Ultra-thin polymer foils for laser-ion acceleration - Aurand, B.; Elkin, B.; Heim, L.-O.; Lommel, B.; Kindler, B.; Tomut, M.; R{\"{o}}del, C.; Kuschel, S.; J{\"{a}ckel, O. and K{\"{u}}hl, T. (Journal of Radioanalytical and Nuclear Chemistry, 2014)
We report on the development of new materials for laser-ion acceleration applicable for the advanced mechanism of radiation-pressure-acceleration. These targets are ultra-thin with thicknesses of just a few nm. For several years, diamond-like carbon foils in this thickness range can be produced. An alternative material containing more than one ion-species has the potential to further improve the acceleration mechanism. The fabrication and characterization of self-supporting polymer-based targets will be described in this paper. Furthermore, we show the significant influence on a radiation-pressure induced acceleration process by experimental data.
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Core-valence double photoionization of atomic mercury - Huttula, M.; Huttula, S.-M.; Fritzsche, S.; Lablanquie, P.; Penent, F.; Palaudoux, J.; Andric, L. (Physical Review A, 2014)
Multielectron coincidence spectroscopy has been used to study core 4f valence 5d, 6s double photoionization of atomic mercury. Multiconfiguration Dirac–Fock calculations were performed to calculate the energies and to estimate the single-photon intensities of the 4f^13(5d^96s^2 + 5d^106s^1) double-ionized states of atomic mercury. Reasonable agreement between the measured and simulated spectra is found if the relaxation effects of the bound-state density is taken into account in the computation of the photoionization amplitudes.
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Design considerations for a high power, ultrabroadband optical parametricchirped-pulse amplifier - Prandolini, M. J.; Riedel, R.; Schulz, M.; Hage, A.; H{\"{o}}ppner, H.; Tavella, F. (Optics Express, 2014)
A conceptual design of a high power, ultrabroadband optical parametric chirped-pulse amplifier(OPCPA) was carried out comparing nonlinear crystals (LBO and BBO) for 810 nm centered, sub-7.0 fspulses with energies above 1 mJ. These amplifiers are only possible with a parallel development ofkilowatt-level OPCPA-pump amplifiers. It is therefore important to know good strategies to use theavailable OPCPA-pump energy efficiently. Numerical simulations, including self- and cross-phasemodulation, were used to investigate the critical parameters to achieve sufficient spectral andspatial quality. At high output powers, thermal absorption in the nonlinear crystals starts todegrade the output beam quality. Strategies to minimize thermal effects and limits to the maximumaverage power are discussed.
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Laser-driven three-stage heavy-ion acceleration from relativistic laser-plasma interaction - Wang, H. Y.; Lin, C.; Liu, B.; Sheng, Z. M.; Lu, H. Y.; Ma, W. J.; Bin, J. H.; Schreiber, J.; He, X. T.; Chen, J. E.; Zepf, M.; Yan, X. (Physical Review E, 2014)
A three-stage heavy ion acceleration scheme for generation of high-energy quasimonoenergetic heavy ion beams is investigated using two-dimensional particle-in-cell simulation and analytical modeling. The scheme is based on the interaction of an intense linearly polarized laser pulse with a compound two-layer target (a front heavy ion layer + a second light ion layer). We identify that, under appropriate conditions, the heavy ions preaccelerated by a two-stage acceleration process in the front layer can be injected into the light ion shock wave in the second layer for a further third-stage acceleration. These injected heavy ions are not influenced by the screening effect from the light ions, and an isolated high-energy heavy ion beam with relatively low-energy spread is thus formed. Two-dimensional particle-in-cell simulations show that ∼100 MeV/u quasimonoenergetic Fe^24+ beams can be obtained by linearly polarized laser pulses at intensities of 1.1×10^21W/cm2.
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Triple-clad large-pitch fibers for compact high-power pulsed fiber laser systems - Gaida, C.; Stutzki, F.; Jansen, F.; Otto, H.-J.; Eidam, T.; Jauregui, C.; Vries, O. d.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2014)
We present a novel ytterbium (Yb)-doped large-pitch fiber design with significantly increased pump absorption and higher energy storage/gain per unit length, which enables high-peak-power fiber laser systems with smaller footprints. Up to now index matching between core and surrounding material in microstructured fibers was achieved by co-doping the active core region with fluorine. Here we carry out the index matching by passively doping the cladding with germanium, thus raising its index of refraction. Hence, the fluorine in the core can be omitted, which leads to an effective increase of the core doping concentration, while detrimental effects such as photo-darkening and lifetime quenching are avoided by maintaining the bulk Yb concentration. Experiments and simulations show that a gain higher than 50 dB/m and an output average power higher than 100 W with excellent beam quality are feasible even with a fiber length of only 40 cm.
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Experimental investigation of hole boring and light sail regimes of RPA by varying laser and target parameters - Kar, S.; Kakolee, K. F.; Cerchez, M.; Doria, D.; Macchi, A.; McKenna, P.; Neely, D.; Osterholz, J.; Quinn, K.; Ramakrishna, B.; Sarri, G.; Willi, O.; et al. (Plasma Physics and Controlled Fusion, 2013)
Temporal evolution of plasma jets from micrometre-scale thick foils following the interaction of intense (3 × 10 20 W cm^−2 ) laser pulses is studied systematically by time resolved optical interferometry. The fluid velocity in the plasma jets is determined by comparing the data with 2D hydrodynamic simulation, which agrees with the expected hole-boring (HB) velocity due to the laser radiation pressure. The homogeneity of the plasma density across the jets has been found to be improved substantially when irradiating the laser at circular polarization compared to linear polarization. While overdense plasma jets were formed efficiently for micrometre thick targets, decreasing the target areal density and/or increasing the irradiance on the target have provided indication of transition from the ’ to the sail (LS)’ regime of RPA, characterized by the appearance of narrow-band spectral features at several MeV/nucleon in proton and carbon spectra.
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Generation of high photon flux coherent soft x-ray radiation with few-cycle pulses - Demmler, S.; Rothhardt, J.; H{\"{a}drich, S. and Krebs, M. and Hage, A. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2013)
We present a tabletop source of coherent soft x-ray radiation with high-photon flux. Two-cycle pulses delivered by a fiber-laser-pumped optical parametric chirped-pulse amplifier operating at 180 kHz repetition rate are upconverted via high harmonic generation in neon to photon energies beyond 200 eV. A maximum photon flux of 1.3 x 10^8 photons/s is achieved within a 1% bandwidth at 125 eV photon energy. This corresponds to a conversion efficiency of 10^−9, which can be reached due to a gas jet simultaneously providing a high target density and phase matching. Further scaling potential toward higher photon flux as well as higher photon energies are discussed.
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Laser-driven generation of collimated ultra-relativistic positron beams - Sarri, G.; Schumaker, W.; Piazza, A. D.; Poder, K.; Cole, J. M.; Vargas, M.; Doria, D.; Kushel, S.; Dromey, B.; Grittani, G.; Gizzi, L.; Dieckmann, M. E.; et al. (Plasma Physics and Controlled Fusion, 2013)
We report on recent experimental results concerning the generation of collimated (divergence of the order of a few mrad) ultra-relativistic positron beams using a fully optical system. The positron beams are generated exploiting a quantum-electrodynamic cascade initiated by the propagation of a laser-accelerated, ultra-relativistic electron beam through high- Z solid targets. As long as the target thickness is comparable to or smaller than the radiation length of the material, the divergence of the escaping positron beam is of the order of the inverse of its Lorentz factor. For thicker solid targets the divergence is seen to gradually increase, due to the increased number of fundamental steps in the cascade, but it is still kept of the order of few tens of mrad, depending on the spectral components in the beam. This high degree of collimation will be fundamental for further injection into plasma-wakefield afterburners.
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Polarization correlations in the elastic Rayleigh scattering of photons by hydrogenlike ions - Surzhykov, A.; Yerokhin, V. A.; Jahrsetz, T.; Amaro, P.; St{\"{o}}hlker, T.; Fritzsche, S. (Physical Review A, 2013)
The (elastic) Rayleigh scattering of hard x rays by hydrogenlike ions has been investigated within the framework of second-order perturbation theory and Dirac's relativistic equation. The focus of this study was, in particular, on two questions: (i) How is the polarization of scattered photons affected if the incident light is itself (linearly) polarized, and (ii) how do the nondipole contributions to the electron-photon interaction and the relativistic contraction of the wave functions influence such a polarization transfer? Detailed calculations were performed for Ne9+, Xe53+, and U91+ targets and for photon energies up to ten times the 1s ionization threshold of the ions. From the comparison of these fully relativistic computations with the (nonrelativistic) dipole approximation we conclude that relativistic and higher-multipole effects often lead to a significant or even complete depolarization for heavy targets and at high photon energies.
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Probing the Complex Ion Structure in Liquid Carbon at 100 GPa - Kraus, D.; Vorberger, J.; Gericke, D. O.; Bagnoud, V.; Blazevic, A.; Cayzac, W.; Frank, A.; Gregori, G.; Ortner, A.; Otten, A.; Roth, F.; Schaumann, G.; et al. (Physical Review Letters, 2013)
We present the first direct experimental test of the complex ion structure in liquid carbon at pressures around 100 GPa, using spectrally resolved x-ray scattering from shock-compressed graphite samples. Our results confirm the structure predicted by ab initio quantum simulations and demonstrate the importance of chemical bonds at extreme conditions similar to those found in the interiors of giant planets. The evidence presented here thus provides a firmer ground for modeling the evolution and current structure of carbon-bearing icy giants like Neptune, Uranus, and a number of extrasolar planets.
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Analysis of passively combined divided-pulse amplification as an energy-scaling concept - Kienel, M.; Klenke, A.; Eidam, T.; Baumgartl, M.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2013)
The energy scaling of ultrashort-pulse systems employing simultaneously the techniques of chirped-pulse amplification and passively combined divided-pulse amplification is analyzed both experimentally and numerically. The maximum achievable efficiency is investigated and fundamental limitations originating from gain saturation, self-phase modulation and depolarization are discussed. A solution to these limitations could be an active stabilization scheme, which would allow for the operation of every single fiber amplifier at higher pulse energies.
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Characterization of 700 μJ T rays generated during high-power laser solid interaction - Gopal, A.; Singh, P.; Herzer, S.; Reinhard, A.; Schmidt, A.; Dillner, U.; May, T.; Meyer, H.-G.; Ziegler, W.; Paulus, G.G. (Optics Letters, 2013)
Laser-produced solid density plasmas are well-known as table-top sources of electromagnetic radiation. Recent studies have shown that energetic broadband terahertz pulses (T rays) can also be generated from laser-driven compact ion accelerators. Here we report the measurement of record-breaking T-Ray pulses with energies no less than 0.7 mJ. The terahertz spectrum has been characterized for frequencies ranging from 0.1–133 THz. The dependence of T-Ray yield on incident laser energy is linear and shows no tendencies of saturation. The noncollinear emission pattern and the high yield reveal that the T rays are generated by the transient field at the rear surface of the solid target.
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Charge transfer of slow highly charged xenon ions in collisions with magnesium atoms - Chen, W.; Vorobyev, G.; Guo, D.; Hillenbrand, P.-M.; Herfurth, F.; Hagmann, S.; Spillmann, U.; Trotsenko, S.; Gumberidze, A.; St{\"{o}}hlker, T. (Physical Review A, 2013)
We report an experimental study of the charge-transfer process in collisions of Xe^q+ ions (16≤q≤20) with magnesium atoms at an energy of 5.5q keV. With charge-selective and time-coincidence techniques, we separated the pure capture and capture accompanied by transfer-ionization processes. The experimental data indicate that the magnesium target is around two times more likely to lose two electrons than one in the collision. This finding is very different compared to the calculation based on the extended classic over-the-barrier model. The Xe^q+-Mg collision also behaves very differently from “traditional” collisions between highly charged ions and noble gases. We suggest a one-step dielectronic mechanism for the capture process. The data also show that autoionization dominates the relaxation process after the capture, and fluctuation of the autoionization fraction versus the projectile charge state indicates that for the relaxation processes, the projectile core structure plays a more important role than the detailed characteristics of the projectile states where the target electrons are initially captured.
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Divided-pulse nonlinear compression - Klenke, A.; Kienel, M.; Eidam, T.; H{\"{a}drich, S. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2013)
We report on the nonlinear pulse compression of temporally divided pulses, which is presented in a proof-of-principle experiment. A single 320 fs pulse is divided into four replicas, spectrally broadened in a solid-core fiber, and subsequently recombined. This approach makes it possible to reduce the nonlinearities in the fiber and therefore to use total input peak power of about 13.3 MW, which is more than three times higher than the self-focusing threshold. Finally, the combined output pulse could be compressed to sub-100 fs pulse duration. This general and universal approach holds promise for overcoming fundamental limitations of the pulse peak power that lead to destruction of the fiber or ionization limitations in high-energy hollow-core compression.
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Efficient burst mode amplifier for ultra-short pulses based on cryogenically cooled Yb3+:CaF2 - K{\"{o}}rner, J.; Hein, J.; Liebetrau, H.; Seifert, R.; Kl{\"{o}}pfel, D.; Kahle, M.; Loeser, M.; Siebold, M.; Schramm, U.; Kaluza, M.C. (Optics Express, 2013)
We present a novel approach for the amplification of high peak power femtosecond laser pulses at a high repetition rate. This approach is based on an all-diode pumped burst mode laser scheme. In this scheme, pulse bursts with a total duration between 1 and 2 ms are be generated and amplified. They contain 50 to 2000 individual pulses equally spaced in time. The individual pulses have an initial duration of 350 fs and are stretched to 50 ps prior to amplification. The amplifier stage is based on Yb3+:CaF2 cooled to 100 K. In this amplifier, a total output energy in excess of 600 mJ per burst at a repetition rate of 10 Hz is demonstrated. For lower repetition rates the total output energy per burst can be scaled up to 915 mJ using a longer pump duration. This corresponds to an efficiency as high as 25% of extracted energy from absorbed pump energy. This is the highest efficiency, which has so far been demonstrated for a pulsed Yb3+:CaF2 amplifier.
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Few-cycle optical probe-pulse for investigation of relativistic laser-plasma interactions - Schwab, M. B.; S{\"{a}vert, A. and J{\"{a}ckel, O. and Polz, J. and Schnell, M. and Rinck, T. and Veisz, L. and M{\"{o}}ller, M. and Hansinger, P. and Paulus, G.G. and Kaluza, M.C. (Applied Physics Letters, 2013)
The development of a few-cycle optical probe-pulse for the investigation of laser-plasma interactions driven by a Ti:sapphire, 30 Terawatt (TW) laser system is described. The probe is seeded by a fraction of the driving laser's energy and is spectrally broadened via self-phase modulation in a hollow core fiber filled with a rare gas, then temporally compressed to a few optical cycles via chirped mirrors. Shadowgrams of the laser-driven plasma wave created in relativistic electron acceleration experiments are presented with few-fs temporal resolution, which is shown to be independent of post-interaction spectral filtering of the probe-beam.
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High-resolution measurement of the time-modulated orbital electron capture and of the β+ decay of hydrogen-like 142Pm^60+ ions - Kienle, P.; Bosch, F.; B{\"{u}}hler, P.; Faestermann, T.; Litvinov, Y.; Winckler, N.; Sanjari, M.; Shubina, D.; Atanasov, D.; Geissel, H.; Ivanova, V.; Yan, X.; et al. (Physics Letters B, 2013)
The periodic time modulations, found recently in the two-body orbital electron capture (EC) decay of both, hydrogen-like 140Pr58+ and 142Pm60+ ions, with periods near to 7 s7 s and amplitudes of about 20%, were re-investigated for the case of 142Pm60+ by using a 245 MHz resonator cavity with a much improved sensitivity and time resolution. We observed that the exponential EC decay is modulated with a period T=7.11(11) s, in accordance with a modulation period T=7.12(11) s as obtained from simultaneous observations with a capacitive pick-up, employed also in the previous experiments. The modulation amplitudes amount to a_R=0.107(24) and a_P=0.134(27) for the 245 MHz resonator and the capacitive pick-up, respectively. These new results corroborate for both detectors exactly our previous findings of modulation periods near to 7 s, though with distinctly smaller amplitudes. Also the three-body β+ decays have been analyzed. For a supposed modulation period near to 7 s we found an amplitude a=0.027(27), compatible with a=0a=0 and in agreement with the preliminary result a=0.030(30) of our previous experiment. These observations could point at weak interaction as origin of the observed 7 s-modulation of the EC decay. Furthermore, the data suggest that interference terms occur in the two-body EC decay, although the neutrinos are not directly observed.
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Long-term stabilization of high power optical parametric chirped-pulse amplifiers - Riedel, R.; Schulz, M.; Prandolini, M. J.; Hage, A.; H{\"{o}}ppner, H.; Gottschall, T.; Limpert, J.; Drescher, M.; Tavella, F. (Optics Express, 2013)
The long-term stability of optical parametric chirped-pulse amplifiers is hindered by thermal path length drifts affecting the temporal pump-to-signal overlap. A kilowatt-pumped burst amplifier is presented delivering broadband 1.4 mJ pulses with a spectral bandwidth supporting sub-7 fs pulse duration. Active temporal overlap control can be achieved by feedback of optical timing signals from cross-correlation or spectral measurements. Using a balanced optical cross-correlator, we achieve a pump-to-signal synchronization with a residual jitter of only (46 ± 2) fs rms. Additionally, we propose passive pump-to-signal stabilization with an intrinsic jitter of (7.0 ± 0.5) fs rms using white-light continuum generation.
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Nuclear physics with unstable ions at storage rings - Bosch, F.; Litvinov, Y. A.; St{\"{o}}hlker, T. (Progress in Particle and Nuclear Physics, 2013)
During the last two decades, ion storage-cooler rings have been proven as powerful devices for addressing precision experiments in the realm of atomic physics, nuclear physics and nuclear astrophysics. Most important, in particular for stored unstable nuclides, is the unrivalled capability of ion cooler-rings to generate brilliant beams of highest phase–space density owing to sophisticated cooling techniques, and to store them for extended periods of time by preserving their charge state. This report focuses on nuclear physics and nuclear astrophysics experiments with in-flight produced exotic ions that were injected into storage-cooler rings. Those experiments were conducted within the last decade mainly at the only operating facilities that are capable to provide and to store exotic ions, namely the ESR in Darmstadt, Germany and the CSRe-ring in Lanzhou, China. The majority of nuclear physics experiments performed at these equipments concerns ground-state properties of nuclei far from stability, such as masses and lifetimes. The rich harvest of these measurements is presented. In particular it is shown that storage-cooler rings are ideal, if not the only, devices where two-body beta decays of exotic highly-charged ions, such as bound-state beta decay and orbital electron capture, can be studied in every detail, based on “single-ion decay spectroscopy”. Furthermore, experiments at the border between atomic and nuclear physics are discussed which render valuable information on nuclear properties by exploiting one of the most precise tools of atomic spectroscopy on stored ions, the “dielectronic recombination”. Ion storage rings with cooled exotic beams and equipped with thin internal gas targets deliver also unrivalled opportunities for addressing with highest precision key reactions in the fields of nuclear astrophysics and nuclear structure. First very promising experiments exploring the potential of ion cooler-rings in this realm have been already conducted. However, in view of the small nuclear cross sections, many of fervently desired experiments in this field will still suffer from the insufficient number of exotic ions that can be delivered and stored at the time being. The realistic hope on a breakthrough in this field is based on the ion storage rings to come, with their estimated improvements in the intensity of exotic ion beams by many orders of magnitude.
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Plasmon measurements with a seeded x-ray laser - Fletcher, L. B.; Galtier, E.; Heimann, P.; Lee, H. J.; Nagler, B.; Welch, J.; Zastrau, U.; Hastings, J. B.; Glenzer, S. H. (Journal of Instrumentation, 2013)
Plasmon measurements hold great promise for providing highly accurate data on the physical properties of plasmas in the high-energy density physics regime. To this end we demonstrate in recent experiments at the Linac Coherent Light Source the first spectrally-resolved measurements of plasmons using a seeded 8-keV x-ray laser beam. Forward x-ray Thomson scattering spectra from isochorically heated solid aluminum show a well-resolved plasmon feature that is down-shifted in energy by 19 eV from the incident 8 keV elastic scattering feature. In this spectral range, the simultaneously measured backscatter spectrum shows no spectral features indicating observation of collective plasmon oscillations on a scattering length comparable to the screening length. This technique is a prerequisite for Thomson scattering measurements in compressed matter where the plasmon shift is a sensitive function of the free electron density and where the plasmon intensity provides information on temperature.
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Sm3+-doped La2O3–Al2O3–SiO2-glasses: structure, fluorescence and thermal expansion - Kuhn, S.; Herrmann, A.; Hein, J.; Kaluza, M.C.; Russel, C. (Journal of Materials Science, 2013)
This paper reports on the effect of the chemical composition on the glass structure, the coefficients of thermal expansion and the fluorescence properties of Sm3+-doped La2O3–Al2O3–SiO2-glasses. The silica concentration was varied between 50 and 70 mol% and the La2O3:Al2O3 ratio between 50:50 and 30:70. The glass formation and the densities are evaluated and FTIR reflectance spectra, coefficients of thermal expansion and fluorescence lifetimes are determined. It is shown that high SiO2 concentrations and low La2O3:Al2O3 ratios result in relatively high fluorescence lifetime (2.19 ms, 4G5/2) and low coefficients of thermal expansion (4.6 × 10^−6 / K). The coefficients of thermal expansion and the fluorescence lifetimes show a linear dependency on the ratio LaO3/2/(AlO3/2 + SiO2).
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Atomic ionization of hydrogen-like ions by twisted photons: angular distribution of emitted electrons - Matula, O.; Hayrapetyan, A. G.; Serbo, V. G.; Surzhykov, A.; Fritzsche, S. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2013)
We investigate the angular distribution of electrons that are emitted in the ionization of hydrogen-like ions by twisted photons. Analysis is performed based on the first-order perturbation theory and the non-relativistic Schr{\"{o}}dinger equation. Special attention is paid to the dependence of the electron emission pattern on the impact parameter b of the ion with respect to the centre of the twisted wave front. In order to explore such a dependence, detailed calculations were carried out for the photoionization of the 1s ground and 2 p_y excited states of neutral hydrogen atoms. Based on these calculations, we argue that for relatively small impact parameters, the electron angular distributions may be strongly affected by altering the position of the atom within the wave front. In contrast, if the atom is placed far from the front centre, the emission pattern of the electrons is independent of the impact parameter b and resembles that observed in the photoionization by plane wave photons.
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Characterization of strongly-bent HAPG crystals for von-H{\'{a}}mos x-ray spectrographs - Zastrau, U.; Woldegeorgis, A.; F{\"{o}}rster, E.; Loetzsch, R.; Marschner, H.; Uschmann, I. (Journal of Instrumentation, 2013)
The properties of two strongly bent Highly Annealed Pyrolytic Graphite (HAPG) crystals with different thicknesses of 40 μm and 100 μm are studied at all possible reflection orders using x-rays at 4.5 keV and 8 keV photon energies. Typical reflecting areas within 50% reflectivity drop boundaries have sizes of about ≤ 1 mm. These domains are mis-oriented by ≤ 1 minutes of arc to each other. The mosaicity was measured to be 0.06° on a 1 × 1 mm 2 scale, whereas it amounts to 0.14° when the probed area becomes > 2 × 1 mm 2 . We find that the integrated reflectivity of the reflection (004) is in good agreement with the kinematical diffraction theory, while a maximum value of 2.3 mrad is achieved for 8 keV and reflection (002). The highest spectral resolution is obtained with an x-ray source of ≤ 50 μm size and a 40 μm thin graphite coating, which amounts to E /Δ E ≥ 1000 for 4.5 keV and 8 keV. In the case of 8 keV and reflection (008), the resolving power exceeds E /Δ E = 2000. In von-H{\'{a}}mos geometry, it was found that > 60% of the reflected photons are confined in a central 500 μm wide profile where high spectral resolution is pertained. Ray tracing simulations reveal that in order to pertain a certain resolution, a larger mosaicity would result in less contributing photons. Thus the efficiency of the crystal drops significantly when the mosaicity is increased and could not be increased by large crystal opening angles.
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Differential L-shell radiative recombination rate coefficients for bare uranium ions interacting with low-energy electrons - Bana{ś}, D.; Pajek, M.; Gumberidze, A.; Surzhykov, A.; St{\"{o}}hlker, T. (The European Physical Journal - Special Topics, 2013)
Results of the calculations of differential L−shell radiative recombination (RR) rate coefficients for bare uranium ions colliding with free electrons using the nonrelativistic dipole approximation and fully relativistic calculations are reported. The rate coefficients were obtained for very low, in the range of meV, relative electron-ion energies. We demonstrate that even for such low relative ion-electron energies the relativistic effects significantly modify the differential RR rate coefficients for the L−subshells and, as a result, the measurements of the relative electron energy dependence of the L-RR rates could be used for studying of the relativistic effects. These effects are strongest for the L_3-subshell, which is discussed here in more details.
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Focusing and transport of high-intensity multi-MeV proton bunches from a compact laser-driven source - Busold, S.; Schumacher, D.; Deppert, O.; Brabetz, C.; Frydrych, S.; Kroll, F.; Joost, M.; Al-Omari, H.; Blazevic, A.; Zielbauer, B.; Hofmann, I.; Bagnoud, V.; et al. (Physical Review ST - Accelerators and Beams, 2013)
Laser ion acceleration provides for compact, high-intensity ion sources in the multi-MeV range. Using a pulsed high-field solenoid, for the first time high-intensity laser-accelerated proton bunches could be selected from the continuous exponential spectrum and delivered to large distances, containing more than 109 particles in a narrow energy interval around a central energy of 9.4 MeV and showing ≤ 30 mrad envelope divergence. The bunches of only a few nanoseconds bunch duration were characterized 2.2 m behind the laser-plasma source with respect to arrival time, energy width, and intensity as well as spatial and temporal bunch profile.
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High-power fibre lasers - Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Nature Photonics, 2013)
Fibre lasers are now associated with high average powers and very high beam qualities. Both these characteristics are required by many industrial, defence and scientific applications, which explains why fibre lasers have become one of the most popular laser technologies. However, this success, which is largely founded on the outstanding characteristics of fibres as an active medium, has only been achieved through researchers around the world striving to overcome many of the limitations imposed by the fibre architecture. This Review focuses on these limitations, both past and current, and the creative solutions that have been proposed for overcoming them. These solutions have enabled fibre lasers to generate the highest diffraction-limited average power achieved to date by solid-state lasers.
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Nonlinear compression to sub-30-fs, 0.5 mJ pulses at 135 W of average power - H{\"{a}drich, S. and Klenke, A. and Hoffmann, A. and Eidam, T. and Gottschall, T. and Rothhardt, J. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2013)
Incorporation of coherent combination into a state-of-the-art fiber-chirped pulse amplification system obtains 1.1 mJ, 340 fs pulses with up to 280 W of average power at 250 kHz repetition rate. Propagation of this laser pulse inside a krypton-filled hollow-core fiber results in significant spectral broadening. Chirped mirrors are used to compress the pulses to 26 fs, 540 μJ (135 W) leading to a peak power of more than 11 GW. This unprecedented combination of high peak and average power ultrashort pulses opens up new possibilities in multidimensional surface science and coherent soft x-ray generation.
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Photon polarization tensor in a homogeneous magnetic or electric field - Karbstein, F. (Physical Review D, 2013)
We revisit the photon polarization tensor in a homogeneous external magnetic or electric field. The starting point of our considerations is the momentum space representation of the one-loop photon polarization tensor in the presence of a homogeneous electromagnetic field, known in terms of a double parameter integral. Our focus is on explicit analytical insights for both on- and off-the-light-cone dynamics in a wide range of well-specified physical parameter regimes, ranging from the perturbative to the manifestly nonperturbative strong field regime. The basic ideas underlying well-established approximations to the photon polarization tensor are carefully examined and critically reviewed. In particular, we systematically keep track of all contributions, both the ones to be neglected and those to be taken into account explicitly, to all orders. This allows us to study their ranges of applicability in a much more systematic and rigorous way. We point out the limitations of such approximations and manage to go beyond at several instances.
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Two-photon decay of inner-shell vacancies in heavy atoms - Surzhykov, A.; Pratt, R. H.; Fritzsche, S. (Physical Review A, 2013)
Based on the second-order perturbation theory, we investigate the two-photon decay of K-shell vacancies in heavy atoms. The many-electron transition amplitude that occurs in the theory is evaluated by means of the independent particle approximation (IPA). By using this approach, computations are performed for the decay of neutral gold and are directly compared with recent experimental data, not relying on any scaling assumptions. The obtained results confirm previously identified discrepancies between the IPA theory and the experiment for the 2s→1s transition, and an apparent “resonance” region of the 3s→1s transition, but they show a moderate agreement with the measured data for the 3d→1s and 4s+4d→1s cases. Moreover, with the help of the IPA we discuss the validity of the nonrelativistic scaling that was employed in the past to estimate the relative two-photon transition probabilities P in heavy atoms based on calculations done for lighter elements and different decay geometries. We find, in particular, that the electric-dipole angular distribution of emitted photons holds rather well even in the high-Z domain, while the assumption that the relative probability P is independent of nuclear charge may result in 10–30% inaccuracy of theoretical predictions.
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A beamline for x-ray laser spectroscopy at the experimental storage ring at GSI - Winters, D. F. A.; Bagnoud, V.; Ecker, B.; Eisenbarth, U.; G{\"{o}}tte, S.; K{\"{u}}hl, T.; Neumayer, P.; Spielmann, C.; St{\"{o}}hlker, T.; Zielbauer, B. (Physica Scripta, 2013)
By combining an x-ray laser (XRL) with a heavy-ion storage ring, precision laser spectroscopy of the fine-structure splitting in heavy Li-like ions will be possible. An initial study has been performed to determine the feasibility of a first experiment at the experimental storage ring at GSI in Darmstadt, which also has great potential for the experiments planned for FAIR. We plan to perform a unique, direct and precise measurement of a fine-structure transition in a heavy Li-like ion. Such a measurement will test state-of-the-art atomic structure calculations in strong fields. This endeavour will require that the existing infrastructure is complemented by a dedicated beamline for the XRL. In this paper, we will discuss the details of this project and outline a proof-of-principle experiment.
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A study of radiative double electron capture in bare chromium ions at the ESR - Winters, N.; Warczak, A.; Tanis, J. A.; Gassner, T.; Gumberidze, A.; Hagmann, S.; Hillenbrand, P. M.; Kozhuharov, C.; Petridis, N.; Reuschl, R.; Schwemlein, M.; Sierpowski, D.; et al. (Physica Scripta, 2013)
Radiative double electron capture is a fundamental atomic process which should be observed in collisions of bare ions with atoms, albeit with a much smaller cross-section than single radiative electron capture. A new experiment{–}to observe this rare process under single-collision conditions{–}has been performed at the internal gas jet target of the experimental storage ring at GSI in Darmstadt. X-ray spectra associated with single and double charge exchange have been observed in 30 MeV u^(−1) collisions of bare chromium ions (Cr^(24+)) with helium and nitrogen target gases.
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Ab initio calculations of the 2p_3/2-2p_1/2 fine-structure splitting in boronlike ions - Artemyev, A. N.; Shabaev, V. M.; Tupitsyn, I. I.; Plunien, G.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2013)
We have performed ab initio QED calculations of the (1s)^2(2s)^22p_3/2-(1s)^2(2s)^22p_1/2 fine-structure splitting along the boron isoelectronic sequence for all ions with 17 ≤ Z ≤ 100. This level splitting was evaluated within the extended Furry picture and by making use of four different screening potentials in order to estimate the effects of interelectronic correlations. The accuracy of the predicted transition energies has been improved significantly when compared with previous computations.
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An extreme ultraviolet Michelson interferometer for experiments at free-electron lasers - Hilbert, V.; Blinne, A.; Fuchs, S.; Feigl, T.; K{\"{a}mpfer, T. and R{\"{o}}del, C. and Uschmann, I. and W{\"{u}}nsche, M. and Paulus, G. and F{\"{o}}rster, E. and Zastrau, U. (Review of Scientific Instruments, 2013)
We present a Michelson interferometer for 13.5 nm soft x-ray radiation. It is characterized in a proof-of-principle experiment using synchrotron radiation, where the temporal coherence is measured to be 13 fs. The curvature of the thin-film beam splitter membrane is derived from the observed fringe pattern. The applicability of this Michelson interferometer at intense free-electron lasers is investigated, particularly with respect to radiation damage. This study highlights the potential role of such Michelson interferometers in solid density plasma investigations using, for instance, extreme soft x-ray free-electron lasers. A setup using the Michelson interferometer for pseudo-Nomarski-interferometry is proposed.
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Bessel beams of laser-driven two-level atoms - Hayrapetyan, A. G.; Fritzsche, S. (Physica Scripta, 2013)
We study Bessel beams of two-level atoms that are coupled to a linearly polarized laser field. For such atom beams, we construct exact Bessel-type solutions of the Schr{\"{o}}dinger equation beyond the paraxial approximation for beam propagation. In particular, we examine the probability density for Bessel beams of neutral two-level atoms driven by a laser field but without the level damping being taken into account. We show how the radial dependence of the probability density (from the beam axis) can be affected by tuning the parameters of the atom–laser system, such as the resonant frequency and amplitude of the laser field and/or the nuclear charge and velocity of the atomic beam.
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Beta decay of highly charged ions - Bosch, F.; Atanasov, D. R.; Brandau, C.; Dillmann, I.; Dimopoulou, C.; Faestermann, T.; Geissel, H.; Hagmann, S.; Hillenbrand, P.-M.; Kienle, P.; Kn{\"{o}}bel, R.; Kozhuharov, C.; et al. (Physica Scripta, 2013)
A brief overview is presented in this paper on some experiments conducted at the Experimental Storage Ring (ESR) of GSI which addressed the β decay of stored and cooled highly charged ions. Special emphasis is placed on the two-body beta decay of bare or few-electron ions: bound-state β− decay (β_b) and its time-mirrored counterpart, orbital electron capture. The former decay mode was detected experimentally 20 years ago at the ESR. The latter could be investigated there for the first time in detail for the simplest quantum systems: hydrogen- and helium-like atoms. The main results of these experiments will be presented. Also their impact on stellar nucleosynthesis, in particular the s -process, is discussed.
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Bremsstrahlung of polarized positrons scattered off atoms - Yerokhin, V. A.; Surzhykov, A.; M{\"{a}rtin, R. and Tashenov, S. and Weber, G. (Physica Scripta, 2013)
The leading-order positron–atom bremsstrahlung is investigated within the rigorous relativistic approach based on the partial-wave representation of the Dirac wave functions in the external atomic field. Approximating the atomic target by an effective local potential, we calculate the Stokes parameters of the emitted photon for different polarizations of the initial positron. The results for positron–atom bremsstrahlung are compared with analogous data for the electron–atom bremsstrahlung.
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Employing digital pulse processing electronics for the readout of a Si(Li){–}Compton{–}polarimeter for the SPARC collaboration - Spillmann, U.; Blumenhagen, K.-H.; Badura, E.; Balzer, M.; Br{\"{a}uning, H. and Hoffmann, J. and Koch, K. and Kurz, N. and M{\"{a}rtin, R. and Minami, S. and Ott, W. and St{\"{o}}hlker, T. and Weber, G. and Weber, M. (Physica Scripta, 2013)
The future x-ray spectroscopy and polarimetry experiment program of the SPARC collaboration at GSI and FAIR relies strongly on the availability of two-dimensional position-sensitive, energy- and time-dispersive thick semiconductor detector systems, including the appropriate signal processing electronics. To meet these demands, the development of a compact and scalable data acquisition system that has higher rate acceptance compared to commercial VME electronics by employing digital pulse processing electronics was started.
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Few-body quantum dynamics of high-Z ions studied at the future relativistic high-energy storage ring - Hagmann, S.; St{\"{o}}hlker, T.; Litvinov, Y.; Kozhuharov, C.; Hillenbrand, P.-M.; Spillmann, U.; Shabaev, V.; Stiebing, K.; Lestinsky, M.; Surzhykov, A.; Voitkiv, A.; Franzke, B.; et al. (Physica Scripta, 2013)
At the FAIR facility for antiprotons and ion research, the high-energy storage ring will provide highly charged heavy ions with Z all the way to Z = 92 for beam energies ranging from 200 A MeV up to energies of approximately 5 A GeV. This opens up a wealth of opportunities for in-ring atomic physics experiments on few-body quantum dynamics ranging from, for example, the correlated dynamics of various e^(+) –e^(−) pair creation processes to quasi-photoionization of inner shells of the highest- Z ions.
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First observation of the ground-state hyperfine transition in (209)^Bi^(80+) - N{\"{o}}rtersh{\"{a}user, W. and Lochmann, M. and J{\"{o}}hren, R. and Geppert, C. and Andelkovic, Z. and Anielski, D. and Botermann, B. and Bussmann, M. and Dax, A. and Fr{\"{o}}mmgen, N. and Hammen, M. and Hannen, V. and K{\"{u}}hl, T. and Litvinov, Y. A. and Volbrecht, J. and St{\"{o}}hlker, T. and Thompson, R. C. and Weinheimer, C. and Wen, W. and Will, E. and Winters, D. and S{\'{a}}nchez, R. M. (Physica Scripta, 2013)
The long sought after ground-state hyperfine transition in lithium-like bismuth (209)^Bi^(80+) was observed for the first time using laser spectroscopy on relativistic ions in the experimental storage ring at the GSI Helmholtz Centre in Darmstadt. Combined with the transition in the corresponding hydrogen-like ion (209)^Bi^(82+) , it will allow extraction of the specific difference between the two transitions that is unaffected by the magnetic moment distribution in the nucleus and can therefore provide a better test of bound-state QED in extremely strong magnetic fields.
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Fully digital readout of segmented solid state detectors - Blumenhagen, K.-H.; Badura, E.; Br{\"{a}uning, H. and Hoffmann, J. and Koch, K. and Kurz, N. and M{\"{a}rtin, R. and Minami, S. and Ott, W. and Spillmann, U. and St{\"{o}}hlker, T. and Weber, G. and Weber, M. (Physica Scripta, 2013)
In this work, the digital readout of semiconductor detectors in combination with digital filters was investigated. Both non-segmented high-purity germanium and segmented planar lithium-drifted silicon detectors were used. In each case, photons from a stationary americium ((241)^Am) gamma source were detected. The resulting preamplifier output pulses were digitized at a fixed sampling frequency and stored entirely. Digital filters were applied to the stored waveforms to extract time and energy information. The performance of different digital filters was compared. The optimum energy resolution obtained was comparable with the value resulting from an analogue readout system based on standard nuclear instrumentation module and versatile module Europe bus electronics.
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Future experiments using forward electron spectroscopy to study the quantum dynamics of high- Z ions at the ESR/CRYRING storage rings - Hillenbrand, P. M.; Hagmann, S.; St{\"{o}}hlker, T.; Litvinov, Y.; Kozhuharov, C.; Spillmann, U.; Shabaev, V.; Stiebing, K.; Lestinsky, M.; Surzhykov, A.; Voitkiv, A.; Franzke, B.; et al. (Physica Scripta, 2013)
At the FAIR facility for antiproton and ion research, the new ESR + CRYRING combination of storage rings CRYRING@ESR opens up a wealth of opportunities for in-ring atomic physics experiments on few-body quantum dynamics. The low-energy storage ring CRYRING will serve in its new location at FAIR/ESR for experiments with decelerated antiprotons and highly charged ions. We will discuss selected new experiments in the field of quantum dynamics of high- Z ions, for example for adiabatic superheavy quasi-molecules transiently formed with bare and H-like projectiles. Such experiments will be for the first time possible at the future CRYRING at ESR.
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Half-life measurements of highly charged radionuclides - Atanasov, D. R.; Balabanski, D.; Batist, L.; Blaum, K.; Bosch, F.; Boutin, D.; Brandau, C.; Dimopoulou, C.; Essel, H. G.; Faestermann, T.; Geissel, H.; Hagmann, S.; et al. (Physica Scripta, 2013)
In recent years several measurements of the orbital electron capture half-lives of few-electron ions have been carried out employing the storage ring ESR at GSI. Hydrogen-like and helium-like (140)^Pr and (142)^Pm as well as hydrogen-like (122)^I were studied. Half-lives of the corresponding fully ionized nuclides provide the three-body β^(+) decay constants.
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High Resolution Energy-Angle Correlation Measurement of Hard X Rays from Laser-Thomson Backscattering - Jochmann, A.; Irman, A.; Bussmann, M.; Couperus, J. P.; Cowan, T. E.; Debus, A. D.; Kuntzsch, M.; Ledingham, K. W. D.; Lehnert, U.; Sauerbrey, R.; Schlenvoigt, H. P.; Seipt, D.; et al. (Physical Review Letters, 2013)
Thomson backscattering of intense laser pulses from relativistic electrons not only allows for the generation of bright x-ray pulses but also for the investigation of the complex particle dynamics at the interaction point. For this purpose a complete spectral characterization of a Thomson source powered by a compact linear electron accelerator is performed with unprecedented angular and energy resolution. A rigorous statistical analysis comparing experimental data to 3D simulations enables, e.g., the extraction of the angular distribution of electrons with 1.5% accuracy and, in total, provides predictive capability for the future high brightness hard x-ray source PHOENIX (photon electron collider for narrow bandwidth intense x rays) and potential gamma-ray sources.
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K-shell differential radiative recombination rates for bare uranium ions interacting with low-energy electrons - Bana{ś}, D.; Pajek, M.; Gumberidze, A.; Surzhykov, A.; St{\"{o}}hlker, T. (Physica Scripta, 2013)
We report on the calculations of the K-shell differential radiative recombination (RR) rate coefficients for very low, in the range of meV, relative electron–ion energies. The rate coefficients were derived for bare uranium ions colliding with free electrons both within the nonrelativistic dipole approximation and using fully relativistic calculations. We show that even for such low relative ion–electron energies, the differential rate coefficient reveals strong relativistic effects. We demonstrate that the measurements of the relative electron energy dependence of the RR rates represent a very sensitive tool for precise studies of the RR process and, in particular, for probing the fine details of the relativistic effects in the RR of ions with electrons. The results are discussed in the context of the first x-ray state-selective RR experiment performed for very low relative electron–ion energies.
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Lifetime measurement of the 2 3^P_0 state in He-like uranium - Reuschl, R.; Gassner, T.; Spillmann, U.; Br{\"{a}uning-Demian, A. and Ananyeva, A. and Beyer, H. and Blumenhagen, K.-H. and Chen, W. and Hagmann, S. and Hegewald, M. and Indelicato, P. and Schwemlein, M. and Toleikis, S. and Trassinelli, M. and Trotsenko, S. and Winters, D. and Winters, N. and St{\"{o}}hlker, T. (Physica Scripta, 2013)
In recent years for the fundamental theory of quantum electrodynamics, considerable progress in the evaluation of higher order corrections has been achieved{–}not only for hydrogen{–}but also for helium-like systems{–}up to very heavy nuclei. We were aiming at a more precise determination of the lifetime of the metastable 2 3^P_0 state in He-like uranium which has a calculated value of 57.3 ps [1, 2]. From the lifetime it is possible to derive the energy of the state. In October 2011 we were able to perform a first test experiment at GSI, Darmstadt to study the feasibility of a new experimental detection technique. This advanced set-up consists of two state-of-the-art energy-, time- and position-sensitive germanium detectors [3] in combination with collimators in a Soller-slit like assembly. A beam of U^(91+) -ions at an energy of 290 MeV u^(−1) is passed through a thin nickel foil in the interaction chamber. From the decrease in intensity as a function of the target distance one may extract a decay curve from which the lifetime can be derived. The advantages of this new set-up, in comparison to former experiments [4, 5] will be discussed and the results of a preliminary data analysis will be presented.
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Measurement and simulations of hollow atom X-ray spectra of solid-density relativistic plasma created by high-contrast PW optical laser pulses - Pikuz, S.; Faenov, A.; Colgan, J.; Dance, R.; Abdallah, J.; Wagenaars, E.; Booth, N.; Culfa, O.; Evans, R.; Gray, R.; K{\"{a}mpfer, T. and Lancaster, K. and McKenna, P. and Rossall, A. and Skobelev, I. and Schulze, K. and Uschmann, I. and Zhidkov, A. and Woolsey, N. (High Energy Density Physics, 2013)
Abstract K-shell spectra of solid Al excited by petawatt picosecond laser pulses have been investigated at the Vulcan PW facility. Laser pulses of ultrahigh contrast with an energy of 160 J on the target allow studies of interactions between the laser field and solid state matter at 10^20 W/cm^2. Intense X-ray emission of KK hollow atoms (atoms without n = 1 electrons) from thin aluminum foils is observed from optical laser plasma for the first time. Specifically for 1.5 μm thin foil targets the hollow atom yield dominates the resonance line emission. It is suggested that the hollow atoms are predominantly excited by the impact of X-ray photons generated by radiation friction to fast electron currents in solid-density plasma due to Thomson scattering and bremsstrahlung in the transverse plasma fields. Numerical simulations of Al hollow atom spectra using the ATOMIC code confirm that the impact of keV photons dominates the atom ionization. Our estimates demonstrate that solid-density plasma generated by relativistic optical laser pulses provide the source of a polychromatic keV range X-ray field of 10^18 W/cm^2 intensity, and allows the study of excited matter in the radiation-dominated regime. High-resolution X-ray spectroscopy of hollow atom radiation is found to be a powerful tool to study the properties of high-energy density plasma created by intense X-ray radiation.
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Optical control of hard X-ray polarization by electron injection in a laser wakefield accelerator - Schnell, M.; S{\"{a}vert, A. and Uschmann, I. and Reuter, M. and Nicolai, M. and K{\"{a}mpfer, T. and Landgraf, B. and J{\"{a}ckel, O. and Jansen, O. and Pukhov, A. and Kaluza, M.C. and Spielmann, C. (Nature Communications, 2013)
Laser-plasma particle accelerators could provide more compact sources of high-energy radiation than conventional accelerators. Moreover, because they deliver radiation in femtosecond pulses, they could improve the time resolution of X-ray absorption techniques. Here we show that we can measure and control the polarization of ultra-short, broad-band keV photon pulses emitted from a laser-plasma-based betatron source. The electron trajectories and hence the polarization of the emitted X-rays are experimentally controlled by the pulse-front tilt of the driving laser pulses. Particle-in-cell simulations show that an asymmetric plasma wave can be driven by a tilted pulse front and a non-symmetric intensity distribution of the focal spot. Both lead to a notable off-axis electron injection followed by collective electron–betatron oscillations. We expect that our method for an all-optical steering is not only useful for plasma-based X-ray sources but also has significance for future laser-based particle accelerators.
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Parity violation in beryllium-like heavy ions - Surzhykov, A.; Maiorova, A. V.; Shabaev, V. M.; St{\"{o}}hlker, T.; Fritzsche, S. (Physica Scripta, 2013)
A novel scheme is proposed for studying the parity-violating (PV) effects in beryllium-like heavy ions. It is based on the application of circularly polarized ultraviolet light for inducing a single-photon transition between the metastable 1s^(2)2s2p^(3)P_0 and the short-lived 1s^(2)2s2p^(3)P_1 states. We argue that the cross section of such a photoabsorption process is sensitive to the mixing between the allowed magnetic dipole (M1) and the PV electric dipole (E1) excitation channels. Based on relativistic calculations, we find that the PV-mixing may influence the cross section at the level of 10^(−5) % for beryllium-like uranium, U^(88+).
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Preparation and characterization of nanometer-thin freestanding polymer foils for laser-ion acceleration - Aurand, B.; Elkin, B.; Heim, L.-O.; Lommel, B.; Kindler, B.; Tomut, M.; R{\"{o}}del, C.; Kuschel, S.; J{\"{a}ckel, O. and Barz, J. and K{\"{u}}hl, T. (Journal of Polymer Science Part B: Polymer Physics, 2013)
We report on the production and characterization of polymer-based ultra-thin (sub 10 nm) foils suited for experiments on laser-ion acceleration in the regime of radiation pressure acceleration. Beside the remarkable mechanical stability compared with commonly used diamond-like-carbon foils, a very homogeneous layer thickness and a small surface roughness have been achieved. We describe the technical issues of the production process as well as detailed studies of the mechanical stability and surface roughness tests. The capability of producing uniform targets of large area is essential for advanced laser-ion acceleration projects which are dealing with high repetition rate and extended measurement series, but might also be useful for other applications which require ultra-thin and freestanding substrates of high quality.
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Probing nuclear properties by resonant atomic collisions between electrons and ions - Brandau, C.; Kozhuharov, C.; M{\"{u}}ller, A.; Bernhardt, D.; Bana{ś}, D.; Bosch, F.; Currell, F. J.; Dimopoulou, C.; Gumberidze, A.; Hagmann, S.; Hillenbrand, P.-M.; Heil, M.; et al. (Physica Scripta, 2013)
The utilization of the resonant atomic electron–ion collision process of dielectronic recombination (DR) as a tool to probe nuclear properties via isotope shifts and hyperfine effects is discussed. Based on DR, this resonance reaction spectroscopy at electron coolers of heavy-ion storage rings denotes a versatile approach to access nuclear parameters such as charge radius, spin, magnetic moment or lifetimes of long-lived excited nuclear states (isomers). The high sensitivity of DR allows for experiments with artificially synthesized rare isotopes and isomers. Recent experimental progress in the preparation of such exotic species at the ESR storage ring in Darmstadt is presented. The DR technique is exemplified for the case of (234)^Pa^(88+) (Z = 91).
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Production and diagnostics of spin-polarized heavy ions in the sequential two-electron radiative recombination - Maiorova, A. V.; Surzhykov, A.; Tashenov, S.; Shabaev, V. M.; St{\"{o}}hlker, T. (Physica Scripta, 2013)
The sequential radiative recombination of initially bare ions, which are collided with two spatially separated electron targets, is studied. It is demonstrated that the magnetic sublevel population of H-like ions, which are formed due to electron capture from the first target, depends on the first radiative recombination (RR) photon emission direction. Such a relative population, which can be parameterized in terms of the polarization parameters, affects then the angular and polarization properties of the second photon emitted in the collision with the second target. The coincidence γ – γ RR measurements may allow us to study, therefore, the process in which (i) H-like ions of some particular polarization are out’ of the beam by detecting first recombination photons and (ii) this polarization is ’ in the second electron capture process. In order to describe the output of the (future) γ – γ correlation measurements, we derive the general expression for angular- and polarization-correlation function. Detailed calculations for the dependence of this function on the experimental setup and collision energy are performed for the RR of bare uranium ions.
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Relativistic calculations of charge transfer probabilities in U⁹²⁺–U⁹¹⁺(1s) collisions using the basis set of cubic Hermite splines - Maltsev, I. A.; Deyneka, G. B.; Tupitsyn, I. I.; Shabaev, V. M.; Kozhedub, Y. S.; Plunien, G.; St{\"{o}}hlker, T. (Physica Scripta, 2013)
A new approach for solving the time-dependent two-center Dirac equation is presented. The method is based on using the finite basis set of cubic Hermite splines on a two-dimensional lattice. The Dirac equation is treated in the rotating reference frame. The collision of U⁹²⁺ (as a projectile) and U⁹¹⁺ (as a target) is considered at energy E_(lab) = 6 MeV u^(−1) . The charge transfer probabilities are calculated for different values of the impact parameter. The results obtained are compared with previous calculations (Tupitsyn et al 2010 Phys. Rev. A 82 042701), where a method based on atomic-like Dirac–Sturm orbitals was employed. This work can provide a new tool for the investigation of quantum electrodynamics effects in heavy-ion collisions near the supercritical regime.
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Relativistic calculations of inner-shell atomic processes in low-energy ion–atom collisions - Kozhedub, Y. S.; Tupitsyn, I. I.; Shabaev, V. M.; Hagmann, S.; Plunien, G.; St{\"{o}}hlker, T. (Physica Scripta, 2013)
Relativistic calculations of inner-shell atomic processes in low-energy Ne–F^(8+)(1s) and Xe–Xe^(53+)(1s) collisions are performed. The method of calculation exploits the active-electron approximation and is based on the coupled-channel approach with atomic-like Dirac–Sturm–Fock orbitals, localized at the ions (atoms). The screening density-functional theory is applied for description of the interaction with passive electrons. The role of relativistic effects is analyzed.
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SPARC experiments at the high-energy storage ring - St{\"{o}}hlker, T.; Litvinov, Y. A.; Bagnoud, V.; Bechstedt, U.; Dimopoulou, C.; Dolinskii, A.; Geppert, C.; Hagmann, S.; Katayama, T.; K{\"{u}}hl, T.; Maier, R.; N{\"{o}}rtersh{\"{a}user, W. and Prasuhn, D. and Schuch, R. and Steck, M. and Stockhorst, H. (Physica Scripta, 2013)
The physics program of the SPARC collaboration at the Facility for Antiproton and Ion Research (FAIR) focuses on the study of collision phenomena in strong and even extreme electromagnetic fields and on the fundamental interactions between electrons and heavy nuclei up to bare uranium. Here we give a short overview on the challenging physics opportunities of the high-energy storage ring at FAIR for future experiments with heavy-ion beams at relativistic energies with particular emphasis on the basic beam properties to be expected.
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Target-thickness effects in electron–atom bremsstrahlung - M{\"{a}rtin, R. and Weber, G. and Barday, R. and Fritzsche, Y. and Enders, J. and Spillmann, U. and St{\"{o}}hlker, T. (Physica Scripta, 2013)
We report on a study of target-thickness effects on the degree of the linear polarization as well as on the emission probability of bremsstrahlung arising in the collision of 100 keV electrons with thin gold targets. For this purpose an experiment at the electron source SPIN at the TU Darmstadt as well as Monte Carlo simulations have been performed. The results indicate that for high- Z targets the degree of linear polarization is significantly altered by straggling of the electrons inside the target.
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XUV coherent diffraction imaging in reflection geometry with low numerical aperture - Z{\"{u}}rch, M.; Kern, C.; Spielmann, C. (Optics Express, 2013)
We present an experimental realization of coherent diffraction imaging in reflection geometry illuminating the sample with a laser driven high harmonic generation (HHG) based XUV source. After recording the diffraction pattern in reflection geometry, the data must be corrected before the image can be reconstructed with a hybrid-input-output (HIO) algorithm. In this paper we present a detailed investigation of sources of spoiling the reconstructed image due to the nonlinear momentum transfer, errors in estimating the angle of incidence on the sample, and distortions by placing the image off center in the computation grid. Finally we provide guidelines for the necessary parameters to realize a satisfactory reconstruction within a spatial resolution in the range of one micron for an imaging scheme with a numerical aperture NA < 0.03.
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Bessel beams of two-level atoms driven by a linearly polarized laser field - Hayrapetyan, A.; Matula, O.; Surzhykov, A.; Fritzsche, S. (The European Physical Journal D, 2013)
We study Bessel beams of two-level atoms that are driven by a linearly polarized laser field. Starting from the Schr{\"{o}}dinger equation, we determine the states of two-level atoms in a plane-wave field respecting propagation directions both of the atom and the field. For such laser-driven two-level atoms, we construct Bessel beams beyond the typical paraxial approximation. We show that the probability density of these atomic beams obtains a non-trivial, Bessel-squared-type behavior and can be tuned under the special choice of the atom and laser parameters, such as the nuclear charge, atom velocity, laser frequency, and propagation geometry of the atom and laser beams. Moreover, we spatially and temporally characterize the beam of hydrogen and selected (neutral) alkali-metal atoms that carry non-zero orbital angular momentum (OAM). The proposed spatiotemporal Bessel states (i) are able to describe, in principle, twisted states of any two-level system which is driven by the radiation field and (ii) have potential applications in atomic and nuclear processes as well as in quantum communication.
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Coherent Control at Its Most Fundamental: Carrier-Envelope-Phase-Dependent Electron Localization in Photodissociation of a H₂+ Molecular Ion Beam Target - Rathje, T.; Sayler, A. M.; Zeng, S.; Wustelt, P.; Figger, H.; Esry, B. D.; Paulus, G.G. (Physical Review Letters, 2013)
Measurements and calculations of the absolute carrier-envelope-phase (CEP) effects in the photodissociation of the simplest molecule, H₂+, with a 4.5-fs Ti:sapphire laser pulse at intensities up to (4 ± 2) × 10^14 W/cm^2 are presented. Localization of the electron with respect to the two nuclei (during the dissociation process) is controlled via the CEP of the ultrashort laser pulses. In contrast to previous CEP-dependent experiments with neutral molecules, the dissociation of the molecular ions is not preceded by a photoionization process, which strongly influences the CEP dependence. Kinematically complete data are obtained by time- and position-resolved coincidence detection. The phase dependence is determined by a single-shot phase measurement correlated to the detection of the dissociation fragments. The experimental results show quantitative agreement with ab initio 3D time-dependent Schr{\"{o}}dinger equation calculations that include nuclear vibration and rotation.
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Halo coupling and cleaning by a space charge resonance in high intensity beams - Hofmann, I. (Physical Review ST - Accelerators and Beams, 2013)
We show that the difference resonance driven by the space charge pseudo-octupole of high intensity beams not only couples the beam core emittances; it can also lead to emittance exchange in the beam halo, which is of relevance for beam loss in high intensity accelerators. With reference to linear accelerators the “main resonance” kz/kx,y=1 (corresponding to the Montague resonance 2Qx−2Qy=0 in circular accelerators) may lead to such a coupling and transfer of halo between planes. Coupling of transverse halo into the longitudinal plane{–}or vice versa{–}can occur even if the core (rms) emittances are exactly or nearly equal. This halo argument justifies additional caution in linac design including consideration of avoiding an equipartitioned design. At the same time, however, this mechanism may also qualify as an active dynamical halo cleaning scheme by coupling a halo from the longitudinal plane into the transverse plane, where local scraping is accessible. We present semianalytical emittance coupling rates and show that previously developed linac stability charts for the core can be extended{–}using the longitudinal to transverse halo emittance ratio{–}to indicate additional regions where halo coupling could be of importance.
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Nonsequential double ionization of N_2 in a near-single-cycle laser pulse - K{\"{u}}bel, M.; Kling, N.; Betsch, K. J.; Camus, N.; Kaldun, A.; Kleineberg, U.; Ben-Itzhak, I.; Jones, R. R.; Paulus, G.G.; Pfeifer, T.; Ullrich, J.; Moshammer, R.; et al. (Physical Review A, 2013)
We present a comparative study of nonsequential double ionization (NSDI) of N_2 and Ar exposed to near-single-cycle laser pulses. The NSDI process is investigated using carrier-envelope-phase-tagged electron-ion coincidence spectroscopy. The measured NSDI spectra of N_2 and Ar exhibit a striking resemblance. In particular, the correlated two-electron momentum distribution arising from NSDI of N_2 also displays a cross-shape very similar to that reported for Ar [Bergues et al., Nat. Commun. 3, 813 (2012)]. We interpret our results in terms of recollision-excitation with subcycle depletion and discuss how this mechanism accounts for the observed similarities and differences in the ionization behavior of the two species.
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Nuclear physics experiments with ion storage rings - Litvinov, Y.; Bishop, S.; Blaum, K.; Bosch, F.; Brandau, C.; Chen, L.; Dillmann, I.; Egelhof, P.; Geissel, H.; Grisenti, R.; Hagmann, S.; Heil, M.; et al. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2013)
In the last two decades a number of nuclear structure and astrophysics experiments were performed at heavy-ion storage rings employing unique experimental conditions offered by such machines. Furthermore, building on the experience gained at the two facilities presently in operation, several new storage ring projects were launched worldwide. This contribution is intended to provide a brief review of the fast growing field of nuclear structure and astrophysics research at storage rings.
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Observation of Gigawatt-Class THz Pulses from a Compact Laser-Driven Particle Accelerator - Gopal, A.; Herzer, S.; Schmidt, A.; Singh, P.; Reinhard, A.; Ziegler, W.; Br{\"{o}}mmel, D.; Karmakar, A.; Gibbon, P.; Dillner, U.; May, T.; Meyer, H.-G.; et al. (Physical Review Letters, 2013)
We report the observation of subpicosecond terahertz (T-ray) pulses with energies ≥ 460 μJ from a laser-driven ion accelerator, thus rendering the peak power of the source higher even than that of state-of-the-art synchrotrons. Experiments were performed with intense laser pulses (up to 5 × 10^19 W/cm^2) to irradiate thin metal foil targets. Ion spectra measured simultaneously showed a square law dependence of the T-ray yield on particle number. Two-dimensional particle-in-cell simulations show the presence of transient currents at the target rear surface which could be responsible for the strong T-ray emission.
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Passive mitigation strategies for mode instabilities in high-power fiber laser systems - Jauregui, C.; Otto, H.-J.; Stutzki, F.; Jansen, F.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2013)
Mode instabilities have quickly become the most limiting effect when it comes to scaling the output average power of fiber laser systems. In consequence, there is an urgent need for effective strategies to mitigate it and, thus, to increase the power threshold at which it appears. Passive mitigation strategies can be classified into intrinsic, which are related to the fiber design, and extrinsic, which require a modification of the setup. In order to evaluate the impact of mitigation strategies, a means to calculate its power threshold and predict its behavior is required. In this paper we present a simple semi-analytic formula that is able to predict the changes of the mode instability threshold by analyzing the strength of the thermally-induced waveguide perturbations. Furthermore, we propose two passive mitigation strategies, one intrinsic and one extrinsic, that should lead to a significant increase of the power threshold of mode instabilities.
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Photoelectron angular distributions for the two-photon sequential double ionization of xenon by ultrashort extreme ultraviolet free electron laser pulses - Mondal, S.; Ma, R.; Motomura, K.; Fukuzawa, H.; Yamada, A.; Nagaya, K.; Yase, S.; Mizoguchi, Y.; Yao, M.; Rouz{\'{e}}e, A.; Hundertmark, A.; Vrakking, M. J. J.; et al. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2013)
Xenon atoms are double-ionized by sequential two-photon absorption by ultrashort extreme ultraviolet free-electron laser pulses with a photon energy of 23.0 and 24.3 eV, produced by the SPring-8 Compact SASE Source test accelerator. The angular distributions of photoelectrons generated by two-photon double ionization are obtained using velocity map imaging. The results are reproduced reasonably well by the present theoretical calculations within the multi-configurational Dirac–Fock approach.
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Quantum Reflection as a New Signature of Quantum Vacuum Nonlinearity - Gies, H.; Karbstein, F.; Seegert, N. (New Journal of Physics, 2013)
We show that photons subject to a spatially inhomogeneous electromagnetic field can experience quantum reflection. Based on this observation, we propose quantum reflection as a novel means to probe the nonlinearity of the quantum vacuum in the presence of strong electromagnetic fields.
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Schottky mass measurements of heavy neutron-rich nuclides in the element range 70 ≤ Z ≤ 79 at the GSI Experimental Storage Ring - Shubina, D.; Cakirli, R. B.; Litvinov, Yu. A.; Blaum, K.; Brandau, C.; Bosch, F.; Carroll, J. J.; Casten, R. F.; Cullen, D. M.; Cullen, I. J.; Deo, A. Y.; Detwiler, B.; et al. (Physical Review C, 2013)
Storage-ring mass spectrometry was applied to neutron-rich 197Au projectile fragments. Masses of 181,183Lu, 185,186Hf, 187,188Ta, 191W, and 192,193Re nuclei were measured for the first time. The uncertainty of previously known masses of 189,190W and 195Os nuclei was improved. Observed irregularities on the smooth two-neutron separation energies for Hf and W isotopes are linked to the collectivity phenomena in the corresponding nuclei.
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Towards imaging of ultrafast molecular dynamics using FELs - Rouz{\'{e}}e, A.; Johnsson, P.; Rading, L.; Hundertmark, A.; Siu, W.; Huismans, Y.; D{\"{u}}sterer, S.; Redlin, H.; Tavella, F.; Stojanovic, N.; Al-Shemmary, A.; L{\'{e}}pine, F.; et al. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2013)
The dissociation dynamics induced by a 100 fs, 400 nm laser pulse in a rotationally cold Br2 sample was characterized by Coulomb explosion imaging (CEI) using a time-delayed extreme ultra-violet (XUV) FEL pulse, obtained from the Free electron LASer in Hamburg (FLASH). The momentum distribution of atomic fragments resulting from the 400 nm-induced dissociation was measured with a velocity map imaging spectrometer and used to monitor the internuclear distance as the molecule dissociated. By employing the simultaneously recorded in-house timing electro-optical sampling data, the time resolution of the final results could be improved to 300 fs, compared to the inherent 500 fs time-jitter of the FEL pulse. Before dissociation, the Br 2 molecules were transiently in space’ using laser-induced alignment. In addition, similar alignment techniques were used on CO2 molecules to allow the measurement of the photoelectron angular distribution (PAD) directly in the molecular frame (MF). Our results on MFPADs in aligned CO2 molecules, together with our investigation of the dissociation dynamics of the Br2 molecules with CEI, show that information about the evolving molecular structure and electronic geometry can be retrieved from such experiments, therefore paving the way towards the study of complex non-adiabatic dynamics in molecules through XUV time-resolved photoion and photoelectron spectroscopy.
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Vacuum-Assisted Generation and Control of Atomic Coherences at X-Ray Energies - Heeg, K.; Wille, H.-C.; Schlage, K.; Guryeva, T.; Schumacher, D.; Uschmann, I.; Schulze, K. S.; Marx, B.; K{\"{a}mpfer, T. and Paulus, G. and R{\"{o}}hlsberger, R. and Evers, J. (Physical Review Letters, 2013)
The control of light-matter interaction at the quantum level usually requires coherent laser fields. But already an exchange of virtual photons with the electromagnetic vacuum field alone can lead to quantum coherences, which subsequently suppress spontaneous emission. We demonstrate such spontaneously generated coherences (SGC) in a large ensemble of nuclei operating in the x-ray regime, resonantly coupled to a common cavity environment. The observed SGC originates from two fundamentally different mechanisms related to cooperative emission and magnetically controlled anisotropy of the cavity vacuum. This approach opens new perspectives for quantum control, quantum state engineering and simulation of quantum many-body physics in an essentially decoherence-free setting.
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Angular distribution of photons for the simultaneous excitation and ionization of He-like uranium ions in relativistic ion-atom collisions - Salem, S.; St{\"{o}}hlker, T.; Br{\"{a}uning-Demian, A. and Hagmann, S. and Kozhuharov, C. and Liesen, D. and Gumberidze, A. (Physical Review A, 2013)
Simultaneous ionization and excitation processes are studied for initially He-like uranium ions in collisions with xenon gaseous targets at relativistic energy, 220MeV/u. The virtue of investigating the process of simultaneous excitation and ionization is that one electron ends up in the continuum, while the other electron ends up in a hydrogen-like final state. Experimentally, this process can be identified by observing the radiative decay of the excited levels in coincidence with ions that lost one electron (U91+). In particular, owing to the large fine-structure splitting of H-like U, the angular distribution of photons for the simultaneous ionization and excitation into the different total angular momentum j=1/2 and j=3/2 states of the L shell is determined directly from the obser-ved yields of Lyα1 and Lyα2 radiation at various observation angles. The experimental data show a progress for the dependence of the alignment on the collision impact parameter. It is shown that the current results confirm the theoretical predictions based on the independent-particle approximation and first-order perturbation, for which the simultaneous ionization and excitation processes occur at small impact parameter.
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Asymmetries of azimuthal photon distributions in nonlinear Compton scattering in ultrashort intense laser pulses - Seipt, D.; K{\"{a}mpfer, B. (Physical Review A, 2013)
Nonlinear Compton scattering in ultrashort intense laser pulses is discussed with the focus on angular distributions of the emitted photon energy. This is an observable which is easily accessible experimentally. Asymmetries of the azimuthal distributions are predicted for both linear and circular polarization. We present a systematic survey of the influence of the laser intensity, the carrier envelope phase, and the laser polarization on the emission spectra for single-cycle and few-cycle laser pulses. For linear polarization, the dominant direction of the emission changes from a perpendicular pattern with respect to the laser polarization at low-intensity to a dominantly parallel emission for high-intensity laser pulses.
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Chlorine atom densities in the (3p^5)^2P_1/2^0 excited spin-orbit state measured by two-photon absorption laser-induced fluorescence in a chlorine inductively coupled plasma - Sirse, N.; Booth, J. P.; Chabert, P.; Surzhykov, A.; Indelicato, P. (Journal of Physics D: Applied Physics, 2013)
Chlorine atom densities in the (3p^5)^2P_1/2^0 spin–orbit excited state were measured by two-photon absorption laser-induced fluorescence (TALIF) in an inductively coupled plasma discharge in pure Cl2. The atoms were excited by two photons at 235.702 nm to the (4p)^4S_3/2^0 state and detected by fluorescence to the (4s) ^4P_5/2 state at 726 nm. The population of this state relative to that in the (3p^5)^2P_3/2^0 ground state, n_P_1/2/n_P_3/2 was determined from the relative TALIF signal intensity from the two states, combined with new calculations of the two-photon absorption cross-sections. n_P_1/2/n_P_3/2 was found to increase continuously with radio-frequency power (50–500 W), whereas with Cl2 pressure (5–90 mTorr) it passes through a maximum at 10 mTorr, reaching 30% at 500 W. This maximum corresponds to the maximum of electron density in the discharge. Combining this density ratio measurement with previous measurements of the absolute ground state chlorine atom density [1] allows the absolute spin-orbit excited state density to be estimated. A significant fraction of the total chlorine atom density is in this excited state which should be included in plasma chemistry models.
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Controlling mode instabilities by dynamic mode excitation with an acousto-optic deflector - Otto, H.-J.; Jauregui, C.; Stutzki, F.; Jansen, F.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2013)
We demonstrate an approach to actively stabilize the beam profile of a fiber amplifier above the mode instability threshold. Both the beam quality and the pointing stability are significantly increased at power levels of up to three times the mode instabilities threshold. The physical working principle is discussed at the light of the recently published theoretical explanations of mode instabilities.
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Magnesium aluminosilicate glasses as potential laser host material for ultrahigh power laser systems - Tiegel, M.; Herrmann, A.; Russel, C.; K{\"{o}}rner, J.; Kl{\"{o}}pfel, D.; Hein, J.; Kaluza, M.C. (Journal of Materials Chemistry C, 2013)
Magnesium aluminosilicate glasses doped with 0.2 mol% Sm2O3 (1 x 10^20 Sm3+ cm^-3) have been prepared in a very broad compositional range. The effect of the MgO, Al2O3 and SiO2 concentrations as well as the effect of partial substitution of MgO by CaO, SrO, BaO, ZnO or MgF2 have been studied. Increasing the network modifier concentration results in decreasing the glass transformation temperature and increasing the coefficient of thermal expansion due to the formation of non-bridging oxygen sites and decreasing glass network connectivity. Although the network connectivity is changed substantially by the addition of network modifier oxides, the maximum phonon energy and the fluorescence lifetime of Sm3+ are not affected. Equimolar replacement of up to 9 mol% MgO by MgF2 results in increasing Sm3+ fluorescence lifetimes without increasing the coefficient of thermal expansion or decreasing the glass forming ability. Glasses with fairly small thermal expansion coefficients (≤ 3.2 x 10^-6 K^-1), low thermal stress values (≤ 0.5 MPa K^-1), broad fluorescence emission peaks and fluorescence lifetimes in the range from 2.4 to 2.8 ms are obtained. Such glasses are interesting candidates for laser host materials in ultrahigh peak power laser systems.
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Temperature dependent emission and absorption cross section of Yb3+ doped yttrium lanthanum oxide (YLO) ceramic and its application in diode pumped amplifier - Banerjee, S.; Koerner, J.; Siebold, M.; Yang, Q.; Ertel, K.; Mason, P. D.; Phillips, P. J.; Loeser, M.; Zhang, H.; Lu, S.; Hein, J.; Schramm, U.; et al. (Optics Express, 2013)
Temperature dependent absorption and emission cross-sections of 5 at-% Yb3+ doped yttrium lanthanum oxide (Yb:YLO) ceramic between 80K and 300K are presented. In addition, we report on the first demonstration of ns pulse amplification in Yb:YLO ceramic. A pulse energy of 102mJ was extracted from a multi-pass amplifier setup. The amplification bandwidth at room temperature confirms the potential of Yb:YLO ceramic for broad bandwidth amplification at cryogenic temperatures.
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530 W, 1.3 mJ, four-channel coherently combined femtosecond fiber chirped-pulse amplification system - Klenke, A.; Breitkopf, S.; Kienel, M.; Gottschall, T.; Eidam, T.; H{\"{a}drich, S. and Rothhardt, J. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2013)
We report on a femtosecond fiber laser system comprising four coherently combined large-pitch fibers as the main amplifier. With this system, a pulse energy of 1.3 mJ and a peak power of 1.8 GW are achieved at 400 kHz repetition rate. The corresponding average output power is as high as 530 W. Additionally, an excellent beam quality and efficiency of the combination have been obtained. To the best of our knowledge, such a parameter combination, i.e., gigawatt pulses with half a kilowatt average power, has not been demonstrated so far with any other laser architecture.
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Development of a Superconducting Transverse-Gradient Undulator for Laser-Wakefield Accelerators - Rodriguez, V.; Bernhard, A.; Keilmann, A.; Peiffer, P.; Rossmanith, R.; Widmann, C.; Baumbach, T.; Nicolai, M.; Kaluza, M.C. (IEEE Transactions on Applied Superconductivity, 2013)
Relativistic electrons with small energy spread propagating through undulators produce monochromatic radiation with high spectral intensity. The working principle of undulators requires a small energy spread of the electron beam in the order of ΔE/E 0.1%. Laser-wakefield accelerators can produce electron bunches with an energy of several 100 MeV within a few millimeters acceleration length, but with a relatively large energy spread (ΔE/E 1 - 10%). In order to produce monochromatic undulator radiation with these electrons, a novel scheme involving transverse-gradient superconducting undulators was proposed in an earlier work. This paper reports on the design-optimization and construction of an iron-free cylindrical superconducting undulator tailored to the particular beam properties of the laser-wakefield electron accelerator at the University of Jena, Germany.
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High-Precision X-Ray Polarimetry - Marx, B.; Schulze, K. S.; Uschmann, I.; K{\"{a}mpfer, T. and L{\"{o}}tzsch, R. and Wehrhan, O. and Wagner, W. and Detlefs, C. and Roth, T. and H{\"{a}rtwig, J. and F{\"{o}}rster, E. and St{\"{o}}hlker, T. and Paulus, G.G. (Physical Review Letters, 2013)
The polarization purity of 6.457- and 12.914-keV x rays has been improved to the level of 2.4×10-10 and 5.7×10-10. The polarizers are channel-cut silicon crystals using six 90° reflections. Their performance and possible applications are demonstrated in the measurement of the optical activity of a sucrose solution.
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Polarization of M2 Line Emitted Following Electron-Impact Excitation of Beryllium-Like Ions - Ying-Long, S.; Chen-Zhong, D.; Xiao-Yun, M.; Zhong-Wen, W.; Lu-You, X.; Fritzsche, S. (Chinese Physics Letters, 2013)
Detailed calculations are carried out for the electron-impact excitation cross sections from the ground state to the individual magnetic sublevels of the 1s2s^(2) 2p_(3/2)J = 2 excited state of highly-charged beryllium-like ions by using a fully relativistic distorted-wave (RDW) method. The contributions of the Breit interaction to the linear polarization of the 1s2s^(2) 2p_(3/2)J = 2 → 1s^(2) 2s^(2)J = 0 magnetic quadrupole (M2) line are investigated systematically for the beryllium isoelectronic sequence with 42 ≤ Z ≤ 92. It is found that the Breit interaction depolarizes significantly the linear polarization of the M2 fluorescence radiation and that these depolarization effects increase as the incident electron energy and/or the atomic number is enlarged.
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Resonant coherent excitation of the lithiumlike uranium ion - A scheme for heavy-ion spectroscopy - Nakano, Y.; Takano, Y.; Ikeda, T.; Kanai, Y.; Suda, S.; Azuma, T.; Br{\"{a}uning, H. and Br{\"{a}uning-Demian, A. and Dauvergne, D. and St{\"{o}}hlker, T. and Yamazaki, Y. (Physical Review A, 2013)
We report our observation of the resonant fluorescence from highly charged uranium ions. Using the resonant coherent excitation (RCE) technique, the 2s-2p_(3/2) transition in 191.68 MeV/u Li-like U^(89+) ions was excited at 4.5 keV with a resonance width of 4.4 eV. The result demonstrated that the RCE can be applied to resonant fluorescence spectroscopy of high-Z ions up to uranium with high efficiency and resolution.
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Table-Top Laser-Based Source of Femtosecond, Collimated, Ultrarelativistic Positron Beams - Sarri, G.; Schumaker, W.; Di Piazza, A.; Vargas, M.; Dromey, B.; Dieckmann, M. E.; Chvykov, V.; Maksimchuk, A.; Yanovsky, V.; He, Z. H.; Hou, B. X.; Nees, J. A.; et al. (Physical Review Letters, 2013)
The generation of ultrarelativistic positron beams with short duration (τ_e+≃30 fs), small divergence (θ_e+≃3 mrad), and high density (n_e+≃10^14–10^15 cm−3) from a fully optical setup is reported. The detected positron beam propagates with a high-density electron beam and γ rays of similar spectral shape and peak energy, thus closely resembling the structure of an astrophysical leptonic jet. It is envisaged that this experimental evidence, besides the intrinsic relevance to laser-driven particle acceleration, may open the pathway for the small-scale study of astrophysical leptonic jets in the laboratory.
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The quest for axions and other new light particles - Baker, K.; Cantatore, G.; Cetin, S. A.; Davenport, M.; Desch, K.; D{\"{o}}brich, B.; Gies, H.; Irastorza, I. G.; J{\"{a}ckel, J. and Lindner, A. and Papaevangelou, T. and Pivovaroff, M. and Raffelt, G. and Redondo, J. and Ringwald, A. and Semertzidis, Y. and Siemko, A. and Sulc, M. and Upadhye, A. and Zioutas, K. (Annalen der Physik, 2013)
Standard Model extensions often predict low-mass and very weakly interacting particles, such as the axion. A number of small-scale experiments at the intensity/precision frontier are actively searching for these elusive particles, complementing searches for physics beyond the Standard Model at colliders. Whilst a next generation of experiments will give access to a huge unexplored parameter space, a discovery would have a tremendous impact on our understanding of fundamental physics.
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Towards isolated attosecond pulses at megahertz repetition rates - Krebs, M.; H{\"{a}drich, S. and Demmler, S. and Rothhardt, J. and Za{\"{i}}r, A. and Chipperfield, L. and Limpert, J. and T{\"{u}}nnermann, A. (Nature Photonics, 2013)
The strong-field process of high-harmonic generation is the foundation for generating isolated attosecond pulses, which are the fastest controllable events ever induced. This coherent extreme-ultraviolet radiation has become an indispensable tool for resolving ultrafast motion in atoms and molecules. Despite numerous spectacular developments in the new field of attoscience the low data-acquisition rates imposed by low-repetition-rate (maximum of 3 kHz) laser systems hamper the advancement of these sophisticated experiments. Consequently, the availability of high-repetition-rate sources will overcome a major obstacle in this young field. Here, we present the first megahertz-level source of extreme-ultraviolet continua with evidence of isolated attosecond pulses using a fibre laser-pumped optical parametric amplifier for high-harmonic generation at 0.6 MHz. This 200-fold increase in repetition rate will enable and promote a vast variety of new applications, such as attosecond-resolution coincidence and photoelectron spectroscopy, or even video-rate acquisition for spatially resolved pump–probe measurements.
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Two-photon energy distribution from the decay of the 2 ¹S₀ state in He-like uranium - Bana{ś}, D.; Gumberidze, A.; Trotsenko, S.; Volotka, A. V.; Surzhykov, A.; Beyer, H. F.; Bosch, F.; Br{\"{a}uning-Demian, A. and Fritzsche, S. and Hagmann, S. and Kozhuharov, C. and Kumar, A. and Ma, X. and Mann, R. and Mokler, P. H. and Sierpowski, D. and Spillmann, U. and Tashenov, S. and Stachura, Z. and Warczak, A. and St{\"{o}}hlker, T. (Physical Review A, 2013)
We have performed a measurement of the spectral shape from the two-photon decay of the 1s2s 1S0 state in He-like uranium. The two-photon emission followed the ionization of initially Li-like uranium ions in collisions with a N2 gas-jet target. The measured shape of the two-photon energy distribution shows good agreement with results of the relativistic calculations that take into account the electron-electron interaction rigorously up to the first order in quantum electrodynamic perturbation expansion. From the full width at half maximum of the measured two-photon energy distribution, we confirm the theoretically predicted modification of the shape due to the relativistic effects.
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X-ray radiographic expansion measurements of isochorically heated thin wire targets - Hochhaus, D. C.; Aurand, B.; Basko, M.; Ecker, B.; K{\"{u}}hl, T.; Ma, T.; Rosmej, F.; Zielbauer, B.; Neumayer, P. (Physics of Plasmas, 2013)
Solid density matter at temperatures ranging from 150 eV to < 5 eV has been created by irradiating thin wire targets with high-energy laser pulses at intensities ≈ 10^18 W/cm^2 . Energy deposition and transport of the laser-produced fast electrons are inferred from spatially resolved Kα-spectroscopy. Time resolved x-ray radiography is employed to image the target mass density up to solid density and proves isochoric heating. The subsequent hydrodynamic evolution of the target is observed for up to 3 ns and is compared to radiation-hydrodynamic simulations. At distances of several hundred micrometers from the laser interaction region, where temperatures of 5–20 eV and small temperature gradients are found, the hydrodynamic evolution of the wire is a near axially symmetric isentropic expansion, and good agreement between simulations and radiography data confirms heating of the wire over hundreds of micrometers.
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Electron- and Proton-Impact Excitation of Hydrogenlike Uranium in Relativistic Collisions - Gumberidze, A.; Thorn, D.; Fontes, C.; Najjari, B.; Zhang, H.; Surzhykov, A.; Voitkiv, A.; Fritzsche, S.; Bana{ś}, D.; Beyer, H.; Chen, W.; DuBois, R.; et al. (Physical Review Letters, 2013)
The K shell excitation of H-like uranium (U91+) in relativistic collisions with different gaseous targets has been studied at the experimental storage ring at GSI Darmstadt. By performing measurements with different targets as well as with different collision energies, we were able to observe for the first time the effect of electron-impact excitation (EIE) process in the heaviest hydrogenlike ion. The large fine-structure splitting in H-like uranium allowed us to unambiguously resolve excitation into different L shell levels. State-of-the-art calculations performed within the relativistic framework which include excitation mechanisms due to both protons (nucleus) and electrons are in good agreement with the experimental findings. Moreover, our experimental data clearly demonstrate the importance of including the generalized Breit interaction in the treatment of the EIE process.
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Hyperfine-induced effects on the linear polarization of Kα₁ emission from heliumlike ions - Surzhykov, A.; Litvinov, Y.; St{\"{o}}hlker, T.; Fritzsche, S. (Physical Review A, 2013)
The linear polarization of the characteristic photon emission from few-electron ions is studied for its sensitivity with regard to the nuclear spin and magnetic moment of the ions. Special attention is paid, in particular, to the Kα1 (1s2p3/2 1,3P1,2→1s2 1S0) decay of selected heliumlike ions following the radiative electron capture into initially hydrogenlike species. Based on the density matrix theory, a unified description is developed that includes both the many-electron and hyperfine interactions as well as the multipole-mixing effects arising from the expansion of the radiation field. It is shown that the polarization of the Kα1 line can be significantly affected by the mutipole mixing between the leading M2 and hyperfine-induced E1 components of 1s2p 3P2,Fi→1s2 1S0,Ff transitions. This E1-M2 mixing strongly depends on the nuclear properties of the considered isotopes and can be addressed experimentally at existing heavy-ion storage rings.
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Ion Beam Driven High Energy Density Physics Studies at FAIR at Darmstadt: The HEDgeHOB Collaboration - Tahir, N. A.; Shutov, A.; Zharkov, A. P.; Spiller, P.; Piriz, A. R.; Rodriguez Prietoc, G.; Deutsch, C.; St{\"{o}}hlker, T. (Contributions to Plasma Physics, 2013)
High Energy Density (HED) physics spans over numerous areas of basic and applied physics, for example, astrophysics, planetary physics, geophysics, inertial fusion and many others. Due to this reason, it has been a subject of very active research over the past many decades. Static as well as dynamic methods have been applied to generate samples of HED matter in the laboratory. The most commonly used tool in the static techniques is the diamond anvil cell while the dynamic methods involve shock compression of matter. During the past fifteen years, great progress has been made on the development of bunched intense particle beams that have emerged as an additional new tool for studying HED physics. In this paper we present two experiment designs that have been worked out for HED physics studies at the Facility for Antiprotons and Ion Research (FAIR) at Darmstadt. This facility has entered into construction phase and will provide one of the largest and most powerful particle accelerators in the world. (© 2013 WILEY-VCH Verlag GmbH { &} Co. KGaA, Weinheim)
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Optical measurement of the longitudinal ion distribution of bunched ion beams in the ESR - Wen, W. Q.; Lochmann, M.; Ma, X.; Bussmann, M.; Winters, D. F. A.; N{\"{o}}rtersh{\"{a}user, W. and Botermann, B. and Geppert, C. and Fr{\"{o}}mmgen, N. and Hammen, M. and Hannen, V. and J{\"{o}}hren, R. and K{\"{u}}hl, T. and Litvinov, Yu. A. and Sanchez, R. and St{\"{o}}hlker, T. and Vollbrecht, J. and Weinheimer, C. and Dimopoulou, C. and Nolden, F. and Steck, M. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2013)
An optical technique to study the longitudinal distribution of ions in a bunched ion beam circulating in a storage ring is presented. It is based on the arrival-time analysis of photons emitted after collisional excitation of residual gas molecules. The beam-induced fluorescence was investigated in the ultraviolet regime with a channeltron and in the visible region using a photomultiplier tube. Both were applied to investigate the longitudinal shape of bunched and electron-cooled (209)^Bi^(80+) ion beams at about 400 MeV/u in the experimental storage ring (ESR) at GSI Helmholtzzentrum f{\"{u}}r Schwerionenforschung in Darmstadt, Germany. Bunch lengths were determined with an uncertainty of about 0.5 m using the UV-sensitive channeltron and with slightly lower accuracy from the photomultiplier data due to the slower transitions in the red region of the spectrum. The Gaussian shape of the longitudinal distribution of ions inside the bunch was confirmed. With the information of the transverse beam size that can be measured simultaneously by a newly installed ionization profile monitor (IPM) at the ESR, an accurate determination of the ion density in the bunched beam will be allowed.
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Beaming of High-Order Harmonics Generated from Laser-Plasma Interactions - Yeung, M.; Dromey, B.; Adams, D.; Cousens, S.; H{\"{o}}rlein, R.; Nomura, Y.; Tsakiris, G. D.; Zepf, M. (Physical Review Letters, 2013)
Beam divergences of high-order extreme ultraviolet harmonics from intense laser interactions with steep plasma density gradients are studied through experiment and Fourier analysis of the harmonic spatial phase. We show that while emission due to the relativistically oscillating mirror mechanism can be explained by ponderomotive surface denting, in agreement with previous results, the divergence of the emission due to the coherent wake emission mechanism requires a combination of the dent phase and an intrinsic emission phase. The temporal dependence of the divergences for both mechanisms is highlighted while it is also shown that the coherent wake emission divergence can be small in circumstances where the phase terms compensate each other.
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Direct Observation of Density-Gradient Effects in Harmonic Generation from Plasma Mirrors - Kahaly, S.; Monchoc{\'{e}}, S.; Vincenti, H.; Dzelzainis, T.; Dromey, B.; Zepf, M.; Martin, Ph.; Qu{\'{e}}r{\'{e}}, F. (Physical Review Letters, 2013)
High-order harmonics and attosecond pulses of light can be generated when ultraintense, ultrashort laser pulses reflect off a solid-density plasma with a sharp vacuum interface, i.e., a plasma mirror. We demonstrate experimentally the key influence of the steepness of the plasma-vacuum interface on the interaction, by measuring the spectral and spatial properties of harmonics generated on a plasma mirror whose initial density gradient scale length L is continuously varied. Time-resolved interferometry is used to separately measure this scale length.
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Improved Modal Reconstruction for Spatially and Spectrally Resolved Imaging - Otto, H.; Jansen, F.; Stutzki, F.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Journal of Lightwave Technology, 2013)
Spatially and spectrally resolved imaging (S2) is a very sensitive, robust and elegant method to measure the power of excited modes in a fiber. For the common reconstruction technique an approximation is necessary, which is based on dominant fundamental mode content. In this work we present several algorithms that significantly improve the accuracy of modal reconstruction for weak excited fundamental mode content. We show that in some cases general analytical solutions exist that can completely overcome the former limitation. In addition, we introduce an iterative procedure to improve the mode modal reconstruction independent of the specific used algorithm.
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Operation of a picosecond narrow-bandwidth Laser–Thomson-backscattering X-ray source - Jochmann, A.; Irman, A.; Lehnert, U.; Couperus, J.; Kuntzsch, M.; Trotsenko, S.; Wagner, A.; Debus, A.; Schlenvoigt, H.-P.; Helbig, U.; Bock, S.; Ledingham, K.; et al. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2013)
A tunable source of intense ultra-short hard X-ray pulses represents a novel tool for the structural analysis of complex systems with unprecedented temporal and spatial resolution. With the simultaneous availability of a high power short-pulse laser system this provides unique opportunities at the forefront of relativistic light–matter interactions. At Helmholtz-Zentrum Dresden-Rossendorf (HZDR) we demonstrated the principle of such a light source (PHOENIX – Photon Electron collider for Narrow bandwidth Intense X-Rays) by colliding picosecond electron bunches from the ELBE linear accelerator with counter-propagating femtosecond laser pulses from the 150 TW Draco Ti:Sapphire laser system. The generated narrowband X-rays are highly collimated and can be reliably adjusted from 12 keV to 20 keV by tuning the electron energy (24–30 MeV). Ensuring the spatial–temporal overlap at the interaction point and suppressing the Bremsstrahlung background a signal to noise ratio of greater than 300 was reached.
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Performance of solenoids versus quadrupoles in focusing and energy selection of laser accelerated protons - Hofmann, I. (Physical Review Special Topics - Accelerators and Beams, 2013)
Using laser accelerated protons or ions for various applications{–}for example in particle therapy or short-pulse radiographic diagnostics{–}requires an effective method of focusing and energy selection. We derive an analytical scaling for the performance of a solenoid compared with a doublet/triplet as function of the energy, which is confirmed by TRACEWIN simulations. Generally speaking, the two approaches are equivalent in focusing capability, if parameters are such that the solenoid length approximately equals its diameter. The scaling also shows that this is usually not the case above a few MeV; consequently, a solenoid needs to be pulsed or superconducting, whereas the quadrupoles can remain conventional. It is also important that the transmission of the triplet is found only 25% lower than that of the equivalent solenoid. Both systems are equally suitable for energy selection based on their chromatic effect as is shown using an initial distribution following the RPA simulation model by Yan et al. [ Phys. Rev. Lett. 103 135001 (2009)].
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Plasma expansion into vacuum assuming a steplike electron energy distribution - Kiefer, T.; Schlegel, T.; Kaluza, M.C. (Physical Review E, 2013)
The expansion of a semi-infinite plasma slab into vacuum is analyzed with a hydrodynamic model implying a steplike electron energy distribution function. Analytic expressions for the maximum ion energy and the related ion distribution function are derived and compared with one-dimensional numerical simulations. The choice of the specific non-Maxwellian initial electron energy distribution automatically ensures the conservation of the total energy of the system. The estimated ion energies may differ by an order of magnitude from the values obtained with an adiabatic expansion model supposing a Maxwellian electron distribution. Furthermore, good agreement with data from experiments using laser pulses of ultrashort durations τ_L ≤ 80 fs is found, while this is not the case when a hot Maxwellian electron distribution is assumed.
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Relativistic electron mirrors from nanoscale foils for coherent frequency upshift to the extreme ultraviolet - Kiefer, D.; Yeung, M.; Dzelzainis, T.; Foster, P. S.; Rykovanov, S. G.; Lewis, C. L. S.; Marjoribanks, R. S.; Ruhl, H.; Habs, D.; Schreiber, J.; Zepf, M.; Dromey, B. (Nature Communications, 2013)
Reflecting light from a mirror moving close to the speed of light has been envisioned as a route towards producing bright X-ray pulses since Einstein’s seminal work on special relativity. For an ideal relativistic mirror, the peak power of the reflected radiation can substantially exceed that of the incident radiation due to the increase in photon energy and accompanying temporal compression. Here we demonstrate for the first time that dense relativistic electron mirrors can be created from the interaction of a high-intensity laser pulse with a freestanding, nanometre-scale thin foil. The mirror structures are shown to shift the frequency of a counter-propagating laser pulse coherently from the infrared to the extreme ultraviolet with an efficiency > 10^4 times higher than in the case of incoherent scattering. Our results elucidate the reflection process of laser-generated electron mirrors and give clear guidance for future developments of a relativistic mirror structure.
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Single-photon multiple ionization forming double vacancies in the 2p subshell of argon - Linusson, P.; Fritzsche, S.; Eland, J. H. D.; Mucke, M.; Feifel, R. (Physical Review A, 2013)
Single-photon ionization leading to two vacancies in the 2p subshell of argon is investigated experimentally using the photoelectron time-of-flight magnetic bottle coincidence technique. Three peaks corresponding to the 3P, 1D, and 1S states of the dication are found in the ionization energy range 535 to 562 eV. Multiconfigurational Dirac-Fock calculations were performed to estimate the single-photon double-ionization cross sections. Reasonable agreement between the measured and simulated spectra is found if single and double excitations are taken into account in the wave-function expansion.
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Single-shot pulse duration monitor for extreme ultraviolet and X-ray free-electron lasers - Riedel, R.; Al-Shemmary, A.; Gensch, M.; Golz, T.; Harmand, M.; Medvedev, N.; Prandolini, M. J.; Sokolowski-Tinten, K.; Toleikis, S.; Wegner, U.; Ziaja, B.; Stojanovic, N.; et al. (Nature Communications, 2013)
The resolution of ultrafast studies performed at extreme ultraviolet and X-ray free-electron lasers is still limited by shot-to-shot variations of the temporal pulse characteristics. Here we show a versatile single-shot temporal diagnostic tool that allows the determination of the extreme ultraviolet pulse duration and the relative arrival time with respect to an external pump-probe laser pulse. This method is based on time-resolved optical probing of the transient reflectivity change due to linear absorption of the extreme ultraviolet pulse within a solid material. In this work, we present measurements performed at the FLASH free-electron laser. We determine the pulse duration at two distinct wavelengths, yielding (184 ± 14) fs at 41.5 nm and (21 ± 19) fs at 5.5 nm. Furthermore, we demonstrate the feasibility to operate the tool as an online diagnostic by using a 20-nm-thin Si_(3)N_(4) membrane as target. Our results are supported by detailed numerical and analytical investigations.
