Between atomic and nuclear physics: radioactive decays of highly-charged ions - Atanasov, D.; Blaum, K.; Bosch, F.; Brandau, C.; B{\"{u}}hler, P.; Chen, X.; Dillmann, I.; Faestermann, T.; Gao, B.; Geissel, H.; Gernh{\"{a}user, R. and Hagmann, S. and Izumikawa, T. and Hillenbrand, P.-M. and Kozhuharov, C. and Kurcewicz, J. and Litvinov, S. and Litvinov, Y. and Ma, X. and M{\"{u}}nzenberg, G. and Najafi, M. and Nolden, F. and Ohtsubo, T. and Ozawa, A. and Ozturk, F. and Patyk, Z. and Reed, M. and Reifarth, R. and Sanjari, M. and Schneider, D. and Steck, M. and St{\"{o}}hlker, T. and Sun, B. and Suzaki, F. and Suzuki, T. and Trageser, C. and Tu, X. and Uesaka, T. and Walker, P. and Wang, M. and Weick, H. and Winckler, N. and Woods, P. and Xu, H. and Yamaguchi, T. and Yan, X. and Zhang, Y. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2015)
Highly charged radioactive ions can be stored for extended periods of time in storage rings which allows for precision measurements of their decay modes. The straightforward motivation for performing such studies is that fully ionised nuclei or few-electron ions can be viewed as clean quantum-mechanical systems, in which the interactions of the many electrons can be either excluded or treated precisely. Thus, the influence of the electron shell on the decay probability can be investigated. Another important motivation is stellar nucleosynthesis, which proceeds at high temperatures and the involved atoms are therefore highly ionised. Presented here is a compact review of the relevant experiments conducted at heavy-ion storage rings. Furthermore, we outline the perspectives for future experiments at new-generation storage-ring facilities.
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Calibration of time of flight detectors using laser-driven neutron source - Mirfayzi, S. R.; Kar, S.; Ahmed, H.; Krygier, A. G.; Green, A.; Alejo, A.; Clarke, R.; Freeman, R. R.; Fuchs, J.; Jung, D.; Kleinschmidt, A.; Morrison, J. T.; et al. (Review of Scientific Instruments, 2015)
Calibration of three scintillators (EJ232Q, BC422Q, and EJ410) in a time-of-flight arrangement using a laser drive-neutron source is presented. The three plastic scintillator detectors were calibrated with gamma insensitive bubble detector spectrometers, which were absolutely calibrated over a wide range of neutron energies ranging from sub-MeV to 20 MeV. A typical set of data obtained simultaneously by the detectors is shown, measuring the neutron spectrum emitted from a petawatt laser irradiated thin foil.
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Compton scattering of twisted light: Angular distribution and polarization of scattered photons - Stock, S.; Surzhykov, A.; Fritzsche, S.; Seipt, D. (Physical Review A, 2015)
Compton scattering of twisted photons is investigated within a nonrelativistic framework using first-order perturbation theory. We formulate the problem in the density-matrix theory, which enables one to gain new insights into scattering processes of twisted particles by exploiting the symmetries of the system. In particular, we analyze how the angular distribution and polarization of the scattered photons are affected by the parameters of the initial beam such as the opening angle and the projection of orbital angular momentum. We present analytical and numerical results for the angular distribution and the polarization of Compton scattered photons for initially twisted light and compare them with the standard case of plane-wave light.
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Direct Observation of the Injection Dynamics of a Laser Wakefield Accelerator Using Few-Femtosecond Shadowgraphy - S{\"{a}vert, A. and Mangles, S. P. D. and Schnell, M. and Siminos, E. and Cole, J. M. and Leier, M. and Reuter, M. and Schwab, M. B. and M{\"{o}}ller, M. and Poder, K. and J{\"{a}ckel, O. and Paulus, G.G. and Spielmann, C. and Skupin, S. and Najmudin, Z. and Kaluza, M.C. (Physical Review Letters, 2015)
We present few-femtