhykov, A. and Trotsenko, S. and Weber, G. and Yerokhin, V. A. and St{\"{o}}hlker, T. (New Journal of Physics, 2016)
We report on the first elastic hard x-ray scattering experiment where the linear polarization characteristics of both the incident and the scattered radiation were observed. Rayleigh scattering was investigated in a relativistic regime by using a high- Z target material, namely gold, and a photon energy of 175 keV. Although the incident synchrotron radiation was nearly 100% linearly polarized, at a scattering angle of θ=90° we observed a strong depolarization for the scattered photons with a degree of linear polarization of +27% ± 12% only. This finding agrees with second-order quantum electrodynamics calculations of Rayleigh scattering, when taking into account a small polarization impurity of the incident photon beam which was determined to be close to 98%. The latter value was obtained independently from the elastic scattering by analyzing photons that were Compton-scattered in the target. Moreover, our results indicate that when relying on state-of-the-art theory, Rayleigh scattering could provide a very accurate method to diagnose polarization impurities in a broad region of hard x-ray energies.
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Table-top milliwatt-class extreme ultraviolet high harmonic light source - Klas, R.; Demmler, S.; Tschernajew, M.; H{\"{a}drich, S. and Shamir, Y. and T{\"{u}}nnermann, A. and Rothhardt, J. and Limpert, J. (Optica, 2016)
Extreme ultraviolet (XUV) lasers are essential for the investigation of fundamental physics. Especially high repetition rate, high photon flux sources are of major interest for reducing acquisition times and improving signal-to-noise ratios in a plethora of applications. Here, an XUV source based on cascaded frequency conversion is presented, which, due to the drastic better single atom response for short wavelength drivers, delivers an average output power of (832±204) μW at 21.7 eV. This is the highest average power produced by any high harmonic generation source in this spectral range, surpassing previous demonstrations by almost an order of magnitude. Furthermore, a narrowband harmonic at 26.6 eV with a relative energy bandwidth of only ΔE/E=1.8·10−3 has been generated that is of high interest for high-precision spectroscopy experiments.
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Two-color above-threshold ionization of atoms and ions in XUV Bessel beams and intense laser light - Seipt, D.; M{\"{u}}ller, R. A.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2016)
The two-color above-threshold ionization (ATI) of atoms and ions is investigated for a vortex Bessel beam in the presence of a strong near-infrared (NIR) light field. While the photoionization is caused by the photons from the weak but extreme ultraviolet (XUV) vortex Bessel beam, the energy and angular distribution of the photoelectrons and their sideband structure are affected by the plane-wave NIR field. We here explore the energy spectra and angular emission of the photoelectrons in such two-color fields as a function of the size and location of the target atoms with regard to the beam axis. In addition, analog to the circular dichroism in typical two-color ATI experiments with circularly polarized light, we define and discuss seven different dichroism signals for such vortex Bessel beams that arise from the various combinations of the orbital and spin angular momenta of the two light fields. For localized targets, it is found that these dichroism signals strongly depend on the size and position of the atoms relative to the beam. For macroscopically extended targets, in contrast, three of these dichroism signals tend to zero, while the other four just coincide with the standard circular dichroism, similar as for Bessel beams with a small opening angle. Detailed computations of the dichroism are performed and discussed for the 4s valence-shell photoionization of Ca+ ions.
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Elastic scattering of vortex electrons provides direct access to the Coulomb phase - Ivanov, I. P.; Seipt, D.; Surzhykov, A.; Fritzsche, S. (Physical Review D, 2016)
Vortex electron beams are freely propagating electron waves carrying adjustable orbital angular momentum with respect to the propagation direction. Such beams were experimentally realized just a few years ago and are now used to probe various electromagnetic processes. So far, these experiments used the single vortex electron beams, either propagating in external fields or impacting a target. Here, we investigate the elastic scattering of two such aligned vortex electron beams and demonstrate that this process allows one to experimentally measure features which are impossible to detect in the usual plane-wave scattering. The scattering amplitude of this process is well approximated by two plane-wave scattering amplitudes with different momentum transfers, which interfere and give direct experimental access to the Coulomb phase. This phase (shift) affects the scattering of all charged particles and has thus received significant theoretical attention but was never probed experimentally. We show that a properly defined azimuthal asymmetry, which has no counterpart in plane-wave scattering, allows one to directly measure the Coulomb phase as function of the scattering angle.
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Photoionization of neutral atoms by X waves carrying orbital angular momentum - M{\"{u}}ller, R.; Seipt, D.; Beerwerth, R.; Ornigotti, M.; Szameit, A.; Fritzsche, S.; Surzhykov, A. (Physical Review A, 2016)
In contrast to plane waves, twisted or vortex beams have a complex spatial structure. Both their intensity and energy flow vary within the wave front. Beyond that, polychromatic vortex beams, such as X waves, have a spatially dependent energy distribution. We propose a method to measure this (local) energy spectrum. The method is based on the measurement of the energy distribution of photoelectrons from alkali-metal atoms. On the basis of our fully relativistic calculations, we argue that even ensembles of atoms can be used to probe the local energy spectrum of short twisted pulses.
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Prominent role of multielectron processes in K-shell double and triple photodetachment of oxygen anions - Schippers, S.; Beerwerth, R.; Abrok, L.; Bari, S.; Buhr, T.; Martins, M.; Ricz, S.; Viefhaus, J.; Fritzsche, S.; M{\"{u}}ller, A. (Physical Review A, 2016)
The photon-ion merged-beam technique was used at a synchrotron light source for measuring the absolute cross sections of the double and triple photodetachment of O− ions. The experimental photon energy range of 524–543 eV comprised the threshold for K-shell ionization. Using resolving powers of up to 13 000, the position, strength, and width of the below-threshold 1s2s22p6 S2 resonance as well as the positions of the 1s2s22p5 P3 and 1s2s22p5 P1 thresholds for K-shell ionization were determined with high precision. In addition, systematically enlarged multiconfiguration Dirac-Fock calculations have been performed for the resonant detachment cross sections. Results from these ab initio computations agree very well with the measurements for the widths and branching fractions for double and triple detachment, if double shakeup (and shakedown) of the valence electrons and the rearrangement of the electron density is taken into account. For the absolute cross sections, however, a previously found discrepancy between measurements and theory is confirmed.
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Tunable filters for precise spectral gain control in ultra-short-pulse laser systems - Keppler, S.; Hornung, M.; Zimmermann, P.; Liebetrau, H.; Hellwing, M.; Hein, J.; Kaluza, M.C. (Optics Letters, 2016)
We present tunable spectral filters (TSFs) as a variable and precisely adjustable method to control the spectral gain of short-pulse laser systems. The TSFs provide a small residual spectral phase and a high damage threshold, and generate no pre- or post-pulses. The method is demonstrated for two different laser materials and can be applied as an intracavity compensation in regenerative amplifiers as well as a method for pre-compensation in high-energy multipass amplifiers. With this method, a full width at half-maximum bandwidth of 23.9 nm could be demonstrated in a diode-pumped, 50 J Yb:CaF2 amplifier.
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Double-slit experiment in momentum space - Ivanov, I. P.; Seipt, D.; Surzhykov, A.; Fritzsche, S. (Europhysics Letters, 2016)
Young's classic double-slit experiment demonstrates the reality of interference when waves and particles travel simultaneously along two different spatial paths. Here, we propose a double-slit experiment in momentum space, realized in the free-space elastic scattering of vortex electrons. We show that this process proceeds along two paths in momentum space, which are well localized and well separated from each other. For such vortex beams, the (plane-wave) amplitudes along the two paths acquire adjustable phase shifts and produce interference fringes in the final angular distribution. We argue that this experiment can be realized with the present-day technology. We show that it gives experimental access to the Coulomb phase, a quantity which plays an important role in all charged particle scattering but which usual scattering experiments are insensitive to.
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Electric dipole polarizabilities of Rydberg states of alkali-metal atoms - Yerokhin, V. A.; Buhmann, S. Y.; Fritzsche, S.; Surzhykov, A. (Physical Review A, 2016)
Calculations of the static electric-dipole scalar and tensor polarizabilities are presented for two alkali-metal atoms, Rb and Cs, for the nS, nP extonehalf,3/2, and nD3/2,5/2 states with large principal quantum numbers up to n=50. The calculations are performed within an effective one-electron approximation, based on the Dirac-Fock Hamiltonian with a semiempirical core-polarization potential. The obtained results are compared with those from a simpler semiempirical approach and with available experimental data.
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Energetic sub-2-cycle laser with 216 W average power - H{\"{a}drich, S. and Kienel, M. and M{\"{u}}ller, M. and Klenke, A. and Rothhardt, J. and Klas, R. and Gottschall, T. and Eidam, T. and Drozdy, A. and J{\'{o}}j{\'{a}}rt, P. and V{\'{a}}rallyay, Z. and Cormier, E. and Osvay, K. and T{\"{u}}nnermann, A. and Limpert, J. (Optics Letters, 2016)
Few-cycle lasers are essential for many research areas such as attosecond physics that promise to address fundamental questions in science and technology. Therefore, further advancements are connected to significant progress in the underlying laser technology. Here, two-stage nonlinear compression of a 660 W femtosecond fiber laser system is utilized to achieve unprecedented average power levels of energetic ultrashort or even few-cycle laser pulses. In a first compression step, 408 W, 320 μJ, 30 fs pulses are achieved, which can be further compressed to 216 W, 170 μJ, 6.3 fs pulses in a second compression stage. To the best of our knowledge, this is the highest average power few-cycle laser system presented so far. It is expected to significantly advance the fields of high harmonic generation and attosecond science.
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First observation of SASE radiation using the compact wide-spectral-range XUV spectrometer at FLASH2 - Tanikawa, T.; Hage, A.; Kuhlmann, M.; Gonschior, J.; Grunewald, S.; Pl{\"{o}}njes, E.; D{\"{u}}sterer, S.; Brenner, G.; Dziarzhytski, S.; Braune, M.; Brachmanski, M.; Yin, Z.; et al. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2016)
The Free-electron LASer in Hamburg (FLASH) has been extended with a new undulator line FLASH2 in 2014. A compact grazing-incident wide-spectral-range spectrometer based on spherical-variable-line-spacing (SVLS) gratings in the extreme ultraviolet (XUV) region was constructed to optimize and characterize the free-electron laser (FEL) performance at FLASH2. The spectrometer is equipped with three different concave SVLS gratings covering a spectral range from 1 to 62 nm to analyze the spectral characteristics of the XUV radiation. Wavelength calibration and evaluation of the spectral resolution were performed at the plane grating monochromator beamline PG2 at FLASH1 before the installation at FLASH2, and compared with analytical simulations. The first light using self-amplified spontaneous emission from FLASH2 was observed by the spectrometer during a simultaneous operation of both undulator lines {–} FLASH1 and FLASH2. In addition, the spectral resolution of the spectrometer was evaluated by comparing the measured spectrum from FLASH2 with FEL simulations.
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High purity x-ray polarimetry with single-crystal diamonds - Bernhardt, H.; Marx-Glowna, B.; Schulze, K.; Grabiger, B.; Haber, J.; Detlefs, C.; L{\"{o}}tzsch, R.; K{\"{a}mpfer, T. and R{\"{o}}hlsberger, R. and F{\"{o}}rster, E. and St{\"{o}}hlker, T. and Uschmann, I. and Paulus, G.G. (Applied Physics Letters, 2016)
We report on the use of synthetic single-crystal diamonds for high purity x-ray polarimetry to improve the polarization purity of present-day x-ray polarimeters. The polarimeter setup consists of a polarizer and an analyzer, each based on two parallel diamond crystals used at a Bragg angle close to 45°. The experiment was performed using one (400) Bragg reflection on each diamond crystal and synchrotron undulator radiation at an x-ray energy of 9838.75 eV. A polarization purity of 8.9 × 10−10 was measured at the European Synchrotron Radiation Facility, which is the best value reported for two-reflection polarizer/analyzer setups. This result is encouraging and is a first step to improve the resolution of x-ray polarimeters further by using diamond crystal polarizers and analyzers with four or six consecutive reflections.
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Influence of detector noise in holographic imaging with limited photon flux - Wahyutama, I. S.; Tadesse, G. K.; T{\"{u}}nnermann, A.; Limpert, J.; Rothhardt, J. (Optics Express, 2016)
Lensless coherent diffractive imaging usually requires iterative phase-retrieval for recovering the missing phase information. Holographic techniques, such as Fourier-transform holography (FTH) or holography with extended references (HERALDO), directly provide this phase information and thus allow for a direct non-iterative reconstruction of the sample. In this paper, we analyze the effect of detector noise on the reconstruction for FTH and HERALDO with linear and rectangular references. We find that HERALDO is more sensitive to this type of noise than FTH, especially if rectangular references are employed. This excessive noise, caused by the necessary differentiation step(s) during reconstruction in case of HERALDO, additionally depends on the numerical implementation. When considering both shot-noise and detector noise, we find that FTH provides a better signal-to-noise ratio (SNR) than HERALDO if the available photon flux from the light source is low. In contrast, at high photon flux HERALDO provides better SNR and resolution than FTH. Our findings will help in designing optimum holographic imaging experiments particularly in the photon-flux-limited regime where most ultrafast experiments operate.
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Physics book: CRYRING@ESR - Lestinsky, M.; Andrianov, V.; Aurand, B.; Bagnoud, V.; Bernhardt, D.; Beyer, H.; Bishop, S.; Blaum, K.; Bleile, A.; Borovik, At.; Bosch, F.; Bostock, C.; et al. (European Physical Journal Special Topics, 2016)
The exploration of the unique properties of stored and cooled beams of highly-charged ions as provided by heavy-ion storage rings has opened novel and fascinating research opportunities in the realm of atomic and nuclear physics research. Since the late 1980s, pioneering work has been performed at the CRYRING at Stockholm and at the Test Storage Ring (TSR) at Heidelberg. For the heaviest ions in the highest charge-states, a real quantum jump was achieved in the early 1990s by the commissioning of the Experimental Storage Ring (ESR) at GSI Helmholtzzentrum f{\"{u}}r Schwerionenforschung (GSI) in Darmstadt where challenging experiments on the electron dynamics in the strong field regime as well as nuclear physics studies on exotic nuclei and at the borderline to atomic physics were performed. Meanwhile also at Lanzhou a heavy-ion storage ring has been taken in operation, exploiting the unique research opportunities in particular for medium-heavy ions and exotic nuclei.
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Probing the energy flow in Bessel light beams using atomic photoionization - Surzhykov, A.; Seipt, D.; Fritzsche, S. (Physical Review A, 2016)
The growing interest in twisted light beams also requires a better understanding of their complex internal structure. Particular attention is currently being given to the energy circulation in these beams as usually described by the Poynting vector field. In the present study we propose to use the photoionization of alkali-metal atoms as a probe process to measure (and visualize) the energy flow in twisted light fields. Such measurements are possible since the angular distribution of photoelectrons, emitted from a small atomic target, appears sensitive to and is determined by the local direction of the Poynting vector. To illustrate the feasibility of the proposed method, detailed calculations were performed for the ionization of sodium atoms by nondiffractive Bessel beams.
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Reply to on `Controlling the spectral shape of nonlinear Thomson scattering with proper laser chirping' - Rykovanov, S. G.; Geddes, C. G. R.; Schroeder, C. B.; Esarey, E.; Leemans, W. P. (Physical Review Accelerators and Beams, 2016)
We reply to Terzic and Krafft’s forgoing Comment [Phys. Rev. Accel. Beams, Comment on “Controlling the spectral shape of nonlinear Thomson scattering with proper laser chirping” 19 (2016)]. We disagree with the conclusion of the Comment regarding the novelty of solutions and the citations presented in our paper.
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The formation of rarefaction waves in semiconductors after ultrashort excitation probed by grazing incidence ultrafast time-resolved x-ray diffraction - H{\"{o}}fer, S.; K{\"{a}mpfer, T. and F{\"{o}}rster, E. and St{\"{o}}hlker, T. and Uschmann, I. (Structural Dynamics, 2016)
We explore the InSb-semiconductor lattice dynamics after excitation of high density electron-hole plasma with an ultrashort and intense laser pulse. By using time resolved x-ray diffraction, a sub-mA ° and sub-ps resolution was achieved. Thus, a strain of 4% was measured in a 3 nm thin surface layer 2 ps after excitation. The lattice strain was observed for the first 5 ps as exponentially decaying, changing rapidly by time and by depth. The observed phenomena can only be understood assuming nonlinear time dependent laser absorption where the absorption depth decreases by a factor of twenty compared to linear absorption.
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Thulium-doped fiber chirped-pulse amplification system with 2 GW of peak power - Gaida, C.; Gebhardt, M.; Stutzki, F.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2016)
Thulium-doped fibers with ultra large mode-field areas offer new opportunities for the power scaling of mid-IR ultrashort-pulse laser sources. Here, we present a laser system delivering a pulse-peak power of 2 GW and a nearly transform-limited pulse duration of 200 fs in combination with 28.7 W of average power. This performance level has been achieved by optimizing the pulse shape, reducing the overlap with atmospheric absorption lines, and incorporating a climate chamber to reduce the humidity of the atmospheric environment.
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Tunable polarization plasma channel undulator for narrow bandwidth photon emission - Rykovanov, S. G.; Wang, J. W.; Kharin, V. Yu.; Lei, B.; Schroeder, C. B.; Geddes, C. G. R.; Esarey, E.; Leemans, W. P. (Physical Review Accelerators and Beams, 2016)
The theory of a plasma undulator excited by a short intense laser pulse in a parabolic plasma channel is presented. The undulator fields are generated either by the laser pulse incident off-axis and/or under the angle with respect to the channel axis. Linear plasma theory is used to derive the wakefield structure. It is shown that the electrons injected into the plasma wakefields experience betatron motion and undulator oscillations. Optimal electron beam injection conditions are derived for minimizing the amplitude of the betatron motion, producing narrow-bandwidth undulator radiation. Polarization control is readily achieved by varying the laser pulse injection conditions.
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1 kW 1 mJ eight-channel ultrafast fiber laser - M{\"{u}}ller, M.; Kienel, M.; Klenke, A.; Gottschall, T.; Shestaev, E.; Pl{\"{o}}tner, M.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2016)
An ultrafast fiber chirped-pulse amplifier comprising eight coherently combined amplifier channels is presented. The laser delivers 1 kW average power at 1 mJ pulse energy and 260 fs pulse duration. Excellent beam quality and low-noise performance are confirmed. The laser has proven suitable for demanding scientific applications. Further power scaling is possible right away using even more amplifier channels.
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CsI–Silicon Particle detector for Heavy ions Orbiting in Storage rings (CsISiPHOS) - Najafi, M.; Dillmann, I.; Bosch, F.; Faestermann, T.; Gao, B.; Gernh{\"{a}user, R. and Kozhuharov, C. and Litvinov, S. and Litvinov, Y. and Maier, L. and Nolden, F. and Popp, U. and Sanjari, M. and Spillmann, U. and Steck, M. and St{\"{o}}hlker, T. and Weick, H. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2016)
Abstract A heavy-ion detector was developed for decay studies in the Experimental Storage Ring (ESR) at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany. This detector serves as a prototype for the in-pocket particle detectors for future experiments with the Collector Ring (CR) at FAIR (Facility for Antiproton and Ion Research). The detector includes a stack of six silicon pad sensors, a double-sided silicon strip detector (DSSD), and a CsI(Tl) scintillation detector. It was used successfully in a recent experiment for the detection of the β+-decay of highly charged 142Pm60+ ions. Based on the Δ E / E technique for particle identification and an energy resolution of 0.9% for Δ E and 0.5% for E (Full Width at Half Maximum (FWHM)), the detector is well-suited to distinguish neighbouring isobars in the region of interest.
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Effect of B2O3 and fluoride additions on the Yb3+ luminescence of an alumosilicate glass in the system Li2O/MgO/Al2O3/SiO2 - Krey, J.; Herrmann, A.; Hein, J.; K{\"{o}}rner, J.; Kaluza, M.C.; Russel, C. (Optical Materials Express, 2016)
A series of glasses in the system Li2O/MgO/La2O3/Al2O3/B2O3/ Yb2O3/SiO2 was prepared and doped with 6·1020 Yb3+ ions per cm3 which corresponds to about 1.3 mol% Yb2O3 in the glass composition. In this series, Al2O3 was partially replaced by B2O3 up to a concentration of 15 mol%. Furthermore, Li2O and MgO were partially substituted for LiF and MgF2. All melts were bubbled with argon gas to ensure low OH- concentrations. B2O3 and fluoride additions strongly decrease the glass transition temperatures and viscosities of the melts enabling lower melting temperatures, lower Pt solubility and better homogenization of the melt. Furthermore, both additions slightly decrease the density and refractive indices of the glasses. The relatively low coefficients of thermal expansion remain largely unaffected. Increasing B2O3 concentrations do not negatively affect the luminescence lifetimes of Yb3+ despite their relatively high phonon energy. However, the fluoride addition and the OH- concentration strongly influence the luminescence lifetimes.
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Global flow of the Higgs potential in a Yukawa model - Borchardt, J.; Gies, H.; Sondenheimer, R. (European Physical Journal C, 2016)
We study the renormalization flow of the Higgs potential as a function of both field amplitude and energy scale. This overcomes limitations of conventional techniques that rely, e.g., on an identification of field amplitude and RG scale, or on local field expansions. Using a Higgs–Yukawa model with discrete chiral symmetry as an example, our global flows in field space clarify the origin of possible metastabilities, the fate of the pseudo-stable phase, and provide new information as regards the renormalization of the tunnel barrier. Our results confirm the relaxation of the lower bound for the Higgs mass in the presence of more general microscopic interactions (higher-dimensional operators) to a high quantitative accuracy.
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High-repetition-rate and high-photon-flux 70 eV high-harmonic source for coincidence ion imaging of gas-phase molecules - Rothhardt, J.; H{\"{a}drich, S. and Shamir, Y. and Tschnernajew, M. and Klas, R. and Hoffmann, A. and Tadesse, G. K. and Klenke, A. and Gottschall, T. and Eidam, T. and Limpert, J. and T{\"{u}}nnermann, A. and Boll, R. and Bomme, C. and Dachraoui, H. and Erk, B. and Fraia, M. D. and Horke, D. A. and Kierspel, T. and Mullins, T. and Przystawik, A. and Savelyev, E. and Wiese, J. and Laarmann, T. and K{\"{u}}pper, J. and Rolles, D. (Optics Express, 2016)
Unraveling and controlling chemical dynamics requires techniques to image structural changes of molecules with femtosecond temporal and picometer spatial resolution. Ultrashort-pulse x-ray free-electron lasers have significantly advanced the field by enabling advanced pump-probe schemes. There is an increasing interest in using table-top photon sources enabled by high-harmonic generation of ultrashort-pulse lasers for such studies. We present a novel high-harmonic source driven by a 100 kHz fiber laser system, which delivers 10^11 photons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The combination of record-high photon flux and high repetition rate paves the way for time-resolved studies of the dissociation dynamics of inner-shell ionized molecules in a coincidence detection scheme. First coincidence measurements on CH3I are shown and it is outlined how the anticipated advancement of fiber laser technology and improved sample delivery will, in the next step, allow pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon sources. These table-top sources can provide significantly higher repetition rates than the currently operating free-electron lasers and they offer very high temporal resolution due to the intrinsically small timing jitter between pump and probe pulses.
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Intensity-sensitive and position-resolving cavity for heavy-ion storage rings - Chen, X.; Sanjari, M.; H{\"{u}}lsmann, P.; Litvinov, Y.; Nolden, F.; Piotrowski, J.; Steck, M.; St{\"{o}}hlker, T.; Walker, P. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2016)
Abstract A heavy-ion storage ring can be adapted for use as an isochronous mass spectrometer if the ion velocity matches the transition energy of the ring. Due to the variety of stored ion species, the isochronous condition cannot be fulfilled for all the ions. In order to eliminate the measurement uncertainty stemming from the velocity spread, an intensity-sensitive and position-resolving cavity is proposed. In this paper we first briefly discuss the correction method for the anisochronism effect in the measurement with the cavity. Then we introduce a novel design, which is operated in the monopole mode and offset from the central beam orbit to one side. The geometrical parameters were optimized by analytic and numerical means in accordance with the beam dynamics of the future collector ring at FAIR. Afterwards, the electromagnetic properties of scaled prototypes were measured on a test bench. The results were in good agreement with the predictions.
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Single-pass high harmonic generation at high repetition rate and photon flux - H{\"{a}drich, S. and Rothhardt, J. and Krebs, M. and Demmler, S. and Klenke, A. and T{\"{u}}nnermann, A. and Limpert, J. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2016)
Sources of short wavelength radiation with femtosecond to attosecond pulse durations, such as synchrotrons or free electron lasers, have already made possible numerous, and will facilitate more, seminal studies aimed at understanding atomic and molecular processes on fundamental length and time scales. Table-top sources of coherent extreme ultraviolet to soft x-ray radiation enabled by high harmonic generation (HHG) of ultrashort pulse lasers have also gained significant attention in the last few years due to their enormous potential for addressing a plethora of applications, therefore constituting a complementary source to large-scale facilities (synchrotrons and free electron lasers). Ti:sapphire based laser systems have been the workhorses for HHG for decades, but are limited in repetition rate and average power. On the other hand, it has been widely recognized that fostering applications in fields such as photoelectron spectroscopy and microscopy, coincidence detection, coherent diffractive imaging and frequency metrology requires a high repetition rate and high photon flux HHG sources. In this article we will review recent developments in realizing the demanding requirement of producing a high photon flux and repetition rate at the same time. Particular emphasis will be put on suitable ultrashort pulse and high average power lasers, which directly drive harmonic generation without the need for external enhancement cavities. To this end we describe two complementary schemes that have been successfully employed for high power fiber lasers, i.e. optical parametric chirped pulse amplifiers and nonlinear pulse compression. Moreover, the issue of phase-matching in tight focusing geometries will be discussed and connected to recent experiments. We will highlight the latest results in fiber laser driven high harmonic generation that currently produce the highest photon flux of all existing sources. In addition, we demonstrate the first promising applications and discuss the future direction and challenges of this new type of HHG source.
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12 mJ kW-class ultrafast fiber laser system using multidimensional coherent pulse addition - Kienel, M.; M{\"{u}}ller, M.; Klenke, A.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2016)
An ultrafast fiber-chirped-pulse amplification system using a combination of spatial and temporal coherent pulse com- bination is presented. By distributing the amplification among eight amplifier channels and four pulse replicas, up to 12 mJ pulse energy with 700 W average power and 262 fs pulse duration have been obtained with a system efficiency of 78% and excellent beam quality. To the best of our knowledge, this is the highest energy achieved by an ultrafast fiber-based laser system to date.
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Angular correlation function of the hypersatellite-satellite x-ray cascade following K-shell electron capture of ⁵⁵Fe - Bergmann, B.; Michel, T.; Surzhykov, A.; Fritzsche, S. (Physical Review C, 2016)
The first measurement of the hypersatellite-satellite two-photon angular correlation function following the electron capture decay of Fe55 was carried out. In particular, two hybrid active pixel detectors were employed to measure the anisotropy parameter β_eff2(exp)=0.097±0.053, which closely agreed with the theoretical value β_eff2(theor)=0.09735, calculated in the electric-dipole approximation. In addition, we also determined the double K-shell vacancy creation probability in this specific electron capture decay with improved accuracy. We found PKK=(1.388±0.037)×10−4, with a systematic error ΔPKK,syst=0.042×10−4.
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Broadband stimulated Raman backscattering - Landgraf, B.; Aurand, B.; Lehmann, G.; Gangolf, T.; Schnell, M.; K{\"{u}}hl, T.; Spielmann, C. (New Journal of Physics, 2016)
Broadband amplification employing stimulated Raman backscattering is demonstrated. Using seed pulses with a bandwidth of about 200 nm, we study the amplification in a wide spectral range in a single laser shot. With chirped pump pulses and a Ne gas jet, we observed under optimized conditions, amplification in range of about 80 nm, which is sufficient to support the amplification of sub-20 fs pulses. This broad amplification range is also in excellent agreement with PIC simulations. The conversion efficiency is at certain wavelengths as high as 1.2% and was measured to be better than 6 × 10^−3 on average.
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Demonstration of passive plasma lensing of a laser wakefield accelerated electron bunch - Kuschel, S.; Hollatz, D.; Heinemann, T.; Karger, O.; Schwab, M. B.; Ullmann, D.; Knetsch, A.; Seidel, A.; R{\"{o}}del, C.; Yeung, M.; Leier, M.; Blinne, A.; et al. (Physical Review Accelerators and Beams, 2016)
We report on the first demonstration of passive all-optical plasma lensing using a two-stage setup. An intense femtosecond laser accelerates electrons in a laser wakefield accelerator (LWFA) to 100 MeV over millimeter length scales. By adding a second gas target behind the initial LWFA stage we introduce a robust and independently tunable plasma lens. We observe a density dependent reduction of the LWFA electron beam divergence from an initial value of 2.3 mrad, down to 1.4 mrad (rms), when the plasma lens is in operation. Such a plasma lens provides a simple and compact approach for divergence reduction well matched to the mm-scale length of the LWFA accelerator. The focusing forces are provided solely by the plasma and driven by the bunch itself only, making this a highly useful and conceptually new approach to electron beam focusing. Possible applications of this lens are not limited to laser plasma accelerators. Since no active driver is needed the passive plasma lens is also suited for high repetition rate focusing of electron bunches. Its understanding is also required for modeling the evolution of the driving particle bunch in particle driven wake field acceleration.
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Electromagnetic wave propagation in spatially homogeneous yet smoothly time-varying dielectric media - Hayrapetyan, A. G.; G{\"{o}}tte, J. B.; Grigoryan, K. K.; Fritzsche, S.; Petrosyan, R. G. (Journal of Quantitative Spectroscopy and Radiative Transfer, 2016)
We explore the propagation and transformation of electromagnetic waves through spatially homogeneous yet smoothly time-dependent media within the framework of classical electrodynamics. By modelling the smooth transition, occurring during a finite period t, as a phenomenologically realistic and sigmoidal change of the dielectric permittivity, an analytically exact solution to Maxwell’s equations is derived for the electric displacement in terms of hypergeometric functions. Using this solution, we show the possibility of amplification and attenuation of waves and associate this with the decrease and increase of the time-dependent permittivity. We demonstrate, moreover, that such an energy exchange between waves and non-stationary media leads to the transformation (or conversion) of frequencies. Our results may pave the way towards controllable light–matter interaction in time-varying structures.
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High contrast, 86 fs, 35 mJ pulses from a diode-pumped Yb:glass double-chirped-pulse amplification laser system - Liebetrau, H.; Hornung, M.; Keppler, S.; Hellwing, M.; Kessler, A.; Schorcht, F.; Hein, J.; Kaluza, M.C. (Optics Letters, 2016)
We demonstrate the generation of 86 fs, 35 mJ, high-contrast laser pulses at 1030 nm with a repetition rate of 1 Hz from a diode-pumped double chirped-pulse amplification setup. The pulses exhibit a spectral bandwidth exceeding 27 nm full width at half-maximum. This could be achieved by using a laser architecture comprising two stages of chirped pulse amplification with a cross-polarized wave generation filter in between, by applying spectral shaping and by increasing the spectral hard-clip of the second stretcher. These are, to the best of our knowledge, the shortest pulses at the mJ level with ultra-high contrast generated with a diode-pumped front end at 1030 nm.
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Optimizing laser-driven proton acceleration from overdense targets - Novo, A. S.; Kaluza, M.C.; Fonseca, R. A.; Silva, L. O. (Scientific Reports, 2016)
We demonstrate how to tune the main ion acceleration mechanism in laser-plasma interactions to collisionless shock acceleration, thus achieving control over the final ion beam properties (e. g. maximum energy, divergence, number of accelerated ions). We investigate this technique with three-dimensional particle-in-cell simulations and illustrate a possible experimental realisation. The setup consists of an isolated solid density target, which is preheated by a first laser pulse to initiate target expansion, and a second one to trigger acceleration. The timing between the two laser pulses allows to access all ion acceleration regimes, ranging from target normal sheath acceleration, to hole boring and collisionless shock acceleration. We further demonstrate that the most energetic ions are produced by collisionless shock acceleration, if the target density is near-critical, ne ≈ 0.5 ncr. A scaling of the laser power shows that 100 MeV protons may be achieved in the PW range.
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Probing vacuum birefringence using x-ray free electron and optical high-intensity lasers - Karbstein, F.; Sundqvist, C. (Physical Review D, 2016)
Vacuum birefringence is one of the most striking predictions of strong field quantum electrodynamics: Probe photons traversing a strong field region can indirectly sense the applied “pump” electromagnetic field via quantum fluctuations of virtual charged particles which couple to both pump and probe fields. This coupling is sensitive to the field alignment and can effectively result in two different indices of refraction for the probe photon polarization modes giving rise to a birefringence phenomenon. In this article, we perform a dedicated theoretical analysis of the proposed discovery experiment of vacuum birefringence at an x-ray free electron laser/optical high-intensity laser facility. Describing both pump and probe laser pulses realistically in terms of their macroscopic electromagnetic fields, we go beyond previous analyses by accounting for various effects not considered before in this context. Our study facilitates stringent quantitative predictions and optimizations of the signal in an actual experiment.
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Pulse front tilt measurement of femtosecond laser pulses - Dimitrov, N.; Stoyanov, L.; Stefanov, I.; Dreischuh, A.; Hansinger, P.; Paulus, G. (Optics Communications, 2016)
In this work we report experimental investigations of an intentionally introduced pulse front tilt on femtosecond laser pulses by using an inverted field correlator/interferometer. A reliable criterion for the precision in aligning (in principle) dispersionless systems for manipulating ultrashort pulses is developed, specifically including cases when the pulse front tilt is a result of a desired spatio-temporal coupling. The results obtained using two low-dispersion diffraction gratings are in good qualitative agreement with the data from a previously developed analytical model and from an independent interferometric measurement.
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Superconducting radio-frequency resonator in magnetic fields up to 6 T - Ebrahimi, M. S.; Stallkamp, N.; Quint, W.; Wiesel, M.; Vogel, M.; Martin, A.; Birkl, G. (Review of Scientific Instruments, 2016)
We have measured the characteristics of a superconducting radio-frequency resonator in an external magnetic field. The magnetic field strength has been varied with 10 mT resolution between zero and 6 T. The resonance frequency and the quality factor of the resonator have been found to change significantly as a function of the magnetic field strength. Both parameters show a hysteresis effect which is more pronounced for the resonance frequency. Quantitative knowledge of such behaviour is particularly important when experiments require specific values of resonance frequency and quality factor or when the magnetic field is changed while the resonator is in the superconducting state.
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Suppression of spin-exchange relaxation in tilted magnetic fields within the geophysical range - Scholtes, T.; Pustelny, S.; Fritzsche, S.; Schultze, V.; Stolz, R.; Meyer, H.-G. (Physical Review A, 2016)
We present a detailed experimental and theoretical study on the relaxation of spin coherence due to the spin-exchange mechanism arising in the electronic ground states of alkali-metal vapor atoms. As opposed to the well-explored formation of a stretched state in a longitudinal geometry (magnetic field parallel to the laser propagation direction) we employ adapted hyperfine-selective optical pumping in order to suppress spin-exchange relaxation. By comparing measurements of the intrinsic relaxation rate of the spin coherence in the ground state of cesium atoms with detailed density-matrix simulations we show that the relaxation due to spin-exchange collisions can be reduced substantially even in a tilted magnetic field of geomagnetic strength, the major application case of scalar magnetic surveying. This explains the observed striking improvement in sensitivity and further deepens the understanding of the light-narrowed Mx magnetometer, which was presented recently. Additionally, new avenues for investigating the dynamics in alkali-metal atoms governed by the spin-exchange interaction and interacting with arbitrary external fields open up.
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Ultra-broadband two beam CARS using femtosecond laser pulses - Matth{\"{a}us, G. and Demmler, S. and Lebugle, M. and K{\"{u}}ster, F. and Limpert, J. and T{\"{u}}nnermann, A. and Nolte, S. and Ackermann, R. (Vibrational Spectroscopy, 2016)
Femtosecond (fs)-CARS is a promising approach for gas spectroscopy under high pressure and temperature conditions, as it allows probing molecular states on a time scale which is significantly shorter than the typical decay time induced by interfering collisions. Usually, fs-CARS is performed in a three beam setup, which requires maintaining spatial and temporal overlap of the pulses at the focal point. This is a challenging task, especially in harsh environments such as in a combustion chamber. In this study, we present an alternative approach, which uses two beams in a collinear configuration. An ultra-broadband, sub 7 fs laser pulse acts as pump and Stokes pulse, and a ∼500 fs pulse is used for probing. We show that this configuration is suitable for measuring the gas temperature and concentration. Furthermore, possible single shot measurements of the gas temperature are evaluated.
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Cu charge radii reveal a weak sub-shell effect at N=40 - Bissell, M. L.; Carette, T.; Flanagan, K. T.; Vingerhoets, P.; Billowes, J.; Blaum, K.; Cheal, B.; Fritzsche, S.; Godefroid, M.; Kowalska, M.; Kr{\"{a}mer, J. and Neugart, R. and Neyens, G. and N{\"{o}}rtersh{\"{a}user, W. and Yordanov, D. T. (Physical Review C, 2016)
Collinear laser spectroscopy on Cu58–75 isotopes was performed at the CERN-ISOLDE radioactive ion beam facility. In this paper we report on the isotope shifts obtained from these measurements. State-of-the-art atomic physics calculations have been undertaken in order to determine the changes in mean-square charge radii δ⟨r2⟩A,A′ from the observed isotope shifts. A local minimum is observed in these radii differences at N=40, providing evidence for a weak N=40 sub-shell effect. However, comparison of δ⟨r2⟩A,A′ with a droplet model prediction including static deformation deduced from the spectroscopic quadrupole moments, points to the persistence of correlations at N=40.
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Laser-subcycle control of sequential double-ionization dynamics of helium - Sch{\"{o}}ffler, M.; Xie, X.; Wustelt, P.; M{\"{o}}ller, M.; Roither, S.; Kartashov, D.; Sayler, A.; Baltuska, A.; Paulus, G.G.; Kitzler, M. (Physical Review A, 2016)
We present measured momentum distributions on the double ionization of helium with intense, near-circularly-polarized few-cycle laser pulses with a known carrier-envelope offset phase (CEP). The capability of obtaining CEP-resolved momentum distributions enables us to observe signatures of the various combinations of laser-half-cycle two-electron emissions. By comparison to semiclassical trajectory simulations, we succeed in assigning the corresponding structures in the measured distributions to certain two-electron emission dynamics. Based on this possibility, we demonstrate that the sequential double-ionization dynamics can be sensitively controlled with the pulse duration and the laser peak intensity. For the shortest pulse durations and not too high intensities we find that the two electrons are dominantly emitted with a delay of roughly a laser half cycle. For a just slightly increased intensity we find evidence that at least one of the two electrons is surprisingly likely emitted in between the peaks of the laser field oscillations rather than at the field maxima. The simulations reproduce the signatures of these kinds of two-electron emissions overall relatively well.
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Level sequence and splitting identification of closely spaced energy levels by angle-resolved analysis of fluorescence light - Wu, Z. W.; Volotka, A. V.; Surzhykov, A.; Dong, C. Z.; Fritzsche, S. (Physical Review A, 2016)
The angular distribution and linear polarization of the fluorescence light following the resonant photoexcitation is investigated within the framework of density matrix and second-order perturbation theory. Emphasis has been placed on “signatures” for determining the level sequence and splitting of intermediate (partially) overlapping resonances, if analyzed as a function of photon energy of incident light. Detailed computations within the multiconfiguration Dirac–Fock method have been performed, especially for the 1s^2 2s^2 2p^6 3s,Ji=1/2+γ1 → (1s^2 2s 2p^6 3s)_1 3p3/2,J=1/2,3/2 → 1s^2 2s^2 2p^6 3s,Jf=1/2+γ2 photoexcitation and subsequent fluorescence emission of atomic sodium. A remarkably strong dependence of the angular distribution and linear polarization of the γ2 fluorescence emission is found upon the level sequence and splitting of the intermediate (1s^2 2s 2p^6 3s)_1 3p3/2,J=1/2,3/2 overlapping resonances owing to their finite lifetime (linewidth). We therefore suggest that accurate measurements of the angular distribution and linear polarization might help identify the sequence and small splittings of closely spaced energy levels, even if they cannot be spectroscopically resolved.
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Photoelectron angular distributions and correlations in sequential double and triple atomic ionization by free electron lasers - Grum-Grzhimailo, A. N.; Gryzlova, E. V.; Fritzsche, S.; Kabachnik, N. M. (Journal of Modern Optics, 2016)
We present a review of theoretical studies of the simplest nonlinear photoprocesses detected in the XUV range with the use of free electron lasers: sequential double and triple ionization of atoms by two and three XUV photons. Photoelectron angular distributions and angular correlations between emitted electrons are considered. A comparison of the calculated results with recent angle-resolved photoelectron spectroscopy experiments is discussed.
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Simultaneous operation of two soft x-ray free-electron lasers driven by one linear accelerator - Faatz, B.; Pl{\"{o}}njes, E.; Ackermann, S.; Agababyan, A.; Asgekar, V.; Ayvazyan, V.; Baark, S.; Baboi, N.; Balandin, V.; Bargen, N. v.; Bican, Y.; Bilani, O.; et al. (New Journal of Physics, 2016)
Extreme-ultraviolet to x-ray free-electron lasers (FELs) in operation for scientific applications are up to now single-user facilities. While most FELs generate around 100 photon pulses per second, FLASH at DESY can deliver almost two orders of magnitude more pulses in this time span due to its superconducting accelerator technology. This makes the facility a prime candidate to realize the next step in FELs {–} dividing the electron pulse trains into several FEL lines and delivering photon pulses to several users at the same time. Hence, FLASH has been extended with a second undulator line and self-amplified spontaneous emission (SASE) is demonstrated in both FELs simultaneously. FLASH can now deliver MHz pulse trains to two user experiments in parallel with individually selected photon beam characteristics. First results of the capabilities of this extension are shown with emphasis on independent variation of wavelength, repetition rate, and photon pulse length.
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Spatially and temporally resolved temperature measurement in laser media - K{\"{o}}rner, J.; Yue, F.; Hein, J.; Kaluza, M.C. (Optics Letters, 2016)
A technique to measure the spatially resolved temperature distribution in a laser medium is presented. It is based on the temperature dependence of the absorption cross section close to the zero-phonon line of the active medium. Since other materials in the beam path exhibit a high (and constant) transmission at this wavelength, the method can easily be applied in realistic amplifier setups. The method was successfully tested on three different samples, which were pumped by a pulsed laser diode with up to 150 W average power: side-cooled Yb:YAG and Yb:fluoride-phosphate glass at room temperature and face-cooled Yb:CaF2 at 120 K.
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Strong higher-order resonant contributions to x-ray line polarization in hot plasmas - Shah, C.; Amaro, P.; Steinbr{\"{u}}gge, R.; Beilmann, C.; Bernitt, S.; Fritzsche, S.; Surzhykov, A.; Crespo L{\'{o}}pez-Urrutia, J.; Tashenov, S. (Physical Review E, 2016)
We studied angular distributions of x rays emitted in resonant recombination of highly charged iron and krypton ions, resolving dielectronic, trielectronic, and quadruelectronic channels. A tunable electron beam drove these processes, inducing x rays registered by two detectors mounted along and perpendicular to the beam axis. The measured emission asymmetries comprehensively benchmarked full-order atomic calculations. We conclude that accurate polarization diagnostics of hot plasmas can only be obtained under the premise of inclusion of higher-order processes that were neglected in earlier work.
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Temporal laser-pulse-shape effects in nonlinear Thomson scattering - Kharin, V. Yu.; Seipt, D.; Rykovanov, S. G. (Physical Review A, 2016)
The influence of the laser-pulse temporal shape on the nonlinear Thomson scattering on-axis photon spectrum is analyzed in detail. Using the classical description, analytical expressions for the temporal and spectral structure of the scattered radiation are obtained for the case of symmetric laser-pulse shapes. The possibility of reconstructing the incident laser pulse from the scattered spectrum averaged over interference fringes in the case of high peak intensity and symmetric laser-pulse shape is discussed.
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Absorption of twisted light by a mesoscopic atomic target - Peshkov, A. A.; Serbo, V. G.; Fritzsche, S.; Surzhykov, A. (Physica Scripta, 2016)
The excitation of a hydrogen-atom target by a twisted Bessel light beam is investigated. The atoms are assumed to have a Gaussian spatial distribution in the target. Theoretical analysis is performed within a nonrelativistic framework using a first-order perturbation approach and density matrix formalism. By using this theory, we derive the expressions for excitation cross sections and for alignment parameters of the excited atomic state. In particular, we make calculations for the 1s -> 2p transition caused by the interaction of Bessel beams with the atomic target. For this transition, we analyze the population of magnetic sublevels for the excited 2p state and study how it is affected by the projection of the total angular momentum of incident light. The calculations indicate that the projection of the total angular momentum of the incident Bessel beam affects the alignment of atoms for sufficiently small targets with size less than 200 nm. This can be observed experimentally by measuring the linear polarization of the subsequent fluorescent light.
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Collective strong coupling of X-rays and nuclei in a nuclear optical lattice - Haber, J.; Schulze, K.; Schlage, K.; Loetzsch, R.; Bocklage, L.; Gurieva, T.; Bernhardt, H.; Wille, H.-C.; R{\"{u}}ffer, R.; Uschmann, I.; Paulus, G.G.; R{\"{o}}hlsberger, R. (Nature Photonics, 2016)
The advent of third-generation synchrotron radiation sources and X-ray free-electron lasers has opened up the opportunity to perform quantum optical experiments with high-energy X-rays. The prime atomic system for experiments in this energy range is the strongly nuclear resonant 57Fe M{\"{o}}ssbauer isotope. Experiments have included measurements of the collective Lamb shift, observation of electromagnetically induced transparency, subluminal propagation of X-rays and spontaneously generated coherences. In these experiments, however, the nuclei were only weakly coupled to the light field. Collective strong coupling of nuclei and X-rays, which is desirable for many quantum optical applications, has eluded researchers so far. Here, we observe collective strong coupling between X-rays and matter excitations in a periodic array of alternating 57Fe and 56Fe layers. Our experiment extends the range of methods for X-ray quantum optics and paves the way for the observation and exploitation of strong-coupling-related phenomena at X-ray energies.
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Complete characterization of single-cycle double ionization of argon from the nonsequential to the sequential ionization regime - K{\"{u}}bel, M.; Burger, C.; Kling, N.; Pischke, T.; Beaufore, L.; Ben-Itzhak, I.; Paulus, G.G.; Ullrich, J.; Pfeifer, T.; Moshammer, R.; Kling, M. F.; Bergues, B. (Physical Review A, 2016)
Selected features of nonsequential double ionization have been qualitatively reproduced by a multitude of different (quantum and classical) approaches. In general, however, the typical uncertainty of laser pulse parameters and the restricted number of observables measured in individual experiments leave room for adjusting theoretical results to match the experimental data. While this has been hampering the assessment of different theoretical approaches leading to conflicting interpretations, comprehensive experimental data that would allow such an ultimate and quantitative assessment have been missing so far. To remedy this situation we have performed a kinematically complete measurement of single-cycle multiple ionization of argon over a one order of magnitude range of intensity. The momenta of electrons and ions resulting from the ionization of the target gas are measured in coincidence, while each ionization event is tagged with the carrier-envelope phase and intensity of the 4-fs laser pulse driving the process. The acquired highly differential experimental data provide a benchmark for a rigorous test of the many competing theoretical models used to describe nonsequential double ionization.
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Control of transmission of right circularly polarized laser light in overdense plasma by applied magnetic field pulses - Ma, G.; Yu, W.; Yu, M. Y.; Luan, S.; Wu, D. (Physical Review E, 2016)
The effect of a transient magnetic field on right-hand circularly polarized (RHCP) laser light propagation in overcritical-density plasma is investigated. When the electron gyrofrequency is larger than the wave frequency, RHCP light can propagate along the external magnetic field in an overcritical density plasma without resonance or cutoff. However, when the magnetic field falls to below the cyclotron resonance point, the propagating laser pulse will be truncated and the local plasma electrons resonantly heated. Particle-in-cell simulation shows that when applied to a thin slab, the process can produce intense two-cycle light pulses as well as long-lasting self-magnetic fields.
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Evidence of high-n hollow-ion emission from Si ions pumped by ultraintense x-rays from relativistic laser plasma - Colgan, J.; Faenov, A. Ya.; Pikuz, S. A.; Tubman, E.; Butler, N. M. H.; jr., J. A.; Dance, R. J.; Pikuz, T. A.; Skobelev, I. Yu.; Alkhimova, M. A.; Booth, N.; Green, J.; et al. (Europhysics Letters, 2016)
We report on the first observation of high- n hollow ions (ions having no electrons in the K or L shells) produced in Si targets via pumping by ultra-intense x-ray radiation produced in intense laser-plasma interactions reaching the radiation dominant kinetics regime (RDKR). The existence of these new types of hollow ions in high-energy density plasma has been found via observation of highly resolved x-ray emission spectra of silicon plasma. This has been confirmed by plasma kinetics calculations, underscoring the ability of powerful radiation sources to fully strip electrons from the innermost shells of light atoms. Hollow-ions spectral diagnostics provide a unique opportunity to characterize powerful x-ray radiation of laboratory and astrophysical plasmas. With the use of this technique we provide evidence for the existence of the RDKR via observation of asymmetry in the observed radiation of hollow ions from the front and rear sides of the target.
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Isotope shifts of the 2p₃/₂-2p₁/₂ transition in B-like ions - Zubova, N. A.; Malyshev, A. V.; Tupitsyn, I. I.; Shabaev, V. M.; Kozhedub, Y. S.; Plunien, G.; Brandau, C.; St{\"{o}}hlker, T. (Physical Review A, 2016)
Isotope shifts of the 2p3/2−>2p1/2 transition in B-like ions are evaluated for a wide range of the nuclear charge number: Z=8–92. The calculations of the relativistic nuclear recoil and nuclear size effects are performed using a large-scale configuration-interaction Dirac-Fock-Sturm method. The corresponding QED corrections are also taken into account. The results of the calculations are compared with the theoretical values obtained with other methods. The accuracy of the isotope shifts of the 2p3/2−>2p1/2 transition in B-like ions is significantly improved.
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Maximum Proton Energy above 85 MeV from the Relativistic Interaction of Laser Pulses with Micrometer Thick CH₂ Targets - Wagner, F.; Deppert, O.; Brabetz, C.; Fiala, P.; Kleinschmidt, A.; Poth, P.; Schanz, V. A.; Tebartz, A.; Zielbauer, B.; Roth, M.; St{\"{o}}hlker, T.; Bagnoud, V. (Physical Review Letters, 2016)
We present a study of laser-driven ion acceleration with micrometer and submicrometer thick plastic targets. Using laser pulses with high temporal contrast and an intensity of the order of 10^20 W/cm2 we observe proton beams with cutoff energies in excess of 85 MeV and particle numbers of 109 in an energy bin of 1 MeV around this maximum. We show that applying the target normal sheath acceleration mechanism with submicrometer thick targets is a very robust way to achieve such high ion energies and particle fluxes. Our results are backed with 2D particle in cell simulations furthermore predicting cutoff energies above 200 MeV for acceleration based on relativistic transparency. This predicted regime can be probed after a few technically feasible adjustments of the laser and target parameters.
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Modeling ultrafast shadowgraphy in laser-plasma interaction experiments - Siminos, E.; Skupin, S.; S{\"{a}vert, A. and Cole, J. M. and Mangles, S. P. D. and Kaluza, M.C. (Plasma Physics and Controlled Fusion, 2016)
Ultrafast shadowgraphy is a new experimental technique that uses few-cycle laser pulses to image density gradients in a rapidly evolving plasma. It enables structures that move at speeds close to the speed of light, such as laser driven wakes, to be visualized. Here we study the process of shadowgraphic image formation during the propagation of a few cycle probe pulse transversely through a laser-driven wake using three-dimensional particle-in-cell simulations. In order to construct synthetic shadowgrams a near-field snapshot of the ultrashort probe pulse is analyzed by means of Fourier optics, taking into account the effect of a typical imaging setup. By comparing synthetic and experimental shadowgrams we show that the generation of synthetic data is crucial for the correct interpretation of experiments. Moreover, we study the dependence of synthetic shadowgrams on various parameters such as the imaging system aperture, the position of the object plane and the probe pulse delay, duration and wavelength. Finally, we show that time-dependent information from the interaction can be recovered from a single shot by using a broadband, chirped probe pulse and subsequent spectral filtering.
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On the effect of time-dependent inhomogeneous magnetic fields in electron–positron pair production - Kohlf{\"{u}}rst, C.; Alkofer, R. (Physics Letters B, 2016)
Electron–positron pair production in space- and time-dependent electromagnetic fields is investigated. Especially, the influence of a time-dependent, inhomogeneous magnetic field on the particle momenta and the total particle yield is analyzed for the first time. The role of the Lorentz invariant E^2-B^2, including its sign and local values, in the pair creation process is emphasized.
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Pair production by Schwinger and Breit–Wheeler processes in bi-frequent fields - Otto, A.; Nousch, T.; Seipt, D.; K{\"{a}mpfer, B. and Blaschke, D. and Panferov, A. D. and Smolyansky, S. A. and Titov, A. I. (Journal of Plasma Physics, 2016)
Counter-propagating and suitably polarized light (laser) beams can provide conditions for pair production. Here, we consider in more detail the following two situations: (i) in the homogeneity regions of anti-nodes of linearly polarized ultra-high intensity laser beams, the Schwinger process is dynamically assisted by a second high-frequency field, e.g. by an XFEL beam; and (ii) a high-energy probe photon beam colliding with a superposition of co-propagating intense laser and XFEL beams gives rise to the laser-assisted Breit–Wheeler process. The prospects of such bi-frequent field constellations with respect to the feasibility of conversion of light into matter are discussed.
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Sum rules for the polarization correlations in photoionization and bremsstrahlung - Pratt, R. H.; M{\"{u}}ller, R. A.; Surzhykov, A. (Physical Review A, 2016)
The polarization correlations in doubly differential cross sections are investigated for photoionization and ordinary bremsstrahlung. These correlations describe the polarization transfer between incident light and ejected photoelectrons as well as between an incoming electron beam and bremsstrahlung light, respectively. They are characterized by a set of seven real parameters Cij. We show that the squares of these parameters are connected by simple “sum rules.” These sum rules can be applied for both one-electron systems and also for atoms, if the latter are described within the independent particle approximation. In particular, they are exact in their simplest form (i) for the photoionization of K-, LI,II-, and MI,II-atomic shells, as well as (ii) for bremsstrahlung in which the electron is scattered into s1/2 or p1/2 states, as in the tip (bremsstrahlung) region. Detailed calculations are performed to verify the derived identities and to discuss their possible applications for the analysis of modern photoionization and bremsstrahlung experiments. In particular, we argue that the sum rules may help to determine the entire set of (significant) polarization correlations in the case when not all Cij are available for experimental observation.
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100 W average power femtosecond laser at 343 nm - Rothhardt, J.; Rothhardt, C.; M{\"{u}}ller, M.; Klenke, A.; Kienel, M.; Demmler, S.; Elsmann, T.; Rothhardt, M.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2016)
We present a femtosecond laser system delivering up to 100 W of average power at 343 nm. The laser system employs a Yb-based femtosecond fiber laser and subsequent second- and third-harmonic generation in beta barium borate (BBO) crystals. Thermal gradients within these BBO crystals are mitigated by sapphire heat spreaders directly bonded to the front and back surface of the crystals. Thus, a nearly diffraction-limited beam quality (M2<1.4) is achieved, despite the high thermal load to the nonlinear crystals. This laser source is expected to push many industrial and scientific applications in the future.
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Avalanche of stimulated forward scattering in high harmonic generation - Serrat, C.; Roca, D.; Budesca, J. M.; Seres, J.; Seres, E.; Aurand, B.; Hoffmann, A.; Namba, S.; Kuehl, T.; Spielmann, C. (Optics Express, 2016)
Optical amplifiers in all ranges of the electromagnetic spectrum exhibit an essential characteristic, namely the input signal during the propagation in the amplifier medium is multiplied by the avalanche effect of the stimulated emission to produce exponential growth. We perform a theoretical study motivated and supported by experimental data on a He gas amplifier driven by intense 30-fs-long laser pulses and seeded with attosecond pulse trains generated in a separated Ne gas jet. We demonstrate that the strong-field theory in the frame of high harmonic generation fully supports the appearance of the avalanche effect in the amplification of extreme ultraviolet attosecond pulse trains. We theoretically separate and identify different physical processes taking part in the interaction and we demonstrate that X-ray parametric amplification dominates over others. In particular, we identify strong-field mediated intrapulse X-ray parametric processes as decisive for amplification at the single-atom level. We confirm that the amplification takes place at photon energies where the amplifier is seeded and when the seed pulses are perfectly synchronized with the driving strong field in the amplifier. Furthermore, propagation effects, phase matching and seed synchronization can be exploited to tune the amplified spectral range within the seed bandwidth.
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CdTe Timepix detectors for single-photon spectroscopy and linear polarimetry of high-flux hard x-ray radiation - Hahn, C.; Weber, G.; M{\"{a}rtin, R. and H{\"{o}}fer, S. and K{\"{a}mpfer, T. and St{\"{o}}hlker, T. (Review of Scientific Instruments, 2016)
Single-photon spectroscopy of pulsed, high-intensity sources of hard X-rays {–} such as laser-generated plasmas {–} is often hampered by the pileup of several photons absorbed by the unsegmented, large-volume sensors routinely used for the detection of high-energy radiation. Detectors based on the Timepix chip, with a segmentation pitch of 55 μm and the possibility to be equipped with high-Zsensor chips, constitute an attractive alternative to commonly used passive solutions such as image plates. In this report, we present energy calibration and characterization measurements of such devices. The achievable energy resolution is comparable to that of scintillators for γ spectroscopy. Moreover, we also introduce a simple two-detector Compton polarimeter setup with a polarimeter quality of (98 ± 1)%. Finally, a proof-of-principle polarimetry experiment is discussed, where we studied the linear polarization of bremsstrahlung emitted by a laser-driven plasma and found an indication of the X-raypolarization direction depending on the polarization state of the incident laser pulse.
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Guided post-acceleration of laser-driven ions by a miniature modular structure - Kar, S.; Ahmed, H.; Prasad, R.; Cerchez, M.; Brauckmann, S.; Aurand, B.; Cantono, G.; Hadjisolomou, P.; Lewis, C.; Macchi, A.; Nersisyan, G.; Robinson, A.; et al. (Nature Communications, 2016)
All-optical approaches to particle acceleration are currently attracting a significant research effort internationally. Although characterized by exceptional transverse and longitudinal emittance, laser-driven ion beams currently have limitations in terms of peak ion energy, bandwidth of the energy spectrum and beam divergence. Here we introduce the concept of a versatile, miniature linear accelerating module, which, by employing laser-excited electromagnetic pulses directed along a helical path surrounding the laser-accelerated ion beams, addresses these shortcomings simultaneously. In a proof-of-principle experiment on a university-scale system, we demonstrate post-acceleration of laser-driven protons from a flat foil at a rate of 0.5 GeV/m, already beyond what can be sustained by conventional accelerator technologies, with dynamic beam collimation and energy selection. These results open up new opportunities for the development of extremely compact and cost-effective ion accelerators for both established and innovative applications.
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High-harmonic generation at 250 MHz with photon energies exceeding 100 eV - Carstens, H.; H{\"{o}}gner, M.; Saule, T.; Holzberger, S.; Lilienfein, N.; Guggenmos, A.; Jocher, C.; Eidam, T.; Esser, D.; Tosa, V.; Pervak, V.; Limpert, J.; et al. (Optica, 2016)
Ultrafast spectroscopy in the extreme ultraviolet demands for ever-higher pulse repetition rates and photon energies. Here, we drive cavity-enhanced high-order harmonic generation (HHG) at a repetition rate of 250 MHz, with 30 fs pulses and an average power of 10 kW. Employing an optimized cavity geometry and a high-pressure gas target, we couple out nanowatt-level harmonics at photon energies around 100 eV. This constitutes an improvement of more than two orders of magnitude over previous megahertz-repetition-rate HHG experiments and paves the way toward high-photon-energy frequency-comb spectroscopy and toward pump-probe photoelectron microscopy and spectroscopy at unprecedented repetition rates.
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Optimizing high-power Yb-doped fiber amplifier systems in the presence of transverse mode instabilities - Jauregui, C.; Otto, H.-J.; Breitkopf, S.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2016)
The average output power of Yb-doped fiber amplifier systems is currently limited by the onset of transverse mode instabilities. Besides, it has been recently shown that the transverse mode instability threshold can be significantly reduced by the presence of photodarkening in the fiber. Therefore, reducing the photodarkening level of the core material composition is the most straightforward way to increase the output average power of fiber amplifier systems but, unfortunately, this is not always easy or possible. In this paper we present guidelines to optimize the output average power of fiber amplifiers affected by transverse mode instabilities and photodarkening. The guidelines derived from the simulations do not involve changes in the composition of the active material (except for its doping concentration), but can still lead to a significant increase of the transverse mode instability threshold. The dependence of this parameter on the active ion concentration and the core conformation, among others, will be studied and discussed.
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Phase stabilization of spatiotemporally multiplexed ultrafast amplifiers - Mueller, M.; Kienel, M.; Klenke, A.; Eidam, T.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2016)
Actively stabilized, simultaneous spatial and temporal coherent beam combination is a promising power-scaling technique for ultrafast laser systems. For a temporal combination based on optical delay lines, multiple stable states of operation arise for common stabilization techniques. A time resolved Jones’ calculus is applied to investigate the issue. A mitigation strategy based on a temporally gated error signal acquisition is derived and demonstrated, enabling to stabilize laser systems with arbitrary numbers of amplifier channels and optical delay lines.
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Photon merging and splitting in electromagnetic field inhomogeneities - Gies, H.; Karbstein, F.; Seegert, N. (Physical Review D, 2016)
We investigate photon merging and splitting processes in inhomogeneous, slowly varying electromagnetic fields. Our study is based on the three-photon polarization tensor following from the Heisenberg-Euler effective action. We put special emphasis on deviations from the well-known constant field results, also revisiting the selection rules for these processes. In the context of high-intensity laser facilities, we analytically determine compact expressions for the number of merged/split photons as obtained in the focal spots of intense laser beams. For the parameter range of typical petawatt class laser systems as pump and probe, we provide estimates for the numbers of signal photons attainable in an actual experiment. The combination of frequency upshifting, polarization dependence and scattering off the inhomogeneities renders photon merging an ideal signature for the experimental exploration of nonlinear quantum vacuum properties.
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Plasma scale-length effects on electron energy spectra in high-irradiance laser plasmas - Culfa, O.; Tallents, G. J.; Rossall, A. K.; Wagenaars, E.; Ridgers, C. P.; Murphy, C. D.; Dance, R. J.; Gray, R. J.; McKenna, P.; Brown, C. D. R.; James, S. F.; Hoarty, D. J.; et al. (Physical Review E, 2016)
An analysis of an electron spectrometer used to characterize fast electrons generated by ultraintense (10^20 Wcm^−2) laser interaction with a preformed plasma of scale length measured by shadowgraphy is presented. The effects of fringing magnetic fields on the electron spectral measurements and the accuracy of density scale-length measurements are evaluated. 2D EPOCH PIC code simulations are found to be in agreement with measurements of the electron energy spectra showing that laser filamentation in plasma preformed by a prepulse is important with longer plasma scale lengths (>8 μm).
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Strong asymmetry of the electron-loss-to-continuum cusp of multielectron U²⁸⁺ projectiles in near-relativistic collisions with gaseous targets - Hillenbrand, P.-M.; Hagmann, S.; Monti, J. M.; Rivarola, R. D.; Blumenhagen, K.-H.; Brandau, C.; Chen, W.; DuBois, R. D.; Gumberidze, A.; Guo, D. L.; Lestinsky, M.; Litvinov, Yu. A.; et al. (Physical Review A, 2016)
The process of electron-loss to the continuum (ELC) has been studied for the collision systems U28+ -> H2 at a collision energy of 50 MeV/u, U28+ -> N2 at 30 MeV/u, and U28+ -> Xe at 50 MeV/u. The energy distributions of cusp electrons emitted at an angle of 0∘ with respect to the projectile beam were measured using a magnetic forward-angle electron spectrometer. For these collision systems far from equilibrium charge state, a significantly asymmetric cusp shape is observed. The experimental results are compared to calculations based on first-order perturbation theory, which predict an almost symmetric cusp shape. Some possible reasons for this discrepancy are discussed.
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White light generated by femtosecond optical vortex beams - Hansinger, P.; Maleshkov, G.; Garanovich, I. L.; Skryabin, D. V.; Neshev, D. N.; Dreischuh, A.; Paulus, G.G. (Journal of the Optical Society of America B, 2016)
In this work we report detailed experimental and numerical investigation of the white light generation by singly and doubly charged optical vortices propagating in a Kerr medium, where spectral broadening and transfer of topological charge (TC) into emerging spectral satellites take place due to self-phase modulation and degenerate four-wave frequency mixing (FWFM). Experiments performed with different pump beams show excellent agreement with theory. Singly and doubly charged white light vortices are observed within more than ±200 nm bandwidth after nonlinear propagation in Argon gas. Our experiment and theory data confirm that the TC transformation of the newly generated spectral components follows a law analogous to the one for energy conservation in the FWFM process. We also present results on the white light vortex stability.
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Analytical results for nonlinear Compton scattering in short intense laser pulses - Seipt, D.; Kharin, V.; Rykovanov, S.; Surzhykov, A.; Fritzsche, S. (Journal of Plasma Physics, 2016)
We study in detail the strong-field QED process of nonlinear Compton scattering in short intense plane wave laser pulses of circular polarization. Our main focus is placed on how the spectrum of the backscattered laser light depends on the shape and duration of the initial short intense pulse. Although this pulse shape dependence is very complicated and highly nonlinear, and has never been addressed explicitly, our analysis reveals that all the dependence on the laser pulse shape is contained in a class of three-parameter master integrals. Here we present completely analytical expressions for the nonlinear Compton spectrum in terms of these master integrals. Moreover, we analyse the universal behaviour of the shape of the spectrum for very high harmonic lines.
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Controlling the spectral shape of nonlinear Thomson scattering with proper laser chirping - Rykovanov, S. G.; Geddes, C. G. R.; Schroeder, C. B.; Esarey, E.; Leemans, W. P. (Physical Review Accelerators and Beams, 2016)
Effects of nonlinearity in Thomson scattering of a high intensity laser pulse from electrons are analyzed. Analytic expressions for laser pulse shaping in frequency (chirping) are obtained which control spectrum broadening for high laser pulse intensities. These analytic solutions allow prediction of the spectral form and required laser parameters to avoid broadening. Results of analytical and numerical calculations agree well. The control over the scattered radiation bandwidth allows narrow bandwidth sources to be produced using high scattering intensities, which in turn greatly improves scattering yield for future x- and gamma-ray sources.
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Critical Schwinger Pair Production - Gies, H.; Torgrimsson, G. (Physical Review Letters, 2016)
We investigate Schwinger pair production in spatially inhomogeneous electric backgrounds. A critical point for the onset of pair production can be approached by fields that marginally provide sufficient electrostatic energy for an off-shell long-range electron-positron fluctuation to become a real pair. Close to this critical point, we observe features of universality which are analogous to continuous phase transitions in critical phenomena with the pair-production rate serving as an order parameter: electric backgrounds can be subdivided into universality classes and the onset of pair production exhibits characteristic scaling laws. An appropriate design of the electric background field can interpolate between power-law scaling, essential Berezinskii-Kosterlitz-Thouless-type scaling, and a power-law scaling with log corrections. The corresponding critical exponents only depend on the large-scale features of the electric background, whereas the microscopic details of the background play the role of irrelevant perturbations not affecting criticality.
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Linear polarization of the characteristic x-ray lines following inner-shell photoionization of tungsten - K{\"{a}mpfer, T. and Uschmann, I. and Wu, Z. W. and Surzhykov, A. and Fritzsche, S. and F{\"{o}}rster, E. and Paulus, G.G. (Physical Review A, 2016)
The linear polarization of the characteristic lines Lα1 (3d5/2→2p3/2) and Lα2 (3d3/2→2p3/2), following inner-shell photoionization of neutral tungsten, is analyzed both experimentally and theoretically. In the experiment, a tungsten target is photoionized by the primary emission of an x-ray tube with incident photon energies in the range of 10.2–30 keV. The σ and π components of the emitted fluorescence are measured by using a spectropolarimeter, based on x-ray diffraction at Bragg angles close to 45∘. The degree of linear polarization of the Lα1 and Lα2 lines is determined to be +(1.6±0.5)% and −(7±2)%, respectively. In addition, this degree of polarization is calculated within the framework of the density-matrix theory as a function of the incident photon energy. These calculations are in good agreement with the experimental results and show only a weak dependence of the degree of polarization on the energy of the incident photoionizing photon.
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Polarization response in extreme nonlinear optics: when can the semiclassical approach be used? - Bogatskaya, A. V.; Volkova, E. A.; Kharin, V. Y.; Popov, A. M. (Laser Physics Letters, 2016)
Availability of the semiclassical approach in strong-field physics and extreme nonlinear optics is analyzed. It is found that it is valid for calculation of the emission to the initially populated bound state only if population of this state is close to unity during the pulse and in the after pulse regime. If the initial level is depleted, the semiclassical approach fails and should be replaced by quantum-electrodynamical calculations. Also it is demonstrated that the bremsstrahlung spectrum cannot be correctly described in the frames of the semiclassical approach.
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Scalability of components for kW-level average power few-cycle lasers - H{\"{a}drich, S. and Rothhardt, J. and Demmler, S. and Tschernajew, M. and Hoffmann, A. and Krebs, M. and Liem, A. and Vries, O. d. and Pl{\"{o}}tner, M. and Fabian, S. and Schreiber, T. and Limpert, J. and T{\"{u}}nnermann, A. (Applied Optics, 2016)
In this paper, the average power scalability of components that can be used for intense few-cycle lasers based on nonlinear compression of modern femtosecond solid-state lasers is investigated. The key components of such a setup, namely, the gas-filled waveguides, laser windows, chirped mirrors for pulse compression and low dispersion mirrors for beam collimation, focusing, and beam steering are tested under high-average-power operation using a kilowatt cw laser. We demonstrate the long-term stable transmission of kW-level average power through a hollow capillary and a Kagome-type photonic crystal fiber. In addition, we show that sapphire substrates significantly improve the average power capability of metal-coated mirrors. Ultimately, ultrabroadband dielectric mirrors show negligible heating up to 1 kW of average power. In summary, a technology for scaling of few-cycle lasers up to 1 kW of average power and beyond is presented.
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Simulations of the energy loss of ions at the stopping-power maximum in a laser-induced plasma - Cayzac, W.; Frank, A.; Ortner, A.; Bagnoud, V.; Basko, M.; Bedacht, S.; Blazevic, A.; Deppert, O.; Gericke, D.; Hallo, L.; Knetsch, A.; Kraus, D.; et al. (Journal of Physics: Conference Series, 2016)
Simulations have been performed to study the energy loss of carbon ions in a hot, laser-generated plasma in the velocity region of the stopping-power maximum. In this parameter range, discrepancies of up to 30% exist between the various stopping theories and hardly any experimental data are available. The considered plasma, created by irradiating a thin carbon foil with two high-energy laser beams, is fully-ionized with a temperature of nearly 200 eV. To study the interaction at the maximum stopping power, Monte-Carlo calculations of the ion charge state in the plasma are carried out at a projectile energy of 0.5 MeV per nucleon. The predictions of various stopping-power theories are compared and experimental campaigns are planned for a first-time theory benchmarking in this low-velocity range.
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Smith–Purcell radiation in the terahertz regime using charged particle beams from laser–matter interactions - Gopal, A.; Woldegeorgis, A. H.; Herzer, S.; Paulus, G.G.; Singh, P.; Ziegler, W.; May., T. (Laser and Particle Beams, 2016)
We report on the experimental observation of Smith–Purcell (SP) radiation generation by charged particle beam from laser–matter interactions. High-power laser pulses were focused onto a thin metal foil target to generate proton beams with energies up to 1.7 MeV via the target normal sheath acceleration (TNSA) process. The particle beam from the TNSA process was sent close to a periodic structure to generate SP radiation. Sub-μJ terahertz pulses were recorded using a pyroelectric detector. Simultaneous measurement of the ion spectra allowed us to estimate the power of the emitted radiation and compare it with the experimental results. The distance between the grating and the particle beam was varied and its effect on the emitted radiation was studied.
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The generation of amplified spontaneous emission in high-power CPA laser systems - Keppler, S.; S{\"{a}vert, A. and K{\"{o}}rner, J. and Hornung, M. and Liebetrau, H. and Hein, J. and Kaluza, M. (Laser { &} Photonics Reviews, 2016)
An analytical model is presented describing the temporal intensity contrast determined by amplified spontaneous emission in high-intensity laser systems which are based on the principle of chirped pulse amplification. The model describes both the generation and the amplification of the amplified spontaneous emission for each type of laser amplifier. This model is applied to different solid state laser materials which can support the amplification of pulse durations ≤350 fs . The results are compared to intensity and fluence thresholds, e.g. determined by damage thresholds of a certain target material to be used in high-intensity applications. This allows determining if additional means for contrast improvement, e.g. plasma mirrors, are required for a certain type of laser system and application. Using this model, the requirements for an optimized high-contrast front-end design are derived regarding the necessary contrast improvement and the amplified “clean” output energy for a desired focussed peak intensity. Finally, the model is compared to measurements at three different high-intensity laser systems based on Ti:Sapphire and Yb:glass. These measurements show an excellent agreement with the model.
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Caustic structures in x-ray Compton scattering off electrons driven by a short intense laser pulse - Seipt, D.; Surzhykov, A.; Fritzsche, S.; K{\"{a}mpfer, B. (New Journal of Physics, 2016)
We study the Compton scattering of x-rays off electrons that are driven by a relativistically intense short optical laser pulse. The frequency spectrum of the laser-assisted Compton radiation shows a broad plateau in the vicinity of the laser-free Compton line due to a nonlinear mixing between x-ray and laser photons. Special emphasis is placed on how the shape of the short assisting laser pulse affects the spectrum of the scattered x-rays. In particular, we observe sharp peak structures in the plateau region, whose number and locations are highly sensitive to the laser pulse shape. These structures are interpreted as spectral caustics by using a semiclassical analysis of the laser-assisted QED matrix element, relating the caustic peak locations to the laser-driven electron motion.
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Determination of the carrier envelope phase for short, circularly polarized laser pulses - Titov, A.; K{\"{a}mpfer, B. and Hosaka, A. and Nousch, T. and Seipt, D. (Physical Review D, 2016)
We analyze the impact of the carrier envelope phase on the differential cross sections of the Breit- Wheeler and the generalized Compton scattering in the interaction of a charged electron (positron) with an intensive ultrashort electromagnetic (laser) pulse. The differential cross sections as a function of the azimuthal angle of the outgoing electron have a clear bump structure, where the bump position coincides with the value of the carrier phase. This effect can be used for the carrier envelope phase determination.
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High-energy-density electron beam from interaction of two successive laser pulses with subcritical-density plasma - Wang, J. W.; Yu, W.; Yu, M. Y.; Xu, H.; Ju, J. J.; Luan, S. X.; Murakami, M.; Zepf, M.; Rykovanov, S. (Physical Review Accelerators and Beams, 2016)
It is shown by particle-in-cell simulations that a narrow electron beam with high energy and charge density can be generated in a subcritical-density plasma by two consecutive laser pulses. Although the first laser pulse dissipates rapidly, the second pulse can propagate for a long distance in the thin wake channel created by the first pulse and can further accelerate the preaccelerated electrons therein. Given that the second pulse also self-focuses, the resulting electron beam has a narrow waist and high charge and energy densities. Such beams are useful for enhancing the target-back space-charge field in target normal sheath acceleration of ions and bremsstrahlung sources, among others.
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Many-electron effects on x-ray Rayleigh scattering by highly charged He-like ions - Volotka, A. V.; Yerokhin, V. A.; Surzhykov, A.; St{\"{o}}hlker, T.; Fritzsche, S. (Physical Review A, 2016)
The Rayleigh scattering of x rays by many-electron highly charged ions is studied theoretically. The many-electron perturbation theory, based on a rigorous quantum electrodynamics approach, is developed and implemented for the case of the elastic scattering of (high-energetic) photons by heliumlike ions. Using this elaborate approach, we here investigate the many-electron effects beyond the independent-particle approximation (IPA) as conventionally employed for describing the Rayleigh scattering. The total and angle-differential cross sections are evaluated for the x-ray scattering by heliumlike Ni^26+, Xe^52+, and Au^77+ ions in their ground state. The obtained results show that, for high-energetic photons, the effects beyond the IPA do not exceed 2% for the scattering by a closed K shell.
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Nanometer resolution optical coherence tomography using broad bandwidth XUV and soft x-ray radiation - Fuchs, S.; R{\"{o}}del, C.; Blinne, A.; Zastrau, U.; W{\"{u}}nsche, M.; Hilbert, V.; Glaser, L.; Viefhaus, J.; Frumker, E.; Corkum, P.; F{\"{o}}rster, E.; Paulus, G.G. (Scientific Reports, 2016)
Optical coherence tomography (OCT) is a non-invasive technique for cross-sectional imaging. It is particularly advantageous for applications where conventional microscopy is not able to image deeper layers of samples in a reasonable time, e.g. in fast moving, deeper lying structures. However, at infrared and optical wavelengths, which are commonly used, the axial resolution of OCT is limited to about 1 μm, even if the bandwidth of the light covers a wide spectral range. Here, we present extreme ultraviolet coherence tomography (XCT) and thus introduce a new technique for non-invasive cross-sectional imaging of nanometer structures. XCT exploits the nanometerscale coherence lengths corresponding to the spectral transmission windows of, e.g., silicon samples. The axial resolution of coherence tomography is thus improved from micrometers to a few nanometers. Tomographic imaging with an axial resolution better than 18 nm is demonstrated for layer-type nanostructures buried in a silicon substrate. Using wavelengths in the water transmission window, nanometer-scale layers of platinum are retrieved with a resolution better than 8 nm. XCT as a nondestructive method for sub-surface tomographic imaging holds promise for several applications in semiconductor metrology and imaging in the water window.
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Picosecond metrology of laser-driven proton bursts - Dromey, B.; Coughlan, M.; Senje, L.; Taylor, M.; Kuschel, S.; Villagomez-Bernabe, B.; Stefanuik, R.; Nersisyan, G.; Stella, L.; Kohanoff, J.; Borghesi, M.; Currell, F.; et al. (Nature Communications, 2016)
Tracking primary radiation-induced processes in matter requires ultrafast sources and high precision timing. While compact laser-driven ion accelerators are seeding the development of novel high instantaneous flux applications, combining the ultrashort ion and laser pulse durations with their inherent synchronicity to trace the real-time evolution of initial damage events has yet to be realized. Here we report on the absolute measurement of proton bursts as short as 3.5±0.7 ps from laser solid target interactions for this purpose. Our results verify that laser-driven ion acceleration can deliver interaction times over a factor of hundred shorter than those of state-of-the-art accelerators optimized for high instantaneous flux. Furthermore, these observations draw ion interaction physics into the field of ultrafast science, opening the opportunity for quantitative comparison with both numerical modelling and the adjacent fields of ultrafast electron and photon interactions in matter.
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Spectral caustics in laser assisted Breit–Wheeler process - Nousch, T.; Seipt, D.; K{\"{a}mpfer, B. and Titov, A. I. (Physics Letters B, 2016)
Electron–positron pair production by the Breit–Wheeler process embedded in a strong laser pulse is analyzed. The transverse momentum spectrum displays prominent peaks which are interpreted as caustics, the positions of which are accessible by the stationary phases. Examples are given for the superposition of an XFEL beam with an optical high-intensity laser beam. Such a configuration is available, e.g., at LCLS at present and at European XFEL in near future. It requires a counter propagating probe photon beam with high energy which can be generated by synchronized inverse Compton backscattering.
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Temporal Structure of Attosecond Pulses from Laser-Driven Coherent Synchrotron Emission - Cousens, S.; Reville, B.; Dromey, B.; Zepf, M. (Physical Review Letters, 2016)
The microscopic dynamics of laser-driven coherent synchrotron emission transmitted through thin foils are investigated using particle-in-cell simulations. For normal incidence interactions, we identify the formation of two distinct electron nanobunches from which emission takes place each half-cycle of the driving laser pulse. These emissions are separated temporally by 130 as and are dominant in different frequency ranges, which is a direct consequence of the distinct characteristics of each electron nanobunch. This may be exploited through spectral filtering to isolate these emissions, generating electromagnetic pulses of duration 70 as.
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Worldline numerics for energy-momentum tensors in Casimir geometries - Sch{\"{a}fer, M. and Huet, I. and Gies, H. (Journal of Physics A: Mathematical and Theoretical, 2016)
We develop the worldline formalism for computations of composite operators such as the fluctuation induced energy-momentum tensor. As an example, we use a fluctuating real scalar field subject to Dirichlet boundary conditions. The resulting worldline representation can be evaluated by worldline Monte-Carlo methods in continuous spacetime. We benchmark this worldline numerical algorithm with the aid of analytically accessible single-plate and parallel-plate Casimir configurations, providing a detailed analysis of statistical and systematic errors. The method generalizes straightforwardly to arbitrary Casimir geometries and general background potentials.
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Comparison of semiclassical and Wigner function methods in pair production in rotating fields - Blinne, A.; Strobel, E. (Physical Review D, 2016)
We present a comparison of two methods to compute the momentum spectrum and the Schwinger pair creation rate for pulsed rotating electric fields: a numerical method based upon the real-time Dirac-Heisenberg-Wigner formalism and a semiclassical approximation based on a scattering ansatz. For the semiclassical method we propose to either perform numerical calculations or an additional approximation based on an analytical solution for the constant rotating field. We find that the two numerical methods are complementary with respect to computation time as well as accuracy. The approximate method shows the same qualitative features while being computationally much faster. We additionally find that the unequal production of pairs in different spin states reported for constant rotating fields with the scattering method is in agreement with the Wigner function method.
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Crystalline ground states in Polyakov-loop extended Nambu-Jona-Lasinio models - Braun, J.; Karbstein, F.; Rechenberger, S.; Roscher, D. (Physical Review D, 2016)
Nambu–Jona-Lasinio-type models have been used extensively to study the dynamics of the theory of the strong interaction at finite temperature and quark chemical potential on a phenomenological level. In addition to these studies, which are often performed under the assumption that the ground state of the theory is homogeneous, searches for the existence of crystalline phases associated with inhomogeneous ground states have attracted a lot of interest in recent years. In this work, we study the Polyakov-loop extended Nambu–Jona-Lasinio model using two prominent parametrizations and find that the existence of a crystalline phase is stable against a variation of the parametrization of the underlying Polyakov loop potential.
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Direct inversion of digital 3D Fraunhofer holography maps - Podorov, S. G.; F{\"{o}}rster, E. (Applied Optics, 2016)
Differential Fourier holography (DFH) gives an exact mathematical solution of the inverse problem of diffraction in the Fraunhofer regime. After the first publication \[Opt. Express15, 9954 (2007)OPEXFF1094-408710.1364/OE.15.009954\], DFH was successfully applied in many experiments to obtain amplitude and phase information about two-dimensional images. In this paper, we demonstrate numerically the possibility to apply DFH also for investigation of unknown three-dimensional objects. The first simulation is made for a double-spiral structure plus a line as a reference object.
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High Field Physics and QED Experiments at ELI-NP - Turcu, I. C. E.; Negoita, F.; Jaroszynski, D. A.; McKenna, P.; Balascuta, S.; Ursescu, D.; Dancus, I.; Cernaianu, M. O.; Tataru, M. V.; Ghenuche, P.; Stutman, D.; Boianu, A.; et al. (Romanian Reports In Physics, 2016)
ELI-NP facility will enable for the first time the use of two 10 PW laser beams for quantum electrodynamics (QED) experiments. The first beam will accelerate electrons to relativistic energies. The second beam will subject relativistic electrons to the strong electromagnetic field generating QED processes: intense gamma ray radiation and electron-positron pair formation. The laser beams will be focused to intensities above 10^21 W/cm^2 and reaching 10^22–10^23 W/cm^2 for the first time. We propose to use this capability to investigate new physical phenomena at the interfaces of plasma, nuclear and particle physics at ELI-NP. This High Power Laser System Technical Design Report (HPLS-TDR2) presents the experimental area E6 at ELI-NP for investigating high field physics and quantum electrodynamics and the production of electron-positron-pairs and of energetic gamma-rays. The scientific community submitted 12 commissioning runs for E6 interaction chamber with two 10 PW laser beams and one proposal for the CETAL interaction chamber with 1 PW laser. The proposals are representative of the international high field physics community being written by 48 authors from 14 European and US organizations. The proposals are classified according to the science area investigated into: Radiation Reaction Physics: Classical and Quantum; Compton and Thomson Scattering Physics: Linear and Non Linear Regimes; QED in Vacuum; Atoms in Extreme Fields. Two pump-probe colliding 10 PW laser beams are proposed for the E6 interaction chamber. The focused pump laser beam accelerates the electrons to relativistic energies. The accelerated electron bunches interact with the very high electro-magnetic field of the focused probe laser beam. We propose two main types of experiments with: (a) gas targets in which the pump laser-beam is focused by a long focal length mirror and drives a wakefield in which the electron bunch is accelerated to multi-GeV energies and then exposed to the EM field of the probe laser which is tightly focused; (b) solid targets in which both the pump and probe laser beams are focused on the solid target, one accelerating the electrons in the solid and the other, delayed, providing the high electric field to which the relativistic electrons are subjected. We propose four main focusing configurations for the pump and probe laser beams, two for each type of target: counter-propagating 10 PW focused laser beams and the two 10 PW laser beams focused in the same direction. For solid targets we propose an additional configuration with plasma-mirror on the pump laser beam: the plasma mirror placed between the focusing mirror and target. It is proposed that the 10 PW laser beams will have polarization control and focus control by means of adaptive optics. Initially only one 10 PW may have polarization control and adaptive optics. In order to accommodate the two laser beams and diagnostics the proposed interaction chamber is quasi-octagonal with a diameter of 4.5 m. A large electron-spectrometer is proposed for multi-GeV electrons. Other diagnostics are requested for: gamma-rays, electrons and positrons, protons and ions, plasma characterization, transmitted and reflected laser beam. Targets will be provided by the ELI-NP Target Laboratory or purchased. The E6 experiments and diagnostics will benefit from the ELI-NP Electronics Laboratory, the Workshop and the Optics Laboratory. In order to ensure radiation-protection, a large beam-dump is planned for both multi-GeV electrons and multi-100 MeV protons.
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Ultrafast transient absorption microscopy: Study of excited state dynamics in PtOEP crystals - Davydova, D.; de la Cadena, A.; Demmler, S.; Rothhardt, J.; Limpert, J.; Pascher, T.; Akimov, D.; Dietzek, B. (Journal of Chemical Physics, 2016)
We report a novel transient absorption microscope based on a tailor-made femtosecond fiber laser system operating at 250 kHz. The setup is applied to study PtOEP crystals embedded in a PBMA polymer matrix by analyzing the excited state dynamics in specific points of the sample as well as by spatially resolved excited state dynamics of the crystals. The results reveal the impact of the distortions of the crystal lattice, such as microcracks or amorphous regions caused by non-thermal melting on a lifetime of the excited triplet states of PtOEP crystals. Although transient absorption studies without any spatial resolution of PtOEP in solution and thin films were reported before, the study of spatially resolved excited state dynamics of micrometer-sized PtOEP crystals is performed for the first time to the best of our knowledge.
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Calibration of BAS-TR image plate response to high energy (3-300 MeV) carbon ions - Doria, D.; Kar, S.; Ahmed, H.; Alejo, A.; Fernandez, J.; Cerchez, M.; Gray, R. J.; Hanton, F.; MacLellan, D. A.; McKenna, P.; Najmudin, Z.; Neely, D.; et al. (Review of Scientific Instruments, 2015)
The paper presents the calibration of Fuji BAS-TR image plate (IP) response to high energy carbon ions of different charge states by employing an intense laser-driven ion source, which allowed access to carbon energies up to 270 MeV. The calibration method consists of employing a Thomson parabola spectrometer to separate and spectrally resolve different ion species, and a slotted CR-39 solid state detector overlayed onto an image plate for an absolute calibration of the IP signal. An empirical response function was obtained which can be reasonably extrapolated to higher ion energies. The experimental data also show that the IP response is independent of ion charge states.
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Electronic structure theory of the superheavy elements - Eliav, E.; Fritzsche, S.; Kaldor, U. (Nuclear Physics A, 2015)
High-accuracy calculations of atomic properties of the superheavy elements (SHE) up to element 122 are reviewed. The properties discussed include ionization potentials, electron affinities and excitation energies, which are associated with the spectroscopic and chemical behavior of these elements, and are therefore of considerable interest. Accurate predictions of these quantities require high-order inclusion of relativity and electron correlation, as well as large, converged basis sets. The Dirac–Coulomb–Breit Hamiltonian, which includes all terms up to second order in the fine-structure constant a, serves as the framework for the treatment; higher-order Lamb shift terms are considered in some selected cases. Electron correlation is treated by either the multiconfiguration self-consistent-field approach or by Fock-space coupled cluster theory. The latter is enhanced by the intermediate Hamiltonian scheme, allowing the use of larger model (P) spaces. The quality of the calculations is assessed by applying the same methods to lighter homologs of the SHEs and comparing with available experimental information. Very good agreement is obtained, within a few hundredths of an eV, and similar accuracy is expected for the SHEs. Many of the properties predicted for the SHEs differ significantly from what may be expected by straightforward extrapolation of lighter homologs, demonstrating that the structure and chemistry of SHEs are strongly affected by relativity. The major scientific challenge of the calculations is to find the electronic structure and basic atomic properties of the SHE and assign its proper place in the periodic table. Significant recent developments include joint experimental–computational studies of the excitation spectrum of Fm and the ionization energy of Lr, with excellent agreement of experiment and theory, auguring well for the future of research in the field.
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Generation of 25-TW Femtosecond Laser Pulses at 515 nm with Extremely High Temporal Contrast - Hornung, M.; Becker, G.; Seidel, A.; Reisl{\"{o}}hner, J.; Liebetrau, H.; Bock, L.; Keppler, S.; Kessler, A.; Zepf, M.; Hein, J.; Kaluza, M. (Applied Sciences, 2015)
We report on the frequency doubling of femtosecond laser pulses at 1030 nm center wavelength generated from the fully diode-pumped laser system POLARIS. The newly generated pulses at a center wavelength of 515 nm have a pulse energy of 3 J with a pulse duration of 120 fs. On the basis of initially ultra-high contrast seed pulses we expect a temporal intensity contrast better 10^17 200 ps before the peak of the main pulse. We analyzed the temporal intensity contrast from milliseconds to femtoseconds with a dynamic range covering more than 20 orders of magnitude. The pulses were focussed with a f/2-focussing parabola resulting in a peak intensity exceeding 10^20 W/cm2. The peak power and intensity are to the best of our knowledge the highest values for 515 nm-laser-pulses achieved so far.
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Spin-orbit interaction in bremsstrahlung and its effect on the electron motion in a strong Coulomb field - Kovtun, O.; Tioukine, V.; Surzhykov, A.; Yerokhin, V.; Cederwall, B.; Tashenov, S. (Physical Review A, 2015)
Linear polarization of bremsstrahlung x rays produced in collisions of longitudinally polarized 2.1-MeV electrons with gold atoms was studied using the Compton scattering technique. We observed that the angle of x-ray polarization is strongly correlated with the incoming electron polarization. This correlation reveals the dominance of the spin-orbit interaction in bremsstrahlung and indicates a striking effect of the electron spin on the electron motion in a strong Coulomb field. The results confirm the validity of the theoretical predictions in a computationally challenging energy regime.
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Temporal Shaping of High Peak Power Pulse Trains from a Burst-Mode Laser System - K{\"{o}}rner, J.; Reiter, J.; Hein, J.; Kaluza, M. (Applied Sciences, 2015)
It has been shown in the past that pulsed laser systems operating in the so-called “burst mode” are a beneficial approach to generate high peak power laser pulses at high repetition rates suitable for various applications. So far, most high-energy burst-mode laser systems put great effort into generating a homogeneous energy distribution across the burst duration, e.g., by shaping the pump pulse. In this work, we present a new shaping technique, which is able to produce arbitrary energy distributions within the burst by pre-shaping the seed pulse burst with a Pockels cell. Furthermore, this technique allows for the precompensation of any static modulations across the burst, which may be introduced during the subsequent amplification process. Therefore, a pulse burst with a uniform energy distribution can also be generated. The method is tested with an ultra-short pulse burst mode laser amplifier system producing bursts of a 1 ms duration with a pulse repetition rate of 1 MHz and a maximum output power of 800 W during the burst. Furthermore, a method to predict the influence of the amplifier on a non-uniformly shaped burst is presented and successfully tested to produce a pre-defined pulse shape after amplification.
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A new data acquisition system for Schottky signals in atomic physics experiments at GSI's and FAIR's storage rings - Trageser, C.; Brandau, C.; Kozhuharov, C.; Litvinov, Y. A.; M{\"{u}}ller, A.; Nolden, F.; Sanjari, S.; St{\"{o}}hlker, T. (Physica Scripta, 2015)
A new continuous and broadband data acquisition system for measurements of Schottky-signals of ions revolving in a storage ring has been implemented. This set-up is capable of recording the radio frequency (RF) signal of the ions that circulate in the storage ring with a sustained acquisition rate of more than 3.5×10⁷ IQ-samples per second. This allows several harmonics of the full momentum acceptance of a storage ring to be measured at the same time. The RF signal analyzer modules are complemented by further electronic modules such as counters, precision clocks and synchronization modules that facilitate a seamless integration with main experimental data acquisitions for atomic and nuclear physics. In this contribution, the setup and first results from a test run at the experimental storage ring at GSI, Darmstadt, Germany, are presented.
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A squid-based beam current monitor for FAIR/CRYRING - Geithner, R.; Kurian, F.; Reeg, H.; Schwickert, M.; Neubert, R.; Seidel, P.; St{\"{o}}hlker, T. (Physica Scripta, 2015)
A SQUID-based beam current monitor was developed for the upcoming FAIR-Project, providing a non-destructive online monitoring of the beam currents in the nA-range. The cryogenic current comparator (CCC) was optimized for lowest possible noise-limited current resolution together with a high system bandwidth. This CCC is foreseen to be installed in the CRYRING facility, working as a test bench for FAIR. In this contribution we present results of the completed CCC for FAIR/CRYRING and also arrangements that have been done for the installation of the CCC at CRYRING, regarding the cryostat design.
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Atomic physics at the future facility for antiproton and ion research: status report 2014 - Gumberidze, A.; St{\"{o}}hlker, T.; Litvinov, Y. (Physica Scripta, 2015)
In this contribution, a brief overview of the Stored Particle Atomic physics Research Collaboration scientific program at the upcoming Facility for Antiproton and Ion Research (FAIR) is given. The program comprises a very broad range of research topics addressing atomic structure and dynamics in hitherto unexplored regimes, light–matter interactions, lepton pair production phenomena, precision tests of quantum electrodynamics and standard model in the regime of extreme fields and many more. We also present the current strategy for the realization of the envisioned physics program within the modularized start version (MSV) of FAIR.
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Atomic physics experiments at the high energy storage ring - St{\"{o}}hlker, T.; Litvinov, Y. A. (Physica Scripta, 2015)
Facility for Antiproton and Ion Research (FAIR), will offer unprecedented experimental opportunities. The Stored Particles Atomic Research Collaboration (SPARC) at FAIR aims at creating a worldwide unique research program with highly charged ions by utilizing storage ring and trapping facilities. The foreseen experiments will address physics at strong, ultra-short electromagnetic fields including the fundamental interactions between electrons and heavy nuclei as well as the experiments at the border between nuclear and atomic physics. In view of the staged construction of the FAIR facility, SPARC worked out an early realization scheme for experiments with highly-charged heavy-ions at relativistic energies to be conducted in the High-Energy Storage Ring.
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Coherent population of magnetic sublevels of 2p₃/₂ state in hydrogenlike uranium by radiative recombination - Tashenov, S.; Bana{ś}, D.; Beyer, H.; Brandau, C.; Fritzsche, S.; Gumberidze, A.; Hagmann, S.; Hillenbrand, P.-M.; J{\"{o}}rg, H.; Kojouharov, I.; Kozhuharov, C.; Lestinsky, M.; et al. (Physica Scripta, 2015)
The x-rays emitted in the process of radiative recombination (RR) of quasi-free electrons into 2p₃/₂ excited state of hydrogenlike uranium ion were studied experimentally. Both the RR x-ray and the subsequently emitted Lyα₁ x-ray were detected in time-coincidences. The angular distribution of the Lyα₁ x-rays varied as a function of the RR x-ray emission direction. This observation revealed the coherent population of magnetic sublevels of the 2p₃/₂ state in the hydrogenlike uranium ion.
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Conceptual design of elliptical cavities for intensity and position sensitive beam measurements in storage rings - Sanjari, M. S.; Chen, X.; H{\"{u}}lsmann, P.; Litvinov, Y. A.; Nolden, F.; Piotrowski, J.; Steck, M.; St{\"{o}}hlker, T. (Physica Scripta, 2015)
Position sensitive beam monitors are indispensable for the beam diagnostics in storage rings. Apart from their applications in the measurements of beam parameters, they can be used in non-destructive in-ring decay studies of radioactive ion beams as well as enhancing precision in the isochronous mass measurement technique. In this work, we introduce a novel approach based on cavities with elliptical cross-section, in order to compensate the limitations of known designs for the application in ion storage rings. The design is aimed primarily for future heavy ion storage rings of the FAIR project. The conceptual design is discussed together with simulation results.
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Effect of bound-state dressing in laser-assisted radiative recombination - M{\"{u}}ller, R.; Seipt, D.; Fritzsche, S.; Surzhykov, A. (Physical Review A, 2015)
We present a theoretical study on the recombination of a free electron into the ground state of a hydrogenlike ion in the presence of an external laser field. Emphasis is placed on the effects caused by the laser dressing of the residual ionic bound state. To investigate how this dressing affects the total and angle-differential cross section of laser-assisted radiative recombination (LARR) we apply first-order perturbation theory and the separable Coulomb-Volkov continuum ansatz. Using this approach, detailed calculations are performed for low-Z hydrogenlike ions and laser intensities in the range from I_L=10^12 to 10^13W/cm2. It is seen that the total cross section as a function of the laser intensity is remarkably affected by the bound-state dressing. Moreover, the laser dressing becomes manifest as asymmetries in the angular distribution and the (energy) spectrum of the emitted recombination photons.
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Experimental concepts of positron spectroscopy at HESR - Hillenbrand, P.-M.; Hagmann, S.; Stiebing, K. E.; Schippers, S.; Litvinov, Y. A.; St{\"{o}}hlker, T. (Physica Scripta, 2015)
In this paper we discuss the unique possibilities of studying atomic electron–positron pair production processes in high-resolution experiments at the high-energy storage ring (HESR) at the future FAIR accelerator complex. The two pair-production processes bound-free pair production and negative-continuum dielectronic recombination are contrasted. An overview of previous studies is given and requirements for experimental parameters of a magnetic forward-angle positron spectrometer at the HESR are depicted.
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Experimental study of the dielectronic recombination into Li-like uranium - Trotsenko, S.; Gumberidze, A.; Gao, Y.; Kozhuharov, C.; Fritzsche, S.; Beyer, H. F.; Hagmann, S.; Hillenbrand, P.-M.; Petridis, N.; Spillmann, U.; Surzhykov, A.; Thorn, D. B.; et al. (Physica Scripta, 2015)
We have measured the x-rays following 116.15 MeV/u U⁸⁹⁺ collisions with H 2 at 35°, 90°, 120° and 150° observation angles with regard to the ion beam direction. From our experimental spectra combined with radiative electron capture calculations, we obtain angular distribution of characteristic x-rays L to K following the resonance transfer and excitation. Our result shows a good qualitative agreement with theoretical predictions.
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Extremely Nonlinear Optics Using Shaped Pulses Spectrally Broadened in an Argon-or Sulfur Hexafluoride-Filled Hollow-Core Fiber - Hoffmann, A.; Z{\"{u}}rch, M.; Spielmann, C. (Applied Sciences, 2015)
In this contribution we present a comparison of the performance of spectrally broadened ultrashort pulses using a hollow-core fiber either filled with argon or sulfur hexafluoride (SF6) for demanding pulse-shaping experiments. The benefits of both gases for pulse-shaping are studied in the highly nonlinear process of high-harmonic generation. In this setup, temporally shaping the driving laser pulse leads to spectrally shaping of the output extreme ultraviolet (XUV) spectrum, where total yield and spectral selectivity in the XUV are the targets of the optimization approach. The effect of using sulfur hexafluoride for pulse-shaping the XUV yield can be doubled compared to pulse compression and pulse-shaping using argon and the spectral range for selective optimization of a single harmonic can be extended. The obtained results are of interest for extending the range of ultrafast science applications drawing on tailored XUV fields.
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Generation of overdense and high-energy electron-positron-pair plasmas by irradiation of a thin foil with two ultraintense lasers - Chang, H. X.; Qiao, B.; Xu, Z.; Xu, X. R.; Zhou, C. T.; Yan, X. Q.; Wu, S. Z.; Borghesi, M.; Zepf, M.; He, X. T. (Physical Review E, 2015)
A scheme for enhanced quantum electrodynamics (QED) production of electron-positron-pair plasmas is proposed that uses two ultraintense lasers irradiating a thin solid foil from opposite sides. In the scheme, under a proper matching condition, in addition to the skin-depth emission of γ-ray photons and Breit-Wheeler creation of pairs on each side of the foil, a large number of high-energy electrons and photons from one side can propagate through it and interact with the laser on the other side, leading to much enhanced γ-ray emission and pair production. More importantly, the created pairs can be collected later and confined to the center by opposite laser radiation pressures when the foil becomes transparent, resulting in the formation of unprecedentedly overdense and high-energy pair plasmas. Two-dimensional QED particle-in-cell simulations show that electron-positron-pair plasmas with overcritical density 10²² cm⁻³ and a high energy of 100s of MeV are obtained with 10 PW lasers at intensities 10²³ W/cm², which are of key significance for laboratory astrophysics studies.
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High average power 2 mum few-cycle optical parametric chirped pulse amplifier at 100-kHz repetition rate - Shamir, Y.; Rothhardt, J.; H{\"{a}drich, S. and Demmler, S. and Tschernajev, M. and Limpert, J. and T{\"{u}}nnermann, A. (Optics letters, 2015)
Sources of long wavelengths few-cycle high repetition rate pulses are becoming increasingly important for a plethora of applications, e.g., in high-field physics. Here, we report on the realization of a tunable optical parametric chirped pulse amplifier at 100 kHz repetition rate. At a central wavelength of 2 µm, the system delivered 33 fs pulses and a 6 W average power corresponding to 60 µJ pulse energy with gigawatt-level peak powers. Idler absorption and its crystal heating is experimentally investigated for a BBO. Strategies for further power scaling to several tens of watts of average power are discussed.
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HILITE{–}ions in intense photon fields - Ringleb, S.; Vogel, M.; Kumar, S.; Quint, W.; Paulus, G.G.; St{\"{o}}hlker, T. (Physica Scripta, 2015)
We are currently devising the open-endcap Penning trap experiment (high-intensity laser ion-trap experiment) as a tool for ion confinement, manipulation and detection to be used at high-energy and/or high-intensity laser facilities. This instrument will allow studies of laser–ion interactions with well-defined ion targets, and to detect the reaction products non-destructively. The ion target may be controlled concerning the constituent species, the density, shape and position with respect to the laser focus. For commissioning experiments, we optimize the focusing parameters to achieve a high number of ionized particles per shot. The detection electronics is designed to measure all charge states of all nuclei up to xenon. We plan first experiments with argon and xenon irradiated by a titanium:sapphire chirped-pulse-amplification laser system with 10 mJ pulse energy and a pulse duration of 30 fs.
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Hyperfine transition in ²⁰⁹Bi⁸⁰⁺ {–} one step forward - Sanchez, R.; Ullmann, J.; Vollbrecht, J.; Andelkovic, Z.; Dax, A.; Geithner, W.; Geppert, C.; Gorges, C.; Hammen, M.; Hannen, V.; Kaufmann, S.; K{\"{o}}nig, K.; et al. (Physica Scripta, 2015)
The hyperfine transitions in lithium-like and hydrogen-like bismuth were remeasured by direct laser spectroscopy at the experimental storage ring. For this we have now employed a voltage divider which enabled us to monitor the electron cooler voltage in situ . This will improve the experimental accuracy by about one order of magnitude with respect to our previous measurement using the same technique.
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Identification and reduction of unwanted stray radiation using an energy- and position-sensitive Compton polarimeter - Blumenhagen, K.-H.; Spillmann, U.; Gassner, T.; Gumberidze, A.; M{\"{a}rtin, R. and Schell, N. and Trotsenko, S. and Weber, G. and St{\"{o}}hlker, T. (Physica Scripta, 2015)
In this work, we report on an experiment that investigated the elastic scattering of linearly polarized 175 keV photons on a gold target. A combined measurement of the angular distribution and the linear polarization of the scattered photons was performed using standard germanium detectors and a double-sided Si(Li) strip polarimeter. Since the data analysis is still in progress, we will show results in forthcoming papers and present here how the polarimeter was used to identify a lack of shielding during the experiment.
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Investigation of the heavy-ion mode in the FAIR High Energy Storage Ring - Kovalenko, O.; Dolinskii, O.; Litvinov, Y. A.; Maier, R.; Prasuhn, D.; St{\"{o}}hlker, T. (Physica Scripta, 2015)
High energy storage ring (HESR) as a part of the future accelerator facility FAIR (Facility for Antiproton and Ion Research) will serve for a variety of internal target experiments with high-energy stored heavy ions (SPARC collaboration). Bare uranium is planned to be used as a primary beam. Since a storage time in some cases may be significant{–}up to half an hour{–}it is important to examine the high-order effects in the long-term beam dynamics. A new ion optics specifically for the heavy ion mode of the HESR is developed and is discussed in this paper. The subjects of an optics design, tune working point and a dynamic aperture are addressed. For that purpose nonlinear beam dynamics simulations are carried out. Also a flexibility of the HESR ion optical lattice is verified with regard to various experimental setups. Specifically, due to charge exchange reactions in the internal target, secondary beams, such as hydrogen-like and helium-like uranium ions, will be produced. Thus the possibility of separation of these secondary ions and the primary U⁹²⁺ beam is presented with different internal target locations.
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Laser cooling of relativistic heavy-ion beams for FAIR - Winters, D.; Beck, T.; Birkl, G.; Dimopoulou, C.; Hannen, V.; K{\"{u}}hl, T.; Lochmann, M.; Loeser, M.; Ma, X.; Nolden, F.; N{\"{o}}rtersh{\"{a}user, W. and Rein, B. and Sanchez, R. and Schramm, U. and Siebold, M. and Spiller, P. and Steck, M. and St{\"{o}}hlker, T. and Ullmann, J. and Walther, T. and Wen, W. and Yang, J. and Zhang, D. and Bussmann, M. (Physica Scripta, 2015)
Laser cooling is a powerful technique to reduce the longitudinal momentum spread of stored relativistic ion beams. Based on successful experiments at the experimental storage ring at GSI in Darmstadt, of which we show some important results in this paper, we present our plans for laser cooling of relativistic ion beams in the future heavy-ion synchrotron SIS100 at the Facility for Antiproton and Ion Research in Darmstadt.
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Noncollinear Polarization Gating of Attosecond Pulse Trains in the Relativistic Regime - Yeung, M.; Bierbach, J.; Eckner, E.; Rykovanov, S.; Kuschel, S.; S{\"{a}vert, A. and F{\"{o}}rster, M. and R{\"{o}}del, C. and Paulus, G. and Cousens, S. and Coughlan, M. and Dromey, B. and Zepf, M. (Physical Review Letters, 2015)
High order harmonics generated at relativistic intensities have long been recognized as a route to the most powerful extreme ultraviolet pulses. Reliably generating isolated attosecond pulses requires gating to only a single dominant optical cycle, but techniques developed for lower power lasers have not been readily transferable. We present a novel method to temporally gate attosecond pulse trains by combining noncollinear and polarization gating. This scheme uses a split beam configuration which allows pulse gating to be implemented at the high beam fluence typical of multi-TW to PW class laser systems. Scalings for the gate width demonstrate that isolated attosecond pulses are possible even for modest pulse durations achievable for existing and planned future ultrashort high-power laser systems. Experimental results demonstrating the spectral effects of temporal gating on harmonic spectra generated by a relativistic laser plasma interaction are shown.
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Nuclear magnetic dipole moment effect on the angular distribution of the K-alpha lines - Wu, Z. W.; Fritzsche, S.; Surzhykov, A. (Physica Scripta, 2015)
We present a theoretical analysis of the fine-structure transitions for helium-like heavy ions with non-zero nuclear spin. The angular distribution of these transitions is studied for its sensitivity with regard to the nuclear magnetic dipole moment. Detailed calculations, performed for the helium-like Sn48+, Xe52+ and Tl79+ ions with nuclear spin I=1/2, indicate that the emission pattern of the fine-structure resolved photons is significantly affected by the nuclear magnetic dipole moment and that this effect can be addressed experimentally at present storage ring facilities.
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Nuclear transfer reaction measurements at the ESR{–}for the investigation of the astrophysical ¹⁵O(α,γ)¹⁹Ne reaction - Doherty, D. T.; Woods, P. J.; Litvinov, Y. A.; Ali Najafi, M.; Bagchi, S.; Bishop, S.; Bo, M.; Brandau, C.; Davinson, T.; Dillmann, I.; Estrade, A.; Egelhof, P.; et al. (Physica Scripta, 2015)
Astrophysical x-ray bursts are thought to be a result of thermonuclear explosions on the atmosphere of an accreting neutron star. Between these bursts, energy is thought to be generated by the hot CNO cycles. The ¹⁵O(α,γ)¹⁹Ne reaction is one reaction that allows breakout from these CNO cycles and into the rp-process to fuel outbursts. The reaction is expected to be dominated by a single 3/2⁺ resonance at 4.033 MeV in ¹⁹Ne, however, limited information is available on this key state. This work reports on a pioneering study of the ²⁰Ne(p,d)¹⁹Ne reaction, performed in inverse kinematics at the experimental storage ring (ESR) as a means of accessing the astrophysically important 4.033 MeV state in ¹⁹Ne. The unique, background free, high luminosity conditions of the storage ring were utilized for this, the first transfer reaction performed at the ESR. The results of this pioneering test experiment are presented along with suggestions for future measurements at storage ring facilities.
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Predictions for the energy loss of light ions in laser-generated plasmas at low and medium velocities - Cayzac, W.; Bagnoud, V.; Basko, M. M.; Blazevic, A.; Frank, A.; Gericke, D. O.; Hallo, L.; Malka, G.; Ortner, A.; Tauschwitz, An.; Vorberger, J.; Roth, M. (Physical Review E, 2015)
The energy loss of light ions in dense plasmas is investigated with special focus on low to medium projectile energies, i.e., at velocities where the maximum of the stopping power occurs. In this region, exceptionally large theoretical uncertainties remain and no conclusive experimental data are available. We perform simulations of beam-plasma configurations well suited for an experimental test of ion energy loss in highly ionized, laser-generated carbon plasmas. The plasma parameters are extracted from two-dimensional hydrodynamic simulations, and a Monte Carlo calculation of the charge-state distribution of the projectile ion beam determines the dynamics of the ion charge state over the whole plasma profile. We show that the discrepancies in the energy loss predicted by different theoretical models are as high as 20-30%, making these theories well distinguishable in suitable experiments.
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Prospects for laser spectroscopy of highly charged ions with high-harmonic XUV and soft x-ray sources - Rothhardt, J.; H{\"{a}drich, S. and Demmler, S. and Krebs, M. and Winters, D. and Kuehl, T. and St{\"{o}}hlker, T. and Limpert, J. and T{\"{u}}nnermann, A. (Physica Scripta, 2015)
We present novel high photon flux XUV and soft x-ray sources based on high harmonic generation (HHG). The sources employ femtosecond fiber lasers, which can be operated at very high (MHz) repetition rate and average power (>100 W). HHG with such lasers results in ∼1013 photons s−1 within a single harmonic line at ∼40 nm (∼30 eV) wavelength, a photon flux comparable to what is typically available at synchrotron beam lines. In addition, resonant enhancement of HHG can result in narrow-band harmonics with high spectral purity{–}well suited for precision spectroscopy. These novel light sources will enable seminal studies on electronic transitions in highly-charged ions. For example, at the experimental storage ring 2s1/2– 2p1/2 transitions in Li-like ions can be excited up to Z=47 (∼100 eV transition energy), which provides unique sensitivity to quantum electro-dynamical effects and nuclear corrections. We estimate fluorescence count rates of the order of tens per second, which would enable studies on short-lived isotopes as well. In combination with the Doppler up-shift available in head-on excitation at future heavy-ion storage rings, such as the high energy storage ring, even multi-keV transitions can potentially be excited. Pump–probe experiments with femtosecond resolution could also be feasible and access the lifetime of short-lived excited states, thus providing novel benchmarks for atomic structure theory.
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Prototype internal target design for storage ring experiments - Petridis, N.; Grisenti, R. E.; Litvinov, Y. A.; St{\"{o}}hlker, T. (Physica Scripta, 2015)
The introduction of cryogenically cooled, few micrometer-sized nozzle geometries and an essential modification of the experimental storage ring (ESR) target station allowed for a reliable operation using low- Z gases at target area densities in the range of 10¹³{–}10¹⁴ cm−². Therefore, a remarkably versatile target source was established, enabling operation over the whole range of desired target gases (from H₂ to Xe) and area densities ( 10¹⁰ to 10¹⁴ cm−²). Moreover, the considerably smaller orifice diameter of the new target source enables a much more compact inlet chamber while, at the same time, maintaining the demanding vacuum requirements of a storage ring. A completely new inlet chamber design is presented here, which, besides the improvements regarding the achievable area densities, will feature a variable beam width down to 1 mm at the ion beam interaction region. This is of paramount importance with respect to the realization of high precision experiments, e.g. by reducing the inaccuracy of the observation angle causing the relativistic Doppler broadening. While being intended for the deployment at the future high energy storage ring within the SPARC collaboration, the new inlet chamber can also replace the current one at the ESR or serve as an internal target for CRYRING.
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Report on a computer-controlled automatic test platform for precision RF cavity characterizations - Chen, X.; Sanjari, M. S.; Piotrowski, J.; H{\"{u}}lsmann, P.; Litvinov, Y. A.; Nolden, F.; Steck, M.; St{\"{o}}hlker, T. (Physica Scripta, 2015)
An automated test platform aiming at accurate and efficient cavity characterizations has recently been set up at GSI, Darmstadt. In this proceeding the composition of such a system, on both hardware and software sides, is described in detail. The amount of necessary human work is significantly reduced to the minimum, while the measurement precision is improved considerably.
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Self-compression in a solid fiber to 24 MW peak power with few-cycle pulses at 2 μm wavelength - Gaida, C.; Gebhardt, M.; Stutzki, F.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2015)
We report on the experimental realization of a compact, fiber-based, ultrashort-pulse laser system in the 2 μm wavelength region delivering 24 fs pulse duration with 24 MW pulse peak power and 24.6 W average power. This performance level has been enabled by the favorable quadratic wavelength-dependence of the self-focusing limit, which has been experimentally verified to be at approximately 24 MW for circular polarization in a solid-core fused-silica fiber operated at a wavelength around 2 μm. The anomalous dispersion in this wavelength region allows for a simultaneous nonlinear spectral broadening and temporal pulse compression. This makes an additional compression stage redundant and facilitates a very simple and power-scalable approach. Simulations that include both the nonlinear pulse evolution and the transverse optical Kerr effect support the experimental results.
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Spatial characterization of the internal gas target at the ESR for the FOCAL experiment - Gassner, T.; Beyer, H. F. (Physica Scripta, 2015)
The FOCAL experiment involves a highly accurate twin crystal spectrometer, designed for the measurement of the ground state Lamb shift of stored highly charged ions, like hydrogen-like Au 78+, via spectroscopy in the hard-x-ray regime with an accuracy down to the few-eV level where higher-order QED contributions become accessible. For this level of accuracy all geometrical parameters including the position of the x-ray source are of crucial importance. In this conference proceeding we present our efforts to characterize the internal gas target at the experiment storage ring at GSI Darmstadt where in 2012 the FOCAL experiment was conducted.
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Storage-ring experiments on dielectronic recombination at the interface of atomic and nuclear physics - Brandau, C.; Kozhuharov, C.; Lestinsky, M.; M{\"{u}}ller, A.; Schippers, S.; St{\"{o}}hlker, T. (Physica Scripta, 2015)
A brief review about topical developments in the exploitation of the resonant electron–ion collision process of dielectronic recombination (DR) as a sensitive spectroscopic tool is given. The focus will be on DR storage-ring experiments of few-electron highly charged ions. Among others, the questions addressed in these studies cover diverse topics from the areas of strong-field quantum electrodynamics, of lifetime studies using DR resonances, and of nuclear physics. Examples from the storage rings CRYRING in Stockholm, TSR in Heidelberg, and ESR in Darmstadt are given. In addition, an overview is provided about the ongoing developments and future perspectives of DR collision spectroscopy at the upcoming Facility for Antiproton and Ion Research in Darmstadt, Germany.
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Towards FAIR: first measurements of metallic magnetic calorimeters for high-resolution x-ray spectroscopy at GSI - Hengstler, D.; Keller, M.; Sch{\"{o}}tz, C.; Geist, J.; Krantz, M.; Kempf, S.; Gastaldo, L.; Fleischmann, A.; Gassner, T.; Weber, G.; M{\"{a}rtin, R. and St{\"{o}}hlker, T. and Enss, C. (Physica Scripta, 2015)
Metallic magnetic calorimeters are energy dispersive particle detectors that are operated at temperatures below 100 mK. Applied to x-ray spectroscopy they combine the high energy resolution of crystal spectrometers with the large energy bandwidth of semiconductor detectors. After the absorption of a photon its energy is converted into heat. A paramagnetic alloy converts the temperature change into a change of magnetization that is read out by a sensitive superconducting quantum interference device magnetometer. With such a metallic magnetic calorimeter we performed two successful measurements at the internal gas target of the experimental storage ring at GSI. In the first beamtime lithium-like Au-ions were targeted on a N2 and a Xe gas target, respectively. In the second beamtime we observed a projectile beam of bare Xe ions interacting with a Xe gas target. In both experiments we achieved an energy resolution below 60 eV from 0 keV to 60 keV . We were able to detect K-lines of Xe ions of different charge states, including the Lyman series up to Ly-η and could resolve the Ly-β-doublet in H-like Xe.
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Divided-pulse amplification for terawatt-class fiber lasers - Eidam, T.; Kienel, M.; Klenke, A.; Limpert, J.; T{\"{u}}nnermann, A. (European Physical Journal Special Topics, 2015)
The coherent combination of ultra short laser pulses is a promising approach for scaling the average and peak power of ultrafast lasers. Fiber lasers and amplifiers are especially suited for this technique due to their simple singe-pass setups that can be easily parallelized. Here we propose the combination of the well-known approach of spatially separated amplification with the technique of divided-pulse amplification, i.e. an additionally performed temporally separated amplification. With the help of this multidimensional pulse stacking, laser systems come into reach capable of emitting 10’s of joules of energy at multi-kW average powers that simultaneously employ a manageable number of fibers.
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Ionization of H₂⁺ molecular ions by twisted Bessel light - Peshkov, A. A.; Fritzsche, S.; Surzhykov, A. (Physical Review A, 2015)
The photoionization of H+2 molecular ions is investigated for Bessel beams of twisted light. In particular, the angle-differential photoionization cross sections are evaluated for a macroscopic target of randomly distributed but initially aligned ions by using the nonrelativistic first-order perturbation theory. Detailed calculations of these cross sections and angular distributions are performed for different setups of the electron detectors and for selected opening angles of the Bessel beams and are compared with those for incident plane-wave radiation. It is shown that the modification in the angular distributions of the photoelectrons can be understood quite easily from the variations in the intensity pattern of the Bessel beams, relative to the size of the H+2 molecular ions.
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Limitations of Extreme Nonlinear Ultrafast Nanophotonics - Kern, C.; Z{\"{u}}rch, M.; Spielmann, C. (Nanophotonics, 2015)
High-harmonic generation (HHG) has been established as an indispensable tool in optical spectroscopy. This effect arises for instance upon illumination of a noble gas with sub-picosecond laser pulses at focussed intensities significantly greater than 1012W/cm2. HHG provides a coherent light source in the extreme ultraviolet (XUV) spectral region, which is of importance in inner shell photo ionization of many atoms and molecules. Additionally, it intrinsically features light fields with unique temporal properties. Even in its simplest realization, XUV bursts of sub-femtosecond pulse lengths are released. More sophisticated schemes open the path to attosecond physics by offering single pulses of less than 100 attoseconds duration. Resonant optical antennas are important tools for coupling and enhancing electromagnetic fields on scales below their free-space wavelength. In a special application, placing field-enhancing plasmonic nano antennas at the interaction site of an HHG experiment has been claimed to boost local laser field strengths, from insufficient initial intensities to sufficient values. This was achieved with the use of arrays of bow-tie-shaped antennas of ∼ 100nm in length. However, the feasibility of this concept depends on the vulnerability of these nano-antennas to the still intense driving laser light.We show, by looking at a set of exemplary metallic structures, that the threshold fluence Fth of laser-induced damage (LID) is a greatly limiting factor for the proposed and tested schemes along these lines.We present our findings in the context of work done by other groups, giving an assessment of the feasibility and effectiveness of the proposed scheme.
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