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Symmetries of the three-heavy-quark system and the color-singlet static energy at next-to-next-to-leading logarithmic order - Brambilla, N.; Karbstein, F.; Vairo, A. (Physical Review D, 2013)
We study the symmetries of the three-heavy-quark system under exchange of the quark fields within the effective field theory framework of potential nonrelativistic QCD. The symmetries constrain the form of the matching coefficients in the effective theory. We then focus on the color-singlet sector and determine the so far unknown leading ultrasoft contribution to the static potential, which is of order α_(s)^(4)ln⁡μ, and consequently to the static energy, which is of order α_(s)^(4)lnα_(s). Finally, in the case of an equilateral geometry, we solve the renormalization group equations and resum the leading ultrasoft logarithms for the static potential of three quarks in a color singlet, octet and decuplet representation.
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Exotic Dense-Matter States Pumped by a Relativistic Laser Plasma in the Radiation-Dominated Regime - Colgan, J.; Abdallah, J.; Faenov, A. Ya.; Pikuz, S. A.; Wagenaars, E.; Booth, N.; Culfa, O.; Dance, R. J.; Evans, R. G.; Gray, R. J.; K{\"{a}mpfer, T. and Lancaster, K. L. and McKenna, P. and Rossall, A. L. and Skobelev, I. Yu. and Schulze, K. S. and Uschmann, I. and Zhidkov, A. G. and Woolsey, N. C. (Physical Review Letters, 2013)
In high-spectral resolution experiments with the petawatt Vulcan laser, strong x-ray radiation of KK hollow atoms (atoms without n=1 electrons) from thin Al foils was observed at pulse intensities of 3 × 10^(20) W/cm^2. The observations of spectra from these exotic states of matter are supported by detailed kinetics calculations, and are consistent with a picture in which an intense polychromatic x-ray field, formed from Thomson scattering and bremsstrahlung in the electrostatic fields at the target surface, drives the KK hollow atom production. We estimate that this x-ray field has an intensity of > 5 × 10^(18) W/cm^2 and is in the 3 keV range.
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High-intensity, high-contrast laser pulses generated from the fully diode-pumped Yb:glass laser system POLARIS - Hornung, M.; Keppler, S.; B{\"{o}}defeld, R.; Kessler, A.; Liebetrau, H.; K{\"{o}}rner, J.; Hellwing, M.; Schorcht, F.; J{\"{a}ckel, O. and S{\"{a}vert, A. and Polz, J. and Arunachalam, A. K. and Hein, J. and Kaluza, M.C. (Optics Letters, 2013)
We report on the first generation of high-contrast, 164 fs duration pulses from the laser system POLARIS reaching focused peak intensities in excess of 2×10^20 W/cm2. To our knowledge, this is the highest peak intensity reported so far that has been achieved with a diode-pumped, solid-state laser. Several passive contrast enhancement techniques have been specially developed and implemented, achieving a relative prepulse intensity smaller than 10^−8 at t=−30 ps before the main pulse. Furthermore a closed-loop adaptive-optics system has been installed. Together with angular chirp compensation, this method has led to a significant reduction of the focal spot size and an increase of the peak intensity.
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Parity-nonconservation effects on the radiative recombination of heavy hydrogenlike ions - Gunst, J.; Surzhykov, A.; Artemyev, A.; Fritzsche, S.; Tashenov, S.; Maiorova, A.; Shabaev, V. M.; St{\"{o}}hlker, T. (Physical Review A, 2013)
Based on the theoretical analysis of the radiative recombination of heavy hydrogen-like ions with unpolarized electrons, a scheme is proposed for observing atomic parity nonconservation (PNC). The scheme employs the sensitivity of the polarization properties of recombination photons on the PNC-induced mixing of opposite-parity ionic levels. For the electron capture into the 1s2p(3)^P_(0) state of helium-like ions, in particular, the PNC leads to a rotation of the photon linear polarization on the angle, directly proportional to the 1s2p (3)^P_(0)–1s2s (1)^S_(0) mixing parameter. Owing to the recent advances in the development of x-ray polarimeters, the observation of such a rotation angle and, hence, the corresponding parity mixing is likely to become feasible in the future.
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Propagation of sound waves through a spatially homogeneous but smoothly time-dependent medium - Hayrapetyan, A.; Grigoryan, K.; Petrosyan, R.; Fritzsche, S. (Annals of Physics, 2013)
The propagation of sound through a spatially homogeneous but non-stationary medium is investigated within the framework of fluid dynamics. For a non-vortical fluid, especially, a generalized wave equation is derived for the (scalar) potential of the fluid velocity distribution in dependence of the equilibrium mass density of the fluid and the sound wave velocity. A solution of this equation for a finite transition period ττ is determined in terms of the hypergeometric function for a phenomenologically realistic, sigmoidal change of the mass density and sound wave velocity. Using this solution, it is shown that the energy flux of the sound wave is not conserved but increases always for the propagation through a non-stationary medium, independent of whether the equilibrium mass density is increased or decreased. It is found, moreover, that this amplification of the transmitted wave arises from an energy exchange with the medium and that its flux is equal to the (total) flux of the incident and the reflected wave. An interpretation of the reflected wave as a propagation of sound backward in time is given in close analogy to Feynman and Stueckelberg for the propagation of anti-particles. The reflection and transmission coefficients of sound propagating through a non-stationary medium is analyzed in more detail for hypersonic waves with transition periods ττ between 15 and 200 ps as well as the transformation of infrasound waves in non-stationary oceans.
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Radiation pressure-assisted acceleration of ions using multi-component foils in high-intensity laser-matter interactions - Aurand, B.; Kuschel, S.; J{\"{a}ckel, O. and R{\"{o}}del, C. and Zhao, H. Y. and Herzer, S. and Paz, A. E. and Bierbach, J. and Polz, J. and Elkin, B. and Paulus, G.G. and Karmakar, A. and Gibbon, P. and K{\"{u}}hl, T. and Kaluza, M.C. (New Journal of Physics, 2013)
Experimental results on the acceleration of protons and carbon ions from ultra-thin polymer foils at intensities of up to 6 × 10^(19) W cm^(−2) are presented revealing quasi-monoenergetic spectral characteristics for different ion species at the same time. For carbon ions and protons, a linear correlation between the cutoff energy and the peak energy is observed when the laser intensity is increased. Particle-in-cell simulations supporting the experimental results imply an ion acceleration mechanism driven by the radiation pressure as predicted for multi-component foils at these intensities.
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Thermal effects in high average power optical parametric amplifiers - Rothhardt, J.; Demmler, S.; H{\"{a}drich, S. and Peschel, T. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2013)
Optical parametric amplifiers (OPAs) have the reputation of being average power scalable due to the instantaneous nature of the parametric process (zero quantum defect). This Letter reveals serious challenges originating from thermal load in the nonlinear crystal caused by absorption. We investigate these thermal effects in high average power OPAs based on beta barium borate. Absorption of both pump and idler waves is identified to contribute significantly to heating of the nonlinear crystal. A temperature increase of up to 148 K with respect to the environment is observed and mechanical tensile stress up to 40 MPa is found, indicating a high risk of crystal fracture under such conditions. By restricting the idler to a wavelength range far from absorption bands and removing the crystal coating we reduce the peak temperature and the resulting temperature gradient significantly. Guidelines for further power scaling of OPAs and other nonlinear devices are given.
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Tunnelling of the 3rd kind: A test of the effective non-locality of quantum field theory - Gardiner, S.; Gies, H.; J{\"{a}ckel, J. and Wallace, C. (Europhysics Letters, 2013)
Integrating out virtual quantum fluctuations in an originally local quantum field theory results in an effective theory which is non-local. In this letter we argue that tunnelling of the 3rd kind - where particles traverse a barrier by splitting into a pair of virtual particles which recombine only after a finite distance - provides a direct test of this non-locality. We sketch a quantum-optical setup to test this effect, and investigate observable effects in a simple toy model.
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Bremsstrahlung polarization correlations and their application for polarimetry of electron beams - Tashenov, S.; B{\"{a}ck, T. and Barday, R. and Cederwall, B. and Enders, J. and Khaplanov, A. and Fritzsche, Yu. and Sch{\"{a}ssburger, K.-U. and Surzhykov, A. and Yerokhin, V. A. and Jakubassa-Amundsen, D. (Physical Review A, 2013)
Linear polarization of hard x rays emitted in the process of atomic-field electron bremsstrahlung has been measured with a polarized electron beam. The correlation between the initial orientation of the electron spin and the angle of photon polarization has been systematically studied by means of Compton and Rayleigh polarimetry techniques applied to a segmented germanium detector. The results are in good agreement with those of fully relativistic calculations. The observed correlations are also explained classically and in a unique way manifest that due to the spin-orbit interaction the electron scattering trajectory is not confined to a single scattering plane. The developed photon polarimetry technique with a passive scatterer is very efficient and accurate and thus allows for additional applications. Bremsstrahlung polarization correlations lead to an alternative method of polarimetry of electron beams. Such a method is sensitive to all three components of the electron spin. It can be applied in a broad range of the electron beam energies from ≈100 keV up to a few tens of MeV. The results of a measurement at 100 keV are shown. The optimum scheme for electron polarimetry is analyzed and the relevant theoretical predictions are presented.
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High-power thermally guiding index-antiguiding-core fibers - Jansen, F.; Stutzki, F.; Otto, H.-J.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2013)
We investigate high-power operation of a very-large-mode-area (VLMA) fiber concept based on an index-antiguiding, thermally guiding core in which an ytterbium-doped region is completely surrounded by silica with a slightly higher refractive index. Experimentally, regimes of antiguidance, single-mode operation, and mode instabilities predominantly with radially symmetric higher-order modes are observed. Fundamental limitations for conventional VLMA step-index fibers are discussed.
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Low temperature permeability and current noise of ferromagnetic pickup coils - Geithner, R.; Heinert, D.; Neubert, R.; Vodel, W.; Seidel, P. (Cryogenics, 2013)
For a non-destructive measurement of intensities of charged particle beams a Cryogenic Current Comparator is used which captures the azimuthal magnetic field of the beam by a superconducting pickup coil at 4.2 K and transforms it into a current which is detected by a SQUID based current sensor. The current noise of the pickup coil and the bandwidth of this transformer depend on the frequency response curve of the complex permeability of the ferromagnetic core material embedded in the pickup coil. A measurement of the series inductance LS and series resistance RS of such a coil allows an indirect evaluation of the current noise contribution of the core using the Fluctuation–Dissipation-Theorem. These measurements were done with a commercial LCR-Meter in a frequency range from 20 Hz to 2 MHz. The current noise density was also directly measured using a SQUID-sensor. A comparison with between the direct and indirect measurement showed a good coincidence. Due to the critical temperature of the LTS-SQUID, noise measurements above 4.2 K are not possible apart from using an anti-cryostat. The measurement of the series inductance LS and series resistance RS with an LCR-Meter works in the whole temperature range and provides a comfortable access to the magnetic properties of core materials. Compared to direct measurements, the indirect measurement thus allows a technologically simpler and broader determination of the core noise.
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Near-monochromatic high-harmonic radiation from relativistic laser-plasma interactions with blazed grating surfaces - Yeung, M.; Dromey, B.; R{\"{o}}del, C.; Bierbach, J.; W{\"{u}}nsche, M.; Paulus, G.; Hahn, T.; Hemmers, D.; Stelzmann, C.; Pretzler, G.; Zepf, M. (New Journal of Physics, 2013)
Intense, femtosecond laser interactions with blazed grating targets are studied through experiment and particle-in-cell (PIC) simulations. The high harmonic spectrum produced by the laser is angularly dispersed by the grating leading to near-monochromatic spectra emitted at different angles, each dominated by a single harmonic and its integer-multiples. The spectrum emitted in the direction of the third-harmonic diffraction order is measured to contain distinct peaks at the 9th and 12th harmonics which agree well with two-dimensional PIC simulations using the same grating geometry. This confirms that surface smoothing effects do not dominate the far-field distributions for surface features with sizes on the order of the grating grooves whilst also showing this to be a viable method of producing near-monochromatic, short-pulsed extreme-ultraviolet radiation.
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Sensitivity calibration of an imaging extreme ultraviolet spectrometer-detector system for determining the efficiency of broadband extreme ultraviolet sources - Fuchs, S.; R{\"{o}}del, C.; Krebs, M.; H{\"{a}drich, S. and Bierbach, J. and Paz, A. E. and Kuschel, S. and W{\"{u}}nsche, M. and Hilbert, V. and Zastrau, U. and F{\"{o}}rster, E. and Limpert, J. and Paulus, G.G. (Review of Scientific Instruments, 2013)
We report on the absolute sensitivity calibration of an extreme ultraviolet (XUV) spectrometer system that is frequently employed to study emission from short-pulse laser experiments. The XUV spectrometer, consisting of a toroidal mirror and a transmission grating, was characterized at a synchrotron source in respect of the ratio of the detected to the incident photon flux at photon energies ranging from 15.5 eV to 99 eV. The absolute calibration allows the determination of the XUV photon number emitted by laser-based XUV sources, e.g., high-harmonic generation from plasma surfaces or in gaseous media. We have demonstrated high-harmonic generation in gases and plasma surfaces providing 2.3 μW and μJ per harmonic using the respective generation mechanisms.
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2.4 mJ, 33 W Q-switched Tm-doped fiber laser with near diffraction-limited beam quality - Stutzki, F.; Jansen, F.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2013)
We report on a high pulse energy and high average power Q-switched Tm-doped fiber oscillator. The oscillator produces 2.4 mJ pulses with 33 W average power (at a repetition rate of 13.9 kHz) and nearly diffraction-limited beam quality. This record performance is enabled by a Tm-doped large-pitch fiber, which allows for large core diameters in combination with effective single-mode operation.
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Magnetically amplified light-shining-through-walls via virtual minicharged particles - D{\"{o}}brich, B.; Gies, H.; Neitz, N.; Karbstein, F. (Physical Review D, 2013)
We show that magnetic fields have the potential to significantly enhance a recently proposed light-shining-through-walls scenario in quantum-field theories with photons coupling to minicharged particles. Suggesting a dedicated laboratory experiment, we demonstrate that this particular tunneling scenario could provide access to a parameter regime competitive with the currently best direct laboratory limits on minicharged fermions below the meV regime. With present day technology, such an experiment has the potential to even overcome the best model-independent cosmological bounds on minicharged fermions with masses below O(10^(-4)) eV.
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58 mJ burst comprising ultrashort pulses with homogenous energy level from an Yb-doped fiber amplifier - Breitkopf, S.; Klenke, A.; Gottschall, T.; Otto, H.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2012)
We report on a laser system producing a burst comprising femtosecond pulses with a total energy of 58 mJ. Every single pulse within this burst has an energy between 27 and 31 μJ. The pump is able to rebuild the inversion fast enough between the pulses, resulting in an almost constant gain for every pulse during the burst. This causes a very homogenous energy distribution during the burst. The output burst has a repetition frequency of 20 Hz, is 200 μs long and, therefore, contains 2000 pulses at a pulse repetition rate of 10 MHz.
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Attosecond-Recollision-Controlled Selective Fragmentation of Polyatomic Molecules - Xie, X.; Doblhoff-Dier, K.; Roither, S.; Sch{\"{o}}ffler, M.; Kartashov, D.; Xu, H.; Rathje, T.; Paulus, G.; Baltuska, A.; Gr{\"{a}fe, S. and Kitzler, M. (Physical Review Letters, 2012)
Control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of intense few-cycle laser pulses is demonstrated experimentally. We show both experimentally and theoretically that the responsible mechanism is inelastic ionization from inner-valence molecular orbitals by recolliding electron wave packets, whose recollision energy in few-cycle ionizing laser pulses strongly depends on the optical waveform. Our work demonstrates an efficient and selective way of predetermining fragmentation and isomerization reactions in polyatomic molecules on subfemtosecond time scales.
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Controlled directional ion emission from several fragmentation channels of CO driven by a few-cycle laser field - Betsch, K. J.; Johnson, N.; Bergues, B.; K{\"{u}}bel, M.; Herrwerth, O.; Senftleben, A.; Ben-Itzhak, I.; Paulus, G.G.; Moshammer, R.; Ullrich, J.; Kling, M. F.; Jones, R. R. (Physical Review A, 2012)
We explore the dissociative ionization of CO with carrier-envelope-phase (CEP) tagged few-cycle laser pulses. We observe the CEP dependence of the directional emission of C^(p+) and O^(q+) fragments from transient CO^([p+q]+) ions, where p + q ≤ 3 and q ≤ 1. At I_0 = 3.5 × 10^(14) W/cm^2, a 180°. phase difference between the C^(+) and O^(+) fragments from the (p=1, q=0) and (p=0, q=1) channels reflects the orientation dependence of the CO ionization. At I_0 = 1.2 × 10^(15) W/cm^2, we find a 35° phase shift between the C^(2+) fragments from the (p = 2, q = 0) and (p = 2, q = 1) channels, in contrast to the 180∘ shift previously observed between the C2+ fragment channels at I_0 = 6 × 10^(14) W/cm^2 [ Phys. Rev. Lett. 106 073004 (2011)].
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Evidence for ultra-fast heating in intense-laser irradiated reduced-mass targets - Neumayer, P.; Aurand, B.; Fraga, R.; Ecker, B.; Grisenti, R. E.; Gumberidze, A.; Hochhaus, D. C.; Kalinin, A.; Kaluza, M.C.; K{\"{u}}hl, T.; Polz, J.; Reuschl, R.; et al. (Physics of Plasmas, 2012)
We report on an experiment irradiating individual argon droplets of 20 μm diameter with laser pulses of several Joule energy at intensities of 10^19 W/cm^2. K-shell emission spectroscopy was employed to determine the hot electron energy fraction and the time-integrated charge-state distribution. Spectral fitting indicates that bulk temperatures up to 160 eV are reached. Modelling of the hot-electron relaxation and generation of K-shell emission with collisional hot-electron stopping only is incompatible with the experimental results, and the data suggest an additional ultra-fast (sub-ps) heating contribution. For example, including resistive heating in the modelling yields a much better agreement with the observed final bulk temperature and qualitatively reproduces the observed charge state distribution.
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Pair production in a plane wave by thermal background photons - King, B.; Gies, H.; Di Piazza, A. (Physical Review A, 2012)
Ever since Schwinger published his influential paper [J. Schwinger Phys. Rev. 82 664 (1951)], the maxim that there can be no pair creation in a plane wave has been often cited. We advance an analysis that indicates that in any real situation, where thermal effects are present, in a single plane-wave field, even in the limit of zero frequency (a constant crossed field), thermal photons can seed pair creation. Interestingly, the pair-production rate is found to depend nonperturbatively on both the amplitude of the constant crossed field and on the temperature.
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Renormalization flow of axion electrodynamics - Eichhorn, A.; Gies, H.; Roscher, D. (Physical Review D, 2012)
We study the renormalization flow of axion electrodynamics, concentrating on the nonperturbative running of the axion-photon coupling and the mass of the axion(-like) particle. Due to a nonrenormalization property of the axion-photon vertex, the renormalization flow is controlled by photon and axion anomalous dimensions. As a consequence, momentum-independent axion self-interactions are not induced by photon fluctuations. The nonperturbative flow towards the ultraviolet exhibits a Landau-pole-type behavior, implying that the system has a scale of maximum UV extension and that the renormalized axion-photon coupling in the deep infrared is bounded from above. Even though gauge invariance guarantees that photon fluctuations do not decouple in the infrared, the renormalized couplings remain finite even in the deep infrared and even for massless axions. Within our truncation, we also observe the existence of an exceptional renormalization group trajectory, which is extendable to arbitrarily high scales, without being governed by a UV fixed point.
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Coherent synchrotron emission from electron nanobunches formed in relativistic laser-plasma interactions - Dromey, B.; Rykovanov, S.; Yeung, M.; H{\"{o}}rlein, R.; Jung, D.; Gautier, D. C.; Dzelzainis, T.; Kiefer, D.; Palaniyppan, S.; Shah, R.; Schreiber, J.; Ruhl, H.; et al. (Nature Physics, 2012)
Extreme ultraviolet (XUV) and X-ray harmonic spectra produced by intense laser - solid interactions have, so far, been consistent with Doppler upshifted reflection from collective relativistic plasma oscillations - the relativistically oscillating mirror mechanism. Recent theoretical work, however, has identified a new interaction regime in which dense electron nanobunches are formed at the plasma–vacuum boundary resulting in coherent XUV radiation by coherent synchrotron emission (CSE). Our experiments enable the isolation of CSE from competing processes, demonstrating that electron nanobunch formation does indeed occur. We observe spectra with the characteristic spectral signature of CSE - a slow decay of intensity, I, with high-harmonic order, n, as I(n) n^(−1.62) before a rapid efficiency rollover. Particle-in-cell code simulations reveal how den se nanobunches of electrons are periodically formed and accelerated during normal-incidence interactions with ultrathin foils and result in CSE in the transmitted direction. This observation of CSE presents a route to high-energy XUV pulses and offers a new window on understanding ultrafast energy coupling during intense laser - solid density interactions.
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Effect of electron heating on self-induced transparency in relativistic-intensity laser-plasma interactions - Siminos, E.; Grech, M.; Skupin, S.; Schlegel, T.; Tikhonchuk, V. T. (Physical Review E, 2012)
The effective increase of the critical density associated with the interaction of relativistically intense laser pulses with overcritical plasmas, known as self-induced transparency, is revisited for the case of circular polarization. A comparison of particle-in-cell simulations to the predictions of a relativistic cold-fluid model for the transparency threshold demonstrates that kinetic effects, such as electron heating, can lead to a substantial increase of the effective critical density compared to cold-fluid theory. These results are interpreted by a study of separatrices in the single-electron phase space corresponding to dynamics in the stationary fields predicted by the cold-fluid model. It is shown that perturbations due to electron heating exceeding a certain finite threshold can force electrons to escape into the vacuum, leading to laser pulse propagation. The modification of the transparency threshold is linked to the temporal pulse profile, through its effect on electron heating.
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Gain lifetime measurement of a Ni-like Ag soft X-ray laser - Ecker, B.; Oliva, E.; Aurand, B.; Hochhaus, D. C.; Neumayer, P.; Zhao, H.; Zielbauer, B.; Cassou, K.; Daboussi, S.; Guilbaud, O.; Kazamias, S.; Le, T. T. T.; et al. (Optics Express, 2012)
Experimental results of a two-stage Ni-like Ag soft X-ray laser operated in a seed-amplifier configuration are presented. Both targets were pumped applying the double-pulse grazing incidence technique with intrinsic travelling wave excitation. The injection of the seed X-ray laser into the amplifier target was realized by a spherical mirror. The results show amplification of the seed X-ray laser and allow for a direct measurement of the gain lifetime. The experimental configuration is suitable for providing valuable input for computational simulations.
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High-power very large mode-area thulium-doped fiber laser - Jansen, F.; Stutzki, F.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2012)
Large-pitch photonic-crystal fibers have demonstrated their unique capability of combining very large mode areas, high output powers and robust single-mode operation at a wavelength of 1 μm. In this Letter, we present the experimental realization of thulium-doped very large mode-area fibers based on the large-pitch fibers with record mode-field diameters exceeding 60 μm and delivering more than 52 W of output power.
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Improving carrier-envelope phase stability in optical parametric chirped-pulse amplifiers by control of timing jitter - H{\"{a}drich, S. and Rothhardt, J. and Krebs, M. and Demmler, S. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2012)
It is shown that timing jitter in optical parametric chirped-pulse amplification induces spectral drifts that transfer to carrier-envelope phase (CEP) instabilities via dispersion. Reduction of this effect requires temporal synchronization, which is realized with feedback obtained from the angularly dispersed idler. Furthermore, a novel method to measure the CEP drifts by utilizing parasitic second harmonic generation within parametric amplifiers is presented. Stabilization of the timing allows the obtainment of a CEP stability of 86 mrad over 40 min at 150 kHz repetition rate.
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Ion Acceleration in Multispecies Targets Driven by Intense Laser Radiation Pressure - Kar, S.; Kakolee, K. F.; Qiao, B.; Macchi, A.; Cerchez, M.; Doria, D.; Geissler, M.; McKenna, P.; Neely, D.; Osterholz, J.; Prasad, R.; Quinn, K.; et al. (Physical Review Letters, 2012)
The acceleration of ions from ultrathin foils has been investigated by using 250 TW, subpicosecond laser pulses, focused to intensities of up to 3 × 10^(20) W cm^(-2). The ion spectra show the appearance of narrow-band features for protons and carbon ions peaked at higher energies (in the 5 - 10 MeV/nucleon range) and with significantly higher flux than previously reported. The spectral features and their scaling with laser and target parameters provide evidence of a multispecies scenario of radiation pressure acceleration in the light sail mode, as confirmed by analytical estimates and 2D particle-in-cell simulations. The scaling indicates that monoenergetic peaks with more than 100 MeV/nucleon are obtainable with moderate improvements of the target and laser characteristics, which are within reach of ongoing technical developments.
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K-shell spectroscopy of silicon ions as diagnostic for high electric fields - L{\"{o}}tzsch, R.; J{\"{a}ckel, O. and H{\"{o}}fer, S. and K{\"{a}mpfer, T. and Polz, J. and Uschmann, I. and Kaluza, M.C. and F{\"{o}}rster, E. and Stambulchik, E. and Kroupp, E. and Maron, Y. (Review of Scientific Instruments, 2012)
We developed a detection scheme, capable of measuring X-ray line shape of tracer ions in μm thick layers at the rear side of a target foil irradiated by ultra intense laser pulses. We performed simulations of the effect of strong electric fields on the K-shell emission of silicon and developed a spectrometer dedicated to record this emission. The combination of a cylindrically bent crystal in von H{\'{a}}mos geometry and a CCD camera with its single photon counting capability allows for a high dynamic range of the instrument and background free spectra. This approach will be used in future experiments to study electric fields of the order of TV/m at high density plasmas close to solid density.
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Sub 25 fs pulses from solid-core nonlinear compression stage at 250 W of average power - Jocher, C.; Eidam, T.; H{\"{a}drich, S. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2012)
We report on a highpower femtosecond fiber chirped-pulse amplification system with an excellent beam quality (M^2 = 1.2) operating at 250 MHz repetition rate. We demonstrate nonlinear compression in a solid-core photonic crystal fiber at unprecedented average power levels. By exploiting self-phase modulation with subsequent chirped-mirror compression we achieve pulse shortening by more than one order of magnitude to 23 fs pulses. The use of circular polarization allows higher than usual peak powers in the broadening fiber resulting in compressed 0.9 μJ pulse energy and a peak power of 34 MW at 250 W of average power (M^2 = 1.3). This system is well suited for driving cavity-enhanced high-repetition rate high-harmonic generation.
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Control of nonlinear spectral phase induced by ultra-broadband optical parametric amplification - Demmler, S.; Rothhardt, J.; H{\"{a}drich, S. and Bromage, J. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2012)
Optical parametric amplifiers (OPAs) impose an optical parametric phase (OPP) onto the amplified signal. It manifests itself as a spectral phase in the case of broadband signals and, therefore, hampers pulse compression. Here we present, for the first time, a complete experimental characterization of this OPP for different ultra-broadband noncollinear OPA configurations. This measurement allows us to compensate the OPP and to achieve Fourier-limited pulses as short as 1.9 optical cycles. A numerical model is in excellent agreement with our measurements and reveals the importance of high order phase compensation in the case of noncollinear phase matching. In contrast, operation at degeneracy enables almost complete compensation of the OPP by second-order dispersion only.
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High-power efficient generation of visible and mid-infrared radiation exploiting four-wave-mixing in optical fibers - Jauregui, C.; Steinmetz, A.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2012)
We report on the generation of 17.6 W of visible radiation at 650 nm using four-wave-mixing in an endlessly single-mode silica fiber. The conversion efficiency was as high as 30%. This high efficiency could be obtained by exploiting the natural absorption of silica for the mid-infrared radiation > 2.5µm. In a separate experiment 1.6 W of mid-IR radiation at 2570 nm were generated simultaneously with 14.4 W at 672 nm. These power levels of picosecond red radiation are among the highest reported so far for a diffraction limited beam quality in this wavelength region.
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Implications for the electron distribution from the stationary hydrodynamic model of a one-dimensional plasma expansion into vacuum - Kiefer, T.; Schlegel, T. (Physics of Plasmas, 2012)
It is shown that the hydrodynamic model of a one-dimensional collisionless plasma expansion is contained in the kinetic description as a special case. This belongs to a specific choice for the electron distribution function. Moreover, the consequences of the use of the hydrodynamic approach regarding the temporal evolution of the electron phase space density are investigated. It turns out that only the case of a hydrodynamic description with the adiabatic constant κ = 3 is physically self-consistent. Numerical simulations confirm this argumentation. The analysis for the case κ = 3 is extended to the kinetics of a relativistic electron gas.
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Strong-field physics with singular light beams - Z{\"{u}}rch, M.; Kern, C.; Hansinger, P.; Dreischuh, A.; Spielmann, C. (Nature Physics, 2012)
Light beams carrying a point singularity with a screw-type phase distribution are associated with an optical vortex. The corresponding momentum flow leads to an orbital angular momentum of the photons. The study of optical vortices has led to applications such as particle micro-manipulation, imaging, interferometry, quantum information and high-resolution microscopy and lithography. Recent analyses showed that transitions forbidden by selection rules seem to be allowed when using optical vortex beams. To exploit these intriguing new applications, it is often necessary to shorten the wavelength by nonlinear frequency conversion. However, during the conversion the optical vortices tend to break up. Here we show that optical vortices can be generated in the extreme ultraviolet (XUV) region using high-harmonic generation. The singularity impressed on the fundamental beam survives the highly nonlinear process. Vortices in the XUV region have the same phase distribution as the driving field, which is in contradiction to previous findings, where multiplication of the momentum by the harmonic order is expected. This approach opens the way for several applications based on vortex beams in the XUV region.
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Carrier-envelope-phase tagging in measurements with long acquisition times - K{\"{u}}bel, M.; Betsch, K. J.; Johnson, N.; Kleineberg, U.; Moshammer, R.; Ullrich, J.; Paulus, G.G.; Kling, M. F.; Bergues, B. (New Journal of Physics, 2012)
We present a detailed analysis of the systematic errors that affect single-shot carrier envelope phase (CEP) measurements in experiments with long acquisition times, for which only limited long-term laser stability can be achieved. After introducing a scheme for eliminating these systematic errors to a large extent, we apply our approach to investigate the CEP dependence of the yield of doubly charged ions produced via non-sequential double ionization of argon in strong near-single-cycle laser pulses. The experimental results are compared to predictions of semiclassical calculations.
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Comparative study of the electron-atom and positron-atom bremsstrahlung - Yerokhin, V.; Surzhykov, A.; M{\"{a}rtin, R. and Tashenov, S. and Weber, G. (Physical Review A, 2012)
Fully relativistic treatment of the electron-atom and positron-atom bremsstrahlung is reported. The calculation is based on the partial-wave expansion of the Dirac scattering states in an external atomic field. A comparison of the electron and positron bremsstrahlung is presented for the single and double differential cross sections and the Stokes parameters of the emitted photon. It is demonstrated that the electron-positron symmetry of the bremsstrahlung spectra, which is nearly exact in the nonrelativistic regime, is to a large extent removed by the relativistic effects.
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Focal aberrations of large-aperture HOPG von-Hamos x-ray spectrometers - Zastrau, U.; Brown, C. R. D.; D{\"{o}}ppner, T.; Glenzer, S. H.; Gregori, G.; Lee, H. J.; Marschner, H.; Toleikis, S.; Wehrhan, O.; F{\"{o}}rster, E. (Journal of Instrumentation, 2012)
Focal aberrations of large-aperture highly oriented pyrolytic graphite (HOPG) crystals in von-H{\`{a}}mos geometry are investigated by experimental and computational methods. A mosaic HOPG crystal film of 100 μm thickness diffracts 8 keV x-rays. This thickness is smaller than the absorption depth of the symmetric 004-reflection, which amounts to 257 μm. Cylindrically bent crystals with 110 mm radius of curvature and up to 100 mm collection width produce a X-shaped halo around the focus. This feature vanishes when the collection aperture is reduced, but axial spectral profiles show that the resolution is not affected. X-ray topography reveals significant inhomogeneous crystallite domains of 2 ± 1 mm diameter along the entire crystal. Rocking curves shift by about ±20 arcmin between domains, while their full width at half-maximum varies between 30 and 50 arcmin. These inhomogeneities are not imprinted at the focal spot, since the monochromatically reflecting area of the crystal is large compared to inhomogeneities. Ray-tracing calculations using a Monte-Carlo-based algorithm developed for mosaic crystals reproduce the X-shaped halo in the focal plane, stemming from the mosaic defocussing in the non-dispersive direction in combination with large apertures. The best achievable resolution is found by analyzing a diversity of rocking curve widths, source sizes and crystal thicknesses for 8 keV x-rays to be ΔE/E 10^(−4). Finally a general analytic expression for the shape of the aberration is derived.
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Harmonic Generation from Relativistic Plasma Surfaces in Ultrasteep Plasma Density Gradients - R{\"{o}}del, C.; an der Br{\"{u}}gge, D.; Bierbach, J.; Yeung, M.; Hahn, T.; Dromey, B.; Herzer, S.; Fuchs, S.; Pour, A.; Eckner, E.; Behmke, M.; Cerchez, M.; et al. (Physical Review Letters, 2012)
Harmonic generation in the limit of ultrasteep density gradients is studied experimentally. Observations reveal that, while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale lengths (L_p/λ < 1), the absolute efficiency of the harmonics declines for the steepest plasma density scale length L_p → 0, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the relativistic oscillating mirror was estimated to be in the range of 10^-4 – 10^-6 of the laser pulse energy for photon energies ranging from 20 – 40 eV, with the best results being obtained for an intermediate density scale length.
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Magnetically Amplified Tunneling of the Third Kind as a Probe of Minicharged Particles - D{\"{o}}brich, B.; Gies, H.; Neitz, N.; Karbstein, F. (Physical Review Letters, 2012)
We show that magnetic fields significantly enhance a new tunneling mechanism in quantum field theories with photons coupling to fermionic minicharged particles (MCPs). We propose a dedicated laboratory experiment of the light-shining-through-walls type that can explore a parameter regime comparable to and even beyond the best model-independent cosmological bounds. With present-day technology, such an experiment is particularly sensitive to MCPs with masses in and below the meV regime as suggested by new-physics extensions of the standard model.
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Measurement of temperature-dependent absorption and emission spectra of Yb:YAG, Yb:LuAG, and Yb:CaF_2 between 20 °C and 200 °C and predictions on their influence on laser performance - K{\"{o}}rner, J.; Vorholt, C.; Liebetrau, H.; Kahle, M.; Kl{\"{o}}pfel, D.; Seifert, R.; Hein, J.; Kaluza, M.C. (Journal of the Optical Society of America B, 2012)
Accurate values of the emission and absorption cross sections of Yb:YAG, Yb:LuAG, and Yb:CaF_2 as a function of temperature between room temperature and 200 °C are presented. For this purpose, absorption and fluorescence spectra were measured using a setup optimized to reduce the effect of radiation trapping. From these data, emission cross sections were retrieved by combining the Fuchtbauer–Ladenburg equation and the reciprocity method. Based on our measurements, simple estimations illustrate the effect of temperature shifts that are likely to occur in typical laser setups. Our results show that even minor temperature variations can have significant impact on the laser performance using Yb:YAG and Yb:LuAG as an active medium, while Yb:CaF_2 appears to be rather insensitive.
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Production and diagnostics of spin-polarized heavy ions in sequential two-electron radiative recombination - Maiorova, A. V.; Surzhykov, A.; Tashenov, S.; Shabaev, V. M.; St{\"{o}}hlker, T. (Physical Review A, 2012)
In the present work we investigate the sequential radiative recombination (RR) of initially bare ions colliding with two spatially separated electron targets. It is shown that magnetic sublevel population of the hydrogenlike ions, produced by the electron capture from the first target, depends on the emission direction of the (first) RR photon. This population, which can be expressed in terms of the polarization parameters, affects then the angular and polarization properties of the radiation emitted in the collision with the second target. The coincidence measurements of two subsequent RR photons may allow one to understand, therefore, the production and diagnostics of the ion spin polarization. In order to describe this polarization production and diagnostic scheme we derive the general expression for the γ-γ correlation function. Detailed calculations for the dependence of this function on the geometry of photon emission and collision energy are performed for the radiative recombination of bare uranium ions.
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Thomson backscattering from laser-generated, relativistically moving high-density electron layers - Paz, A.; Kuschel, S.; R{\"{o}}del, C.; Schnell, M.; J{\"{a}ckel, O. and Kaluza, M.C. and Paulus, G.G. (New Journal of Physics, 2012)
We show experimentally that extreme ultraviolet radiation is produced when a laser pulse is Thomson backscattered from sheets of relativistic electrons that are formed at the rear surface of a foil irradiated on its front side with a high-intensity laser. An all-optical setup is realized using the Jena titanium:sapphire ten-terawatt laser system with an enhanced amplified spontaneous emission background of 10^−12. The main pulse is split into two: one of them accelerates electrons from thin aluminium foil targets to energies of the order of some MeV and the other, counterpropagating probe pulse Thomson-backscatters off these electrons when they exit the target rear side. The process produced photons within a wide spectral range of some tens of eV as a result of the broad electron energy distribution. The highest scattering intensity is observed when the probe pulse arrives at the target rear surface 100 fs after irradiation of the target front side by the pump pulse, corresponding to the maximum flux of hot electrons at the interaction region. These results can provide time-resolved information about the evolution of the rear-surface electron sheath and hence about the dynamics of the electric fields responsible for the acceleration of ions from the rear surface of thin, laser-irradiated foils.
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A Penning trap for advanced studies with particles in extreme laser fields - Vogel, M.; Quint, W.; Paulus, G.G.; St{\"{o}}hlker, T. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2012)
We present a Penning trap as a tool for advanced studies of particles in extreme laser fields. Particularly, trap-specific manipulation techniques allow control over the confined particles’ localization and spatial density by use of trap electrodes as tweezers’ and by application of a wall’, respectively. It is thereby possible to select and prepare well-defined ion ensembles and to optimize the laser–particle interaction. Non-destructive detection of reaction educts and products with up to single-ion sensitivity supports advanced studies by maintaining the products for further studies at extended confinement times of minutes and above. The trap features endcaps with conical openings for applications with strongly focused lasers. We show that such a modification of a cylindrical trap is possible while harmonicity and tunability are maintained.
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All-reflective, highly accurate polarization rotator for high-power short-pulse laser systems - Keppler, S.; Hornung, M.; B{\"{o}}defeld, R.; Kahle, M.; Hein, J.; Kaluza, M.C. (Optics Express, 2012)
We present the setup of a polarization rotating device and its adaption for high-power short-pulse laser systems. Compared to conventional halfwave plates, the all-reflective principle using three zero-phase shift mirrors provides a higher accuracy and a higher damage threshold. Since plan-parallel plates, e.g. these halfwave plates, generate postpulses, which could lead to the generation of prepulses during the subsequent laser chain, the presented device avoids parasitic pulses and is therefore the preferable alternative for high-contrast applications. Moreover the device is easily scalable for large beam diameters and its spectral reflectivity can be adjusted by an appropriate mirror coating to be well suited for ultra-short laser pulses.
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Coherent spectral enhancement of carrier-envelope-phase stable continua with dual-gas high harmonic generation - Willner, A.; Hage, A.; Riedel, R.; Grgura{{s}}, I.; Simoncig, A.; Schulz, M.; Dzelzainis, T.; H{\"{o}}ppner, H.; Huber, S.; Prandolini, M. J.; Dromey, B.; Zepf, M.; et al. (Optics Letters, 2012)
Attosecond science is enabled by the ability to convert femtosecond near-infrared laser light into coherent harmonics in the extreme ultraviolet spectral range. While attosecond sources have been utilized in experiments that have not demanded high intensities, substantially higher photon flux would provide a natural link to the next significant experimental breakthrough. Numerical simulations of dual-gas high harmonic generation indicate that the output in the cutoff spectral region can be selectively enhanced without disturbing the single-atom gating mechanism. Here, we summarize the results of these simulations and present first experimental findings to support these predictions.
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Electron emission from highly charged ions - signatures of magnetic interactions and retardation in strong fields - Fritzsche, S.; Surzhykov, A.; Gumberidze, A.; St{\"{o}}hlker, T. (New Journal of Physics, 2012)
The electron emission of highly charged ions has been reanalyzed with the goal of separating the magnetic and retardation contributions to the electron-electron (e-e) interaction from the static Coulomb repulsion in strong fields. A remarkable change in the electron angular distribution due to the relativistic terms in the e-e interaction is found, especially for the autoionization of beryllium-like projectiles, following a 1s → 2p_(3/2) Coulomb excitation in collision with some target nuclei. For low-energetic, high-Z projectiles with Tp ≤ 10 MeV u^(−1), a diminished (electron) emission in the forward direction as well as oscillations in the electron angular distribution due to the magnetic and retarded interactions are predicted for the autoionization of the 1s 2s^(2) 2p_(3/2) 3^P_2 resonance into the 1s^2 2s^2 S_(1/2) ground and the 1s^2 2p^2P_(1/2) excited levels of the finally lithium-like ions, and in contrast to a pure Coulomb repulsion between the bound and emitted electrons. The proposed excitation-autoionization process can be observed at existing storage rings and will provide a novel insight into the dynamics of electrons in strong fields.
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Low-Emittance Electron Bunches from a Laser-Plasma Accelerator Measured using Single-Shot X-Ray Spectroscopy - Plateau, G. R.; Geddes, C. G. R.; Thorn, D. B.; Chen, M.; Benedetti, C.; Esarey, E.; Gonsalves, A. J.; Matlis, N. H.; Nakamura, K.; Schroeder, C. B.; Shiraishi, S.; Sokollik, T.; et al. (Physical Review Letters, 2012)
X-ray spectroscopy is used to obtain single-shot information on electron beam emittance in a low-energy-spread 0.5 GeV-class laser-plasma accelerator. Measurements of betatron radiation from 2 to 20 keV used a CCD and single-photon counting techniques. By matching x-ray spectra to betatron radiation models, the electron bunch radius inside the plasma is estimated to be 0.1 μm. Combining this with simultaneous electron spectra, normalized transverse emittance is estimated to be as low as 0.1 mm mrad, consistent with three-dimensional particle-in-cell simulations. Correlations of the bunch radius with electron beam parameters are presented.
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Observation of energetic terahertz pulses from relativistic solid density plasmas - Gopal, A.; May, T.; Herzer, S.; Reinhard, A.; Minardi, S.; Schubert, M.; Dillner, U.; Pradarutti, B.; Polz, J.; Gaumnitz, T.; Kaluza, M.C.; J{\"{a}ckel, O. and Riehemann, S. and Ziegler, W. and Gem{\"{u}}nd, H.-P. and Meyer, H.-G. and Paulus, G.G. (New Journal of Physics, 2012)
We report the first experimental observation of terahertz (THz) radiation from the rear surface of a solid target while interacting with an intense laser pulse. Experimental and two-dimensional particle-in-cell simulations show that the observed THz radiation is mostly emitted at large angles to the target normal. Numerical results point out that a large part of the emission originates from a micron-scale plasma sheath at the rear surface of the target, which is also responsible for the ion acceleration. This opens a perspective for the application of THz radiation detection for on-site diagnostics of particle acceleration in laser-produced plasmas.
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Plasma switch as a temporal overlap tool for pump-probe experiments at FEL facilities - Harmand, M.; Murphy, C. D.; Brown, C. R. D.; Cammarata, M.; D{\"{o}}ppner, T.; D{\"{u}}sterer, S.; Fritz, D.; F{\"{o}}rster, E.; Galtier, E.; Gaudin, J.; Glenzer, S. H.; G{\"{o}}de, S.; et al. (Journal of Instrumentation, 2012)
We have developed an easy-to-use and reliable timing tool to determine the arrival time of an optical laser and a free electron laser (FEL) pulses within the jitter limitation. This timing tool can be used from XUV to X-rays and exploits high FELs intensities. It uses a shadowgraph technique where we optically (at 800 nm) image a plasma created by an intense XUV or X-ray FEL pulse on a transparent sample (glass slide) directly placed at the pump - probe sample position. It is based on the physical principle that the optical properties of the material are drastically changed when its free electron density reaches the critical density. At this point the excited glass sample becomes opaque to the optical laser pulse. The ultra-short and intense XUV or X-ray FEL pulse ensures that a critical electron density can be reached via photoionization and subsequent collisional ionization within the XUV or X-ray FEL pulse duration or even faster. This technique allows to determine the relative arrival time between the optical laser and the FEL pulses in only few single shots with an accuracy mainly limited by the optical laser pulse duration and the jitter between the FEL and the optical laser. Considering the major interest in pump-probe experiments at FEL facilities in general, such a femtosecond resolution timing tool is of utmost importance.
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Simultaneous imaging electron- and ion-feature Thomson scattering measurements of radiatively heated Xe - Pollock, B. B.; Meinecke, J.; Kuschel, S.; Ross, J. S.; Shaw, J. L.; Stoafer, C.; Divol, L.; Tynan, G. R.; Glenzer, S. H. (Review of Scientific Instruments, 2012)
Uniform density and temperature Xe plasmas have been produced over > 4 mm scale-lengths using x-rays generated in a cylindrical Pb cavity. The cavity is 750 μm in depth and diameter, and is heated by a 300 J, 2 ns square, 1054 nm laser pulse focused to a spot size of 200 μm at the cavity entrance. The plasma is characterized by simultaneous imaging Thomson scattering measurements from both the electron and ion scattering features. The electron feature measurement determines the spatial electron density and temperature profile, and using these parameters as constraints in the ion feature analysis allows an accurate determination of the charge state of the Xe ions. The Thomson scattering probe beam is 40 J, 200 ps, and 527 nm, and is focused to a 100 μm spot size at the entrance of the Pb cavity. Each system has a spatial resolution of 25 μm, a temporal resolution of 200 ps (as determined by the probe duration), and a spectral resolution of 2 nm for the electron feature system and 0.025 nm for the ion feature system. The experiment is performed in a Xe filled target chamber at a neutral pressure of 3–10 Torr, and the x-rays produced in the Pb ionize and heat the Xe to a charge state of 20 ± 4 at up to 200 eV electron temperatures.
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Branching optical signals by fractional vortex dipoles - Maleshkov, G.; Hansinger, P.; Dimitrov, N.; Dreischuh, A.; Paulus, G. (Optics Communications, 2012)
We study the evolution and interaction of semi-infinite dark beams carrying phase dislocations, where step- and screw-like phase profiles are combined. Similar to dark beams with a finite length, semi-infinite dark beams tend to move in transversal direction with respect to their background beam. In addition, they develop a snake-like instability and optical vortices detach from their bending ends. We are looking for appropriate conditions to control the process of concatenating and crossing the ends of several such semi-infinite dark beams in a way suitable for probe-beam branching and routing in self-defocusing Kerr nonlinear media. Colinear and perpendicular probe beam propagations in the optically-induced guiding structures are modeled and analyzed with respect to the branching efficiency to respective virtual output channels.
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Carrier-envelope phase-tagged imaging of the controlled electron acceleration from SiO2 nanospheres in intense few-cycle laser fields - Zherebtsov, S.; S{\"{u}}ssmann, F.; Peltz, C.; Plenge, J.; Betsch, K. J.; Znakovskaya, I.; Alnaser, A. S.; Johnson, N. G.; K{\"{u}}bel, M.; Horn, A.; Mondes, V.; Graf, C.; et al. (New Journal of Physics, 2012)
Waveform-controlled light fields offer the possibility of manipulating ultrafast electronic processes on sub-cycle timescales. The optical lightwave control of the collective electron motion in nanostructured materials is key to the design of electronic devices operating at up to petahertz frequencies. We have studied the directional control of the electron emission from 95 nm diameter SiO_2 nanoparticles in few-cycle laser fields with a well-defined waveform. Projections of the three-dimensional (3D) electron momentum distributions were obtained via single-shot velocity-map imaging (VMI), where phase tagging allowed retrieving the laser waveform for each laser shot. The application of this technique allowed us to efficiently suppress background contributions in the data and to obtain very accurate information on the amplitude and phase of the waveform-dependent electron emission. The experimental data that are obtained for 4 fs pulses centered at 720 nm at different intensities in the range (1 - 4) × 10^13 W cm^(−2) are compared to quasi-classical mean-field Monte-Carlo simulations. The model calculations identify electron backscattering from the nanoparticle surface in highly dynamical localized fields as the main process responsible for the energetic electron emission from the nanoparticles. The local field sensitivity of the electron emission observed in our studies can serve as a foundation for future research on propagation effects for larger particles and field-induced material changes at higher intensities.
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Chromatic energy filter and characterization of laser-accelerated proton beams for particle therapy - Hofmann, I.; Meyer-ter-Vehn, J.; Yan, X.; Al-Omari, H. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2012)
The application of laser accelerated protons or ions for particle therapy has to cope with relatively large energy and angular spreads as well as possibly significant random fluctuations. We suggest a method for combined focusing and energy selection, which is an effective alternative to the commonly considered dispersive energy selection by magnetic dipoles. Our method is based on the chromatic effect of a magnetic solenoid (or any other energy dependent focusing device) in combination with an aperture to select a certain energy width defined by the aperture radius. It is applied to an initial 6D phase space distribution of protons following the simulation output from a Radiation Pressure Acceleration model. Analytical formula for the selection aperture and chromatic emittance are confirmed by simulation results using the TRACEWIN code. The energy selection is supported by properly placed scattering targets to remove the imprint of the chromatic effect on the beam and to enable well-controlled and shot-to-shot reproducible energy and transverse density profiles.
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Classical model of strong-field parametric amplification of soft x rays - Seres, J.; Seres, E.; Spielmann, C. (Physical Review A, 2012)
We present a detailed theoretical description of laser driven x-ray parametric amplification, which has been experimentally demonstrated by Seres et al. [Nature Phys. 6 455 (2010)] together with a supporting basic model. The process is based on the parametric interaction of an x-ray photon with a laser accelerated electron in a Coulomb field. With the extended model we are able to estimate the gain cross section also for a finite energy distribution of the interacting electrons as well as to consider dephasing between the electrons and the x-ray field. The improved model is capable of describing the recent experimental findings much more accurately.
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Classical radiation effects on relativistic electrons in ultraintense laser fields with circular polarization - Schlegel, T.; Tikhonchuk, V. (New Journal of Physics, 2012)
The propagation of a relativistic electron with initial energy ≥ 100 MeV in a number of simple one-dimensional laser field configurations with circular polarization is studied by solving the relativistic equation of motion in the Landau–Lifschitz approach to account for the radiation friction force. The radiation back-reaction on the electron dynamics becomes visible at dimensionless field amplitudes a ≥ 10 at these high particle energies. Analytical expressions are derived for the energy and the longitudinal momentum of the electron, the frequency shift of the light scattered by the electron and the particle trajectories. These findings are compared with the numerical solutions of the basic equations. A strong radiation damping effect results in reduced light scattering, forming at the same time a broad quasi-continuous spectrum. In addition, the electron dynamics in the strong field of a quasistationary laser piston is investigated. Analytical solutions for the electron trajectories in this complex field pattern are obtained and compared with the numerical solutions. The radiation friction force may stop a relativistic electron after propagation over several laser wavelengths at high laser field strengths, which supports the formation of a stable piston.
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Dependence of high-order-harmonic-generation yield on driving-laser ellipticity - M{\"{o}}ller, M.; Cheng, Y.; Khan, S.; Zhao, B.; Zhao, K.; Chini, M.; Paulus, G.G.; Chang, Z. (Physical Review A, 2012)
High-order-harmonic-generation yield is remarkably sensitive to driving laser ellipticity, which is interesting from a fundamental point of view as well as for applications. The most well-known example is the generation of isolated attosecond pulses via polarization gating. We develop an intuitive semiclassical model that makes use of the recently measured initial transverse momentum of tunneling ionization. The model is able to predict the dependence of the high-order-harmonic yield on driving laser ellipticity and is in good agreement with experimental results and predictions from a numerically solved time-dependent Schr{\"{o}}dinger equation.
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Temporal dynamics of mode instabilities in high-power fiber lasers and amplifiers - Otto, H.-J.; Stutzki, F.; Jansen, F.; Eidam, T.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2012)
The temporal behavior of mode instabilities in active large mode area fibers is experimentally investigated in detail. Thus, apart from the onset threshold of mode instabilities, the output beam is characterized using both high-speed camera measurements with 20,000 frames per second and photodiode traces. Based on these measurements, an empiric definition of the power threshold of mode instabilities is introduced. Additionally, it is shown that the temporal dynamics show a transition zone between the stable and the unstable regimes where well-defined periodic temporal fluctuations on ms-timescale can be observed. Finally, it is experimentally shown that the larger the mode-field area, the slower the mode-instability fluctuation is. The observations support the thermal origin of mode instabilities.
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Can we see quantum gravity? Photons in the asymptotic-safety scenario - D{\"{o}}brich, B.; Eichhorn, A. (Journal of High Energy Physics, 2012)
In the search for a quantum theory of gravity it is crucial to find experimental access to quantum gravitational effects. Since these are expected to be very small at observationally accessible scales it is advantageous to consider processes with no tree-level contribution in the standard model, such as photon-photon scattering. We examine the implications of asymptotically safe quantum gravity in a setting with extra dimensions for this case, and point out that various near-future photon-collider setups, employing either electron or muon colliders, or even a purely laser-based setup, could provide a first observational window into the quantum gravity regime.
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Generation of 10 µW relativistic surface high-harmonic radiation at a repetition rate of 10 Hz - Bierbach, J.; R{\"{o}}del, C.; Yeung, M.; Dromey, B.; Hahn, T.; Pour, A.; Fuchs, S.; Paz, A. E.; Herzer, S.; Kuschel, S.; J{\"{a}ckel, O. and Kaluza, M.C. and Pretzler, G. and Zepf, M. and Paulus, G.G. (New Journal of Physics, 2012)
Experimental results on relativistic surface HHG at a repetition rate of 10 Hz are presented. Average powers in the 10 μW range are generated in the spectral range of 51 to 26 nm (24 - 48 eV). The surface harmonic radiation is produced by focusing the second-harmonic of a high-power laser onto a rotating glass surface to moderately relativistic intensities of 3 × 10^19 W cm^−2. The harmonic emission exhibits a divergence of 26 mrad. Together with absolute photon numbers recorded by a calibrated spectrometer, this allows for the determination of the extreme ultraviolet (XUV) yield. The pulse energies of individual harmonics are reaching up to the μJ level, equivalent to an efficiency of 10^−5. The capability of producing stable and intense high-harmonic radiation from relativistic surface plasmas may facilitate experiments on nonlinear ionization or the seeding of free-electron lasers.
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Physical origin of mode instabilities in high-power fiber laser systems - Jauregui, C.; Eidam, T.; Otto, H.-J.; Stutzki, F.; Jansen, F.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2012)
Mode instabilities, i.e. the rapid fluctuations of the output beam of an optical fiber that occur after a certain output power threshold is reached, have quickly become one of the most limiting effects for the further power scaling of fiber laser systems. Even though much work has been done over the last year, the exact origin of the temporal dynamics of this phenomenon is not fully understood yet. In this paper we show that the origin of mode instabilities can be explained by taking into account the interplay between the temporal evolution of the three-dimensional temperature profile inside of the active fiber and the related waveguide changes that it produces via the thermo-optical effect. In particular it is proposed that non-adiabatic waveguide changes play an important role in allowing energy transfer from the fundamental mode into the higher order mode. As it is discussed in the paper, this description of mode instabilities can explain many of the experimental observations reported to date.
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Polarization Transfer of Bremsstrahlung Arising from Spin-Polarized Electrons - M{\"{a}rtin, R. and Weber, G. and Barday, R. and Fritzsche, Y. and Spillmann, U. and Chen, W. and DuBois, R. D. and Enders, J. and Hegewald, M. and Hess, S. and Surzhykov, A. and Thorn, D. B. and Trotsenko, S. and Wagner, M. and Winters, D. F. A. and Yerokhin, V. A. and St{\"{o}}hlker, T. (Physical Review Letters, 2012)
We report on a study of the polarization transfer between transversely polarized incident electrons and the emitted x rays for electron-atom bremsstrahlung. By means of Compton polarimetry we performed for the first time an energy-differential measurement of the complete properties of bremsstrahlung emission related to linear polarization, i.e., the degree of linear polarization as well as the orientation of the polarization axis. For the high-energy end of the bremsstrahlung continuum the experimental results for both observables show a high sensitivity on the initial electron spin polarization and prove that the polarization orientation is virtually independent of the photon energy.
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66 W average power from a microjoule-class sub-100 fs fiber oscillator - Baumgartl, M.; Lecaplain, C.; Hideur, A.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2012)
Performance scaling of passively mode-locked ultrashort-pulse fiber oscillators in terms of average power, peak power, and pulse energy is demonstrated. A very-large-mode-area fiber laser in an all-positive group-velocity-dispersion ring cavity configuration with intracavity spectral filter, mode-locked by nonlinear polarization evolution, emits 66 W of average power at 76 MHz repetition rate, corresponding to 0.9 μJ pulse energy. The pulses are dechirped to 91 fs outside the cavity with an average power of 60 W remaining after the compressor. The generated pulse peak power is as high as 7 MW.
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Attosecond tracing of correlated electron-emission in non-sequential double ionization - Bergues, B.; K{\"{u}}bel, M.; Johnson, N.; Fischer, B.; Camus, N.; Betsch, K.; Herrwerth, O.; Senftleben, A.; Sayler, A.; Rathje, T.; Pfeifer, T.; Ben-Itzhak, I.; et al. (Nature Communications, 2012)
Despite their broad implications for phenomena such as molecular bonding or chemical reactions, our knowledge of multi-electron dynamics is limited and their theoretical modelling remains a most difficult task. From the experimental side, it is highly desirable to study the dynamical evolution and interaction of the electrons over the relevant timescales, which extend into the attosecond regime. Here we use near-single-cycle laser pulses with well-defined electric field evolution to confine the double ionization of argon atoms to a single laser cycle. The measured two-electron momentum spectra, which substantially differ from spectra recorded in all previous experiments using longer pulses, allow us to trace the correlated emission of the two electrons on sub-femtosecond timescales. The experimental results, which are discussed in terms of a semiclassical model, provide strong constraints for the development of theories and lead us to revise common assumptions about the mechanism that governs double ionization.
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Compton polarimeters for the study of hard X-rays arising from energetic collisions of electrons and ions with matter - Weber, G.; Br{\"{a}uning, H. and Fritzsche, S. and Gumberidze, A. and M{\"{a}rtin, R. and Reuschl, R. and Schwemlein, M. and Spillmann, U. and Surzhykov, A. and Winters, D. F. A. and St{\"{o}}hlker, T. (AIP Conference Proceedings, 2012)
Novel position-sensitive x-ray detectors are presented that, when applied as Compton polarimeters, enable precise and efficient linear polarization studies of hard x-rays up to several 100 keV. We give an analytical formula which yields a rough estimate of the polarimeter efficiencies of such detector systems. Moreover, we briefly summarize a recent linear polarization measurement of the Lyman-α1 radiation in a H-like high-Z system, namely U91+.
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Fiber-based source for multiplex-CARS microscopy based on degenerate four-wave mixing - Gottschall, T.; Baumgartl, M.; Sagnier, A.; Rothhardt, J.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2012)
We present a fiber-based laser source for multiplex coherent anti-Stokes Raman scattering (CARS) microscopy. This source is very compact and potentially alignment-free. The corresponding pump and Stokes pulses for the CARS process are generated by degenerate four-wave mixing (FWM) in photonic-crystal fibers. In addition, an ytterbium-doped fiber laser emitting spectrally narrow 100 ps pulses at 1035 nm wavelength serves as pump for the FWM frequency conversion. The FWM process delivers narrow-band pulses at 648 nm and drives a continuum-like spectrum ranging from 700 to 820 nm. With the presented source vibrational resonances with energies between 1200 cm^(−1) and 3200 cm^(−1) can be accessed with a resolution of 10 cm^(−1). Additionally, the temporal characteristics of the FWM output have been investigated by a cross-correlation setup, revealing the suitability of the emitted pulses for CARS microscopy. This work marks a significant step towards a simple and powerful all-fiber, maintenance-free multiplex-CARS source for real-world applications outside a laboratory environment.
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Generation of the simplest rotational wave packet in a diatomic molecule: Tracing a two-level superposition in the time domain - Przystawik, A.; Kickermann, A.; Al-Shemmary, A.; D{\"{u}}sterer, S.; Ellis, A. M.; von Haeften, K.; Harmand, M.; Ramakrishna, S.; Redlin, H.; Schroedter, L.; Schulz, M.; Seideman, T.; et al. (Physical Review A, 2012)
We introduce a time-domain approach to explore rotational dynamics caused by intramolecular coupling or the interaction with dissipative media. It pushes the time resolution toward the ultimate limit determined by the rotational period. Femtosecond pulses create a coherent superposition of two rotational states of carbon monoxide. The wave-packet motion is observed by subsequent Coulomb explosion, which results in a time-dependent asymmetry of spatial fragmentation patterns. The asymmetry oscillation prevails for at least 1 ns, covering more than 300 periods with no decoherence. Long time scans will allow weak perturbations of the order of ΔE/E = 10^(−4) to be discerned. Our conclusions are confirmed by a fully quantum-mechanical model.
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High stability soliton frequency-shifting mechanisms for laser synchronization applications - Rothhardt, J.; Heidt, A. M.; H{\"{a}drich, S. and Demmler, S. and Limpert, J. and T{\"{u}}nnermann, A. (Journal of the Optical Society of America B, 2012)
We analyze frequency-shifting mechanisms in photonic crystal fibers (PCFs). In contrast to the generally used approach of launching pulses in the negative group velocity dispersion (GVD) region of PCFs, we suggest employing a fiber with a higher zero dispersion wavelength that is pumped in the positive GVD region. Results of a numerical optimization reveal that the amplitude stability of the frequency-shifted pulses can be improved by more than 1 order of magnitude and the timing jitter arising from input fluctuations by 2 orders of magnitude by a proper choice of the fiber dispersion. The presented approach and optimization will improve the performance of timing- and amplitude-sensitive applications, such as nonlinear microscopy and spectroscopy or optical synchronization for optical parametric chirped pulse amplification significantly.
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Impact of dispersion on pulse dynamics in chirped-pulse fiber lasers - Baumgartl, M.; Ortac, B.; Limpert, J.; T{\"{u}}nnermann, A. (Applied Physics B, 2012)
We report on a systematic study of an environmentally stable mode-locked Yb-doped fiber laser operating in the chirped-pulse regime. The linear cavity chirped-pulse fiber laser is constructed with a saturable absorber mirror as nonlinear mode-locking mechanism and a nonlinearity-free transmission-grating-based stretcher/compressor for dispersion management. Mode-locked operation and pulse dynamics from strong normal to strong anomalous total cavity dispersion in the range of +2.5 to -1.6 ps^2 is experimentally studied. Strongly positively chirped pulses from 4.3 ps (0.01 ps^2) to 39 ps (2.5 ps^2) are obtained at normal net-cavity dispersion. In the anomalous dispersion regime, the laser generates average soliton feature negatively chirped pulses with autocorrelation pulse durations from 0.8 ps (−0.07 ps^2) to 3.9 ps (-1.6 ps^2). The lowered peak power due to the pulse stretching allows one to increase the double pulse threshold. Based on the numerical simulation, different regimes of mode locking are obtained by varying the intra-cavity dispersion, and the characteristics of average soliton, stretched-pulse, wave-breaking-free and chirped-pulse regimes are discussed.
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Interference Carpets in Above-Threshold Ionization: From the Coulomb-Free to the Coulomb-Dominated Regime - Korneev, Ph. A.; Popruzhenko, S. V.; Goreslavski, S. P.; Yan, T.-M.; Bauer, D.; Becker, W.; K{\"{u}}bel, M.; Kling, M. F.; R{\"{o}}del, C.; W{\"{u}}nsche, M.; Paulus, G.G. (Physical Review Letters, 2012)
The velocity map recorded in above-threshold ionization of xenon at 800 nm exhibits a distinct carpetlike pattern of maxima and minima for emission in the direction approximately perpendicular to the laser polarization. The pattern is well reproduced by a numerical solution of the time-dependent Schr{\"{o}}dinger equation. In terms of the simple-man model and the strong-field approximation, it is explained by the constructive and destructive interference of the contribution of the long and the short orbit. Strictly perpendicular emission is caused by ionization at the two peaks of the laser field per cycle, which results in a 2ℏω separation of the above-threshold ionization rings.
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PEBSI – A Monte Carlo simulator for bremsstrahlung arising from electrons colliding with thin solid-state targets - Weber, G.; M{\"{a}rtin, R. and Surzhykov, A. and Yasuda, M. and Yerokhin, V. A. and St{\"{o}}hlker, T. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2012)
We present a Monte Carlo code dedicated to the simulation of bremsstrahlung arising in collisions of polarized electrons with thin target foils. The program consists of an electron transport algorithm taking into account elastic electron-nucleus scattering and inelastic collisions with target electrons as well as a treatment of polarized-electron bremsstrahlung emission. Good agreement is found between the predictions of the electron transport code and data stemming from other simulation programs and experiments. In addition, we present first results from the bremsstrahlung simulation which indicate a significant decrease in the degree of linear polarization of bremsstrahlung even for the thinnest gold targets considered.
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Storage ring at HIE-ISOLDE Technical design report - Grieser, M.; Litvinov, Yu. A.; Raabe, R.; Blaum, K.; Blumenfeld, Y.; Butler, P. A.; Wenander, F.; Woods, P. J.; Aliotta, M.; Andreyev, A.; Artemyev, A.; Atanasov, D.; et al. (European Journal of Physics - Special Topics, 2012)
We propose to install a storage ring at an ISOL-type radioactive beam facility for the first time. Specifically, we intend to setup the heavy-ion, low-energy ring TSR at the HIE-ISOLDE facility in CERN, Geneva. Such a facility will provide a capability for experiments with stored secondary beams that is unique in the world. The envisaged physics programme is rich and varied, spanning from investigations of nuclear ground-state properties and reaction studies of astrophysical relevance, to investigations with highly-charged ions and pure isomeric beams. The TSR might also be employed for removal of isobaric contaminants from stored ion beams and for systematic studies within the neutrino beam programme. In addition to experiments performed using beams recirculating within the ring, cooled beams can also be extracted and exploited by external spectrometers for high-precision measurements. The existing TSR, which is presently in operation at the Max-Planck Institute for Nuclear Physics in Heidelberg, is well-suited and can be employed for this purpose. The physics cases as well as technical details of the existing ring facility and of the beam and infrastructure requirements at HIE-ISOLDE are discussed in the present technical design report.
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Low energy prepulse for 10 Hz operation of a soft-x-ray laser - Habib, J.; Guilbaud, O.; Zielbauer, B.; Zimmer, D.; Pittman, M.; Kazamias, S.; Montet, C.; K{\"{u}}hl, T.; Ros, D. (Optics Express, 2012)
The influence on Nickel-like Molybdenum soft-x-ray laser performance and stability of a low energy laser prepulse arriving prior to the main laser pumping pulses is experimentally investigated. A promising regime for 10 Hz operation has been observed. A four times increase in soft-x-ray laser operation time with a same target surface is demonstrated. This soft-x-ray laser operation mode corresponds to an optimum delay between the prepulse and the main pulses and to a prepulse energy greater than 20 mJ. We also show that this regime is not associated with a weaker degradation of the target or any reduced ablation rate. Therefore the role of preplasma density gradient in this effect is discussed.
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Octave-spanning OPCPA system delivering CEP-stable few-cycle pulses and 22 W of average power at 1 MHz repetition rate - Rothhardt, J.; Demmler, S.; H{\"{a}drich, S. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Express, 2012)
We report on an OPCPA system delivering CEP-stable pulses with a pulse duration of only 1.7 optical cycles at 880 nm wavelength. This pulse duration is achieved by the generation, optical parametric amplification and compression of a full optical octave of bandwidth. The system is pumped by a high average power Yb-fiber laser system, which allows for operation of the OPCPA at up to 1 MHz repetition rate and 22 W of average output power. Further scaling towards single-cycle pulses, higher energy and output power is discussed.
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Review of attosecond resolved measurement and control via carrier-envelope phase tagging with above-threshold ionization - Rathje, T.; Johnson, N.; M{\"{o}}ller, M.; S{\"{u}}ssmann, F.; Adolph, D.; K{\"{u}}bel, M.; Kienberger, R.; Kling, M. F.; Paulus, G.G.; Sayler, A. M. (Journal of Physics B, 2012)
A precise, real-time, single-shot carrier–envelope phase (CEP) tagging technique for few-cycle pulses was developed and combined with cold-target recoil-ion momentum spectroscopy and velocity-map imaging to investigate and control CEP-dependent processes with attosecond resolution. The stability and precision of these new techniques have allowed for the study of intense, few-cycle, laser-matter dynamics with unprecedented detail. Moreover, the same stereo above-threshold ionization (ATI) measurement was expanded to multi-cycle pulses and allows for CEP locking and pulse-length determination. Here we review these techniques and their first applications to waveform characterization and control, non-sequential double ionization of argon, ATI of xenon and electron emission from SiO_2 nanospheres.
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Yb-doped large-pitch fibres: effective single-mode operation based on higher-order mode delocalisation - Limpert, J.; Stutzki, F.; Jansen, F.; Otto, H.-J.; Eidam, T.; Jauregui, C.; T{\"{u}}nnermann, A. (Light: Science { &} Applications, 2012)
Rare earth-doped fibres are a diode-pumped, solid-state laser architecture that is highly scalable in average power. The performance of pulsed fibre laser systems is restricted due to nonlinear effects. Hence, fibre designs that allow for very large mode areas at high average powers with diffraction-limited beam quality are of enormous interest. Ytterbium-doped, rod-type, large-pitch fibres (LPF) enable extreme fibre dimensions, i.e., effective single-mode fibres with mode sizes exceeding 100 times the wavelength of the guided radiation, by exploiting the novel concept of delocalisation of higher-order transverse modes. The non-resonant nature of the operating principle makes LPF suitable for high power extraction. This design allows for an unparalleled level of performance in pulsed fibre lasers.
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26 mJ, 130 W Q-switched fiber-laser system with near-diffraction-limited beam quality - Stutzki, F.; Jansen, F.; Liem, A.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2012)
We demonstrate a Q-switched fiber laser system emitting sub-60 ns pulses with 26 mJ pulse energy and near-diffraction-limited beam quality (M^2 < 1.3). In combination with a repetition rate of 5 kHz, a corresponding average output power of 130 W is achieved. This record performance is enabled by a large-pitch fiber with a core diameter of 135 µm. This fiber allows for effective single-mode operation with mode field diameters larger than 90 µm even at average output powers exceeding 100 W.
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Extreme ultraviolet light source based on intracavity high harmonic generation in a mode locked Ti:sapphire oscillator with 9.4 MHz repetition rate - Seres, E.; Seres, J.; Spielmann, C. (Optics Express, 2012)
We report on the realization of an intracavity high harmonic source with a cutoff above 30 eV. The EUV source is based on a high power, hard-aperture, Kerr-lens mode-locked Ti:sapphire oscillator with a repetition rate of 9.4 MHz. The laser is operated in the net negative dispersion regime resulting in intracavity pulses as short as 17 fs with 1 µJ pulse energy. In a second intracavity focus, intensity more than 10^(14) W/cm^2 has been achieved, which is sufficient for high harmonic generation in a Xenon gas jet.
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Note: A large aperture four-mirror reflective wave-plate for high-intensity short-pulse laser experiments - Aurand, B.; R{\"{o}}del, C.; Zhao, H.; Kuschel, S.; W{\"{u}}nsche, M.; J{\"{a}ckel, O. and Heyer, M. and Wunderlich, F. and Kaluza, M.C. and Paulus, G.G. and K{\"{u}}hl, T. (Review of Scientific Instruments, 2012)
We report on a four-mirror reflective wave-plate system based on a phase-shifting mirror (PSM) for a continuous variation of elliptical polarization without changing the beam position and direction. The system presented and characterized here can replace a conventional retardation plate providing all advantages of a PSM, such as high damage-threshold, large scalability, and low dispersion. This makes reflective wave-plates an ideal tool for ultra-high power laser applications.
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Optical coherence tomography using broad-bandwidth XUV and soft X-ray radiation - Fuchs, S.; Blinne, A.; R{\"{o}}del, C.; Zastrau, U.; Hilbert, V.; W{\"{u}}nsche, M.; Bierbach, J.; Frumker, E.; F{\"{o}}rster, E.; Paulus, G.G. (Applied Physics B, 2012)
We present a novel approach to extend optical coherence tomography (OCT) to the extreme ultraviolet (XUV) and soft X-ray (SXR) spectral range. With a simple setup based on Fourier-domain OCT and adapted for the application of XUV and SXR broadband radiation, cross-sectional images of semiconductors and organic samples becomes feasible with current synchrotron or laser-plasma sources. For this purpose, broadband XUV radiation is focused onto the sample surface, and the reflected spectrum is recorded by an XUV spectrometer. The proposed method has the particular advantage that the axial spatial resolution only depends on the spectral bandwidth. As a consequence, the theoretical resolution limit of XUV coherence tomography (XCT) is in the order of nanometers, e.g., 3 nm for wavelengths in the water window (280 - 530 eV). We proved the concept of XCT by calculating the reflectivity of one-dimensional silicon and boron carbide samples containing buried layers and found the expected properties with respect to resolution and penetration depth confirmed.
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Relativistic calculations of the K-K charge transfer and K-vacancy production probabilities in low-energy ion-atom collisions - Tupitsyn, I. I.; Kozhedub, Y. S.; Shabaev, V. M.; Bondarev, A. I.; Deyneka, G. B.; Maltsev, I. A.; Hagmann, S.; Plunien, G.; St{\"{o}}hlker, T. (Physical Review A, 2012)
The previously developed technique for evaluation of charge transfer and electron-excitation processes in low-energy heavy-ion collisions [ Tupitsyn et al. Phys. Rev. A 82 042701 (2010)] is extended to collisions of ions with neutral atoms. The method employs the active-electron approximation, in which only the active-electron participates in the charge transfer and excitation processes while the passive electrons provide the screening density-functional theory (DFT) potential. The time-dependent Dirac wave function of the active electron is represented as a linear combination of atomic-like Dirac-Fock-Sturm orbitals, localized at the ions (atoms). The screening DFT potential is calculated using the overlapping densities of each ion (atom), derived from the atomic orbitals of the passive electrons. The atomic orbitals are generated by solving numerically the one-center Dirac-Fock and Dirac-Fock-Sturm equations by means of a finite-difference approach with the potential taken as the sum of the exact reference ion (atom) Dirac-Fock potential and of the Coulomb potential from the other ion within the monopole approximation. The method developed is used to calculate the K-K charge transfer and K-vacancy production probabilties for the Ne(1s^(2) 2s^(2) 2p^(6))-F^(8+)(1s) collisions at the F^(8+)(1s) projectile energies 130 and 230 keV/u. The obtained results are compared with experimental data and other theoretical calculations. The K-K charge transfer and K-vacancy production probabilities are also calculated for the Xe-Xe^(53+)(1s) collision.
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Spatially resolved twin domain distribution and lattice parameter variations in the near-surface region of SrTiO_3 single crystals - L{\"{o}}tzsch, R.; Uschmann, I.; F{\"{o}}rster, E. (Applied Physics B, 2012)
We investigated the twin domain distribution and lattice parameter variations associated with the displacive phase transition in SrTiO_3 by means of X-ray diffraction with high spatial resolution. By using 4.5-keV photons, the probed region is the first micrometer near the surface. We find a very inhomogeneous domain distribution, showing both regions with large monodomains and highly twinned regions, as well as large needle domains. Also, the lattice parameters in these different regions vary substantially.
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XUV spectroscopic characterization of warm dense aluminum plasmas generated by the free-electron-laser FLASH - Zastrau, U.; Burian, T.; Chalupsky, J.; D{\"{o}}ppner, T.; Dzelzainis, T. W. J.; F{\"{a}ustlin, R. R. and Fortmann, C. and Galtier, E. and Glenzer, S. H. and Gregori, G. and Juha, L. and Lee, H. J. and Lee, R. W. and Lewis, C. L. S. and Medvedev, N. and Nagler, B. and Nelson, A. J. and Riley, D. and Rosmej, F. B. and Toleikis, S. and Tschentscher, T. and Uschmann, I. and Vinko, S. M. and Wark, J. S. and Whitcher, T. and F{\"{o}}rster, E. (Laser and Particle Beams, 2012)
We report on experiments aimed at the generation and characterization of solid density plasmas at the free-electron laser FLASH in Hamburg. Aluminum samples were irradiated with XUV pulses at 13.5 nm wavelength (92 eV photon energy). The pulses with duration of a few tens of femtoseconds and pulse energy up to 100 µJ are focused to intensities ranging between 10^(13) and 10^(17) W/cm^2. We investigate the absorption and temporal evolution of the sample under irradiation by use of XUV and optical spectroscopy. We discuss the origin of saturable absorption, radiative decay, bremsstrahlung and atomic and ionic line emission. Our experimental results are in good agreement with simulations.
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All-fiber laser source for CARS microscopy based on fiber optical parametric frequency conversion - Baumgartl, M.; Chemnitz, M.; Jauregui, C.; Meyer, T.; Dietzek, B.; Popp, J.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2012)
A novel approach for an all-fiber mono-laser source for CARS microscopy is presented. An Yb-fiber laser generates 100 ps pulses, which later undergo narrowband in-fiber frequency conversion based on degenerate four-wave-mixing. The frequency conversion is optimized to access frequency shifts between 900 and 3200 cm^(−1), relevant for vibrational imaging. Inherently synchronized pump and Stokes pulses are available at one fiber end, readily overlapped in space and time. The source is applied to CARS spectroscopy and microscopy experiments in the CH-stretching region around 3000 cm^(−1). Due to its simplicity and maintenance-free operation, the laser scheme holds great potential for bio-medical applications outside laser laboratories.
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Compact cryogenic source of periodic hydrogen and argon droplet beams for relativistic laser-plasma generation - Fraga, R.; Kalinin, A.; K{\"{u}}hnel, M.; Hochhaus, D. C.; Schottelius, A.; Polz, J.; Kaluza, M.C.; Neumayer, P.; Grisenti, R. E. (Review of Scientific Instruments, 2012)
We present a cryogenic source of periodic streams of micrometer-sized hydrogen and argon droplets as ideal mass-limited target systems for fundamental intense laser-driven plasma applications. The highly compact design combined with a high temporal and spatial droplet stability makes our injector ideally suited for experiments using state-of-the-art high-power lasers in which a precise synchronization between the laser pulses and the droplets is mandatory. We show this by irradiating argon droplets with multi-terawatt pulses.
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Deducing the Electron-Beam Diameter in a Laser-Plasma Accelerator Using X-Ray Betatron Radiation - Schnell, M.; S{\"{a}vert, A. and Landgraf, B. and Reuter, M. and Nicolai, M. and J{\"{a}ckel, O. and Peth, C. and Thiele, T. and Jansen, O. and Pukhov, A. and Willi, O. and Kaluza, M.C. and Spielmann, C. (Physical Review Letters, 2012)
We investigate the properties of a laser-plasma electron accelerator as a bright source of keV x-ray radiation. During the interaction, the electrons undergo betatron oscillations and from the carefully measured x-ray spectrum the oscillation amplitude of the electrons can be deduced which decreases with increasing electron energies. From the oscillation amplitude and the independently measured x-ray source size of (1.8 ± 0.3) μm we are able to estimate the electron bunch diameter to be (1.6 ± 0.3) μm.
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Half-life measurements of stored fully ionized and hydrogen-like I-122 ions - Atanasov, D. R.; Winckler, N.; Balabanski, D.; Batist, L.; Bosch, F.; Boutin, D.; Brandau, C.; Dimopoulou, C.; Essel, H. G.; F{\"{a}stermann, T. and Geissel, H. and Hachiuma, I. and Hess, S. and Izumikawa, T. and Kienle, P. and Kn{\"{o}}bel, R. and Kozhuharov, C. and Kurcewicz, J. and Kuzminchuk, N. and Litvinov, S. A. and Litvinov, Yu. A. and Mao, R. S. and M{\"{a}rtin, R. and Mazzocco, M. and M{\"{u}}nzenberg, G. and Namihira, K. and Nolden, F. and Ohtsubo, T. and Patyk, Z. and Reuschl, R. and Sanjari, M. S. and Scheidenberger, C. and Shubina, D. and Spillmann, U. and Steck, M. and St{\"{o}}hlker, T. and Sun, B. and Suzuki, T. and Trassinelli, M. and Tupitsyn, I. I. and Weick, H. and Winkler, M. and Winters, D. F. A. and Yamaguchi, T. (European Physical Journal A, 2012)
The half-lives of fully ionized and hydrogen-like (H-like) I-122 ions have been measured in a heavy-ion storage ring. The β^(+)-decay constants for both charge states and the electron capture (EC) decay constant of H-like ions have been determined. The EC-decay constant in H-like I-122 ions λ^(H-like)_(EC) = 7.35(33) · 10^(−4) s^(−1) is, within the uncertainty, the same as the one in neutral atoms. This result is in agreement with the estimates of recent theoretical considerations on the EC-decay of few-electron ions that explicitly take into account the conservation of the total angular momentum of the nucleus plus lepton(s) system and its projections. No firm confirmation could be concluded from our results on the predicted effect that allowed Gamow-Teller transitions become forbidden if the initial and final total angular momenta are not equal.
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Improving high-order harmonic yield using wavefront-controlled ultrashort laser pulses - Eyring, S.; Kern, C.; Z{\"{u}}rch, M.; Spielmann, C. (Optics Express, 2012)
In this work we show that it is possible to increase the high-order harmonic yield when using wavefront-shaped laser beams. The investigation of the beam profile near the interaction region shows that the optimized beam is asymmetric and has a larger diameter. Thus, the optimized beam leads to a higher yield even if the peak intensity is lower compared to an unoptimized beam. This indicates that the wavefront of the fundamental laser beam and, accordingly, the focal profile play an important role in the efficient generation of high-order harmonic radiation.
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Pulsed operation of a high average power Yb:YAG thin-disk multipass amplifier - Schulz, M.; Riedel, R.; Willner, A.; Duesterer, S.; Prandolini, M. J.; Feldhaus, J.; Faatz, B.; Rossbach, J.; Drescher, M.; Tavella, F. (Optics Express, 2012)
An Yb:YAG thin-disk multipass laser amplifier system was developed operating in a 10 Hz burst operation mode with 800 µs burst duration and 100 kHz intra-burst repetition rate. Methods for the suppression of parasitic amplified spontaneous emission are presented. The average output pulse energy is up to 44.5 mJ and 820 fs compressed pulse duration. The average power of 4.45 kW during the burst is the highest reported for this type of amplifier.
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Thermally induced waveguide changes in active fibers - Jansen, F.; Stutzki, F.; Otto, H.-J.; Eidam, T.; Liem, A.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2012)
Thermally induced waveguide changes become significant for very large mode area fibers. This results in a reduction of the mode-field diameter, but simultaneously in an improvement of the beam quality. In this work the first systematic experimental characterization of the reduction of the mode-field diameter in various fibers during high-power operation is carried out. It is shown that the reduction of the mode-field diameter shows a characteristic behavior that scales with the core size but that is independent of the particular fiber design. Furthermore, the strength of the actual index change is experimentally estimated, and its use to overcome avoided crossings is discussed and experimentally demonstrated.
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Lambda_MS from the static potential for QCD with n_f=2 dynamical quark flavors - Jansen, K.; Karbstein, F.; Nagy, A.; Wagner, M. (Journal of High Energy Physics, 2012)
We determine Lambda((MS)over-bar) for QCD with n_f = 2 dynamical quark flavors by fitting the QQ̅ static potential known analytically in the perturbative regime up to terms of O(α^4_s) and α^4_s ln(α_s) to corresponding results obtained from lattice simulations. This has become possible, due to recent advances in both perturbative calculations, namely the determination and publication of the last missing contribution to the QQ̅ static potential at O(α^4_s), and lattice simulations with n_f = 2 dynamical quark flavors performed at the rather fine lattice spacing of a ≈ 0.042 fm. Imposing conservative error estimates we obtain Lambda((MS)over-bar) = 315(30) MeV.
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Temperature-induced index gratings and their impact on mode instabilities in high-power fiber laser systems - Jauregui, C.; Eidam, T.; Otto, H.-J.; Stutzki, F.; Jansen, F.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2012)
Mode-interference along an active fiber in high-power operation gives rise to a longitudinally oscillating temperature profile which, in turn, is converted into a strong index grating via the thermo-optic effect. In the case of mode beating between the fundamental mode and a radially anti-symmetric mode such a grating exhibits two periodic features: a main one which is radially symmetric and has half the period of the modal beating, and a second one that closely follows the mode interference pattern and has its same period. In the case of modal beating between two radially symmetric modes the thermally induced grating only has radially symmetric features and exhibits the same period of the mode interference. The relevance of such gratings in the context of the recently observed mode instabilities of high-power fiber laser systems is discussed.
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1 MHz repetition rate hollow fiber pulse compression to sub-100-fs duration at 100 W average power - Rothhardt, J.; H{\"{a}drich, S. and Carstens, H. and Herrick, N. and Demmler, S. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2011)
We report on nonlinear pulse compression at very high average power. A high-power fiber chirped pulse amplification system based on a novel large pitch photonic crystal fiber delivers 700 fs pulses with 200 μJ pulse energy at a 1 MHz repetition rate, resulting in 200 W of average power. Subsequent spectral broadening in a xenon-filled hollow-core fiber and pulse compression with chirped mirrors is employed for pulse shortening and peak power enhancement. For the first time, to our knowledge, more than 100 W of average power are transmitted through a noble-gas-filled hollow fiber. After pulse compression of 81 fs, 93 μJ pulses are obtained at a 1 MHz repetition rate.
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Basic considerations on coherent combining of ultrashort laser pulses - Klenke, A.; Seise, E.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2011)
Coherent combining is a novel approach to scale the performance of laser amplifiers. The use of ultrashort pulses in a coherent combining setup results in new challenges compared to continuous wave operation or to pulses on the nanosecond timescale, because temporal and spectral effects such as self-phase modulation, dispersion and the optical path length difference between the pulses have to be considered. In this paper the impact of these effects on the combining process has been investigated and simple analytical equations for the evaluation of this impact have been obtained. These formulas provide design guidelines for laser systems using coherent combining. The results show that, in spite of the temporal and spectral effects mentioned above, for a carefully adjusted and stabilized system an excellent efficiency of the combining process can still be achieved.
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Carrier-envelope phase effect in the yield of sequential ionization by an intense few-cycle laser pulse - Shvetsov-Shilovski, N. I.; Sayler, A. M.; Rathje, T.; Paulus, G.G. (New Journal of Physics, 2011)
The relative yield of highly charged atomic ions produced by a short (4–6 fs at FWHM) intense (10^14 – 5 × 10^18 W cm^(−2)) laser pulse was investigated by numerical solution of the rate equations. We predict oscillations of the ion yield as a function of the absolute phase. A distinctive property of this phase dependence is that it can only be observed when at least two ions have comparable yields. It is shown that with currently available laser systems the effect should be experimentally detectable for various rare gas atoms: Xe, Kr, Ar and Ne.
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Energy dispersion in radiation pressure accelerated ion beams - Grech, M.; Skupin, S.; Diaw, A.; Schlegel, T.; Tikhonchuk, V. T. (New Journal of Physics, 2011)
We address the problem of energy dispersion of radiation pressure accelerated (RPA) ion beams emerging from a thin target. Two different acceleration regimes, namely phase-stable acceleration and multistage acceleration, are considered by means of analytical modeling and one-dimensional particle-in-cell simulations. Our investigations offer a deeper understanding of RPA and allow us to derive some guidelines for generating monoenergetic ion beams.
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High-speed modal decomposition of mode instabilities in high-power fiber lasers - Stutzki, F.; Otto, H.; Jansen, F.; Gaida, C.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2011)
A high-speed mode analysis technique is required to gain fundamental understanding of mode instabilities in high-power fiber laser systems. In this work a technique, purely based on the intensity profile of the beam, is demonstrated to be ideally suited to analyze fiber laser dynamics. This technique, together with a high-speed camera, has been applied to the study of the temporal dynamics of mode instabilities at high average powers with up to 20,000 frames per second. These measurements confirm that energy transfer between the fluctuating transversal modes takes place in millisecond-time-scale.
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Coherently-combined two channel femtosecond fiber CPA system producing 3 mJ pulse energy - Klenke, A.; Seise, E.; Demmler, S.; Rothhardt, J.; Breitkopf, S.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2011)
We present a fiber CPA system consisting of two coherently combined fiber amplifiers, which have been arranged in an actively stabilized Mach-Zehnder interferometer. Pulse durations as short as 470 fs and pulse energies of 3 mJ, corresponding to 5.4 GW of peak power, have been achieved at an average power of 30 W.
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Efficient control of quantum paths via dual-gas high harmonic generation - Willner, A.; Tavella, F.; Yeung, M.; Dzelzainis, T.; Kamperidis, C.; Bakarezos, M.; Adams, D.; Riedel, R.; Schulz, M.; Hoffmann, M. C.; Hu, W.; Rossbach, J.; et al. (New Journal of Physics, 2011)
The accurate control of the relative phase of multiple distinct sources of radiation produced by high harmonic generation is of central importance in the continued development of coherent extreme UV (XUV) and attosecond sources. Here, we present a novel approach which allows extremely accurate phase control between multiple sources of high harmonic radiation generated within the Rayleigh range of a single-femtosecond laser pulse using a dual-gas, multi-jet array. Fully ionized hydrogen acts as a purely passive medium and allows highly accurate control of the relative phase between each harmonic source. Consequently, this method allows quantum path selection and rapid signal growth via the full coherent superposition of multiple HHG sources (the so-called quasi-phase-matching). Numerical simulations elucidate the complex interplay between the distinct quantum paths observed in our proof-of-principle experiments.
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Energy loss and cooling of relativistic highly charged uranium ions interacting with an internal hydrogen droplet target beam - Petridis, N.; Kalinin, A.; Popp, U.; Gostishchev, V.; Litvinov, Y.; Dimopoulou, C.; Nolden, F.; Steck, M.; Kozhuharov, C.; Thorn, D. B.; Gumberidze, A.; Trotsenko, S.; et al. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2011)
We present measurements of the energy loss of relativistic highly charged uranium ions interacting with a target beam of near-liquid density hydrogen droplets at the experimental storage ring (ESR) at GSI. Our results reveal that a liquid droplet target beam virtually behaves like a homogeneous gas jet target with respect to both energy loss and ion beam cooling. We also provide first results on ion beam cooling efficiency at high hydrogen area target densities, which are consistent with numerical estimations based on a simple model of the cooling force.
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Parity nonconservation effect in resonance recombination of polarized electrons with heavy hydrogen-like ions - Maiorova, A. V.; Shabaev, V. M.; Volotka, A. V.; Zaytsev, V. A.; Plunien, G.; St{\"{o}}hlker, T. (Journal of Physics B, 2011)
Parity nonconservation (PNC) effect in recombination of a polarized electron with a heavy H-like ion in the case of resonance with a doubly excited state of the corresponding He-like ion is studied. It is assumed that photons of the energy corresponding to the one-photon decay of the doubly excited state into the 2 1S0 or 2 3P0 state are detected at a given angle with respect to the incident electron momentum. Calculations are performed for helium-like thorium (Z = 90) and gadolinium (Z = 64), where the 2 1^S_0 and 2 3^P_0 levels are near to cross and, therefore, the PNC effect is strongly enhanced.
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Proton probe measurement of fast advection of magnetic fields by hot electrons - Willingale, L.; Thomas, A. G. R.; Nilson, P. M.; Kaluza, M.C.; Bandyopadhyay, S.; Dangor, A. E.; Evans, R. G.; Fernandes, P.; Haines, M. G.; Kamperidis, C.; Kingham, R. J.; Minardi, S.; et al. (Plasma Physics and Controlled Fusion, 2011)
A laser generated proton beam was used to measure the megagauss strength self-generated magnetic fields from a nanosecond laser interaction with an aluminum target. At intensities of 10^15 W cm^−2 , the significant hot electron production and strong heat fluxes result in non-local transport becoming important to describe the magnetic field dynamics. Two-dimensional implicit Vlasov–Fokker–Planck modeling shows that fast advection of the magnetic field from the focal region occurs via the Nernst effect at significantly higher velocities than the sound speed, v_N / c_s ≈ 10.
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88 W 0.5 mJ femtosecond laser pulses from two coherently combined fiber amplifiers - Seise, E.; Klenke, A.; Breitkopf, S.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2011)
The generation of 0.5 mJ femtosecond laser pulses by coherent combining of two high power high energy fiber chirped-pulse amplifiers is reported. The system is running at a repetition frequency of 175 kHz producing 88 W of average power after the compressor unit. Polarizing beam splitters have been used to realize an amplifying Mach–Zehnder interferometer, which has been stabilized with a H{\"{a}nsch–Couillaud measurement system. The stabilized system possesses a measured residual rms phase difference fluctuation between the two branches as low as λ/70 rad at the maximum power level. The experiment proves that coherent addition of femtosecond fiber lasers can be efficiently and reliably performed at high B-integral and considerable thermal load in the individual amplifiers.
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Coherent Control of High Harmonic Generation via Dual-Gas Multijet Arrays - Willner, A.; Tavella, F.; Yeung, M.; Dzelzainis, T.; Kamperidis, C.; Bakarezos, M.; Adams, D.; Schulz, M.; Riedel, R.; Hoffmann, M. C.; Hu, W.; Rossbach, J.; et al. (Physical Review Letters, 2011)
High harmonic generation (HHG) is a central driver of the rapidly growing field of ultrafast science. We present a novel quasiphase-matching (QPM) concept with a dual-gas multijet target leading, for the first time, to remarkable phase control between multiple HHG sources (>2) within the Rayleigh range. The alternating jet structure with driving and matching zones shows perfect coherent buildup for up to six QPM periods. Although not in the focus of the proof-of-principle studies presented here, we achieved competitive conversion efficiencies already in this early stage of development.
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Ellipticity dependence of 400 nm-driven high harmonic generation - Khan, S.; Cheng, Y.; M{\"{o}}ller, M.; Zhao, K.; Zhao, B.; Chini, M.; Paulus, G.; Chang, Z. (Applied Physics Letters, 2011)
We studied the dependence of high harmonic generation efficiency on the ellipticity of 400 nm driving laser pulses at 7.7 × 10^14 W/cm2 and compared it with the 800 nm driving laser under the same conditions. The measured decrease of high harmonic yield with the ellipticity of the 400 nm laser is ∼1.5 times slower that of the 800 nm, which agrees well with theoretical predictions based on a semi-classical model. The results indicate that it is feasible to use the generalized double optical gating with 400 nm lasers for extracting single attosecond pulses with high efficiency.
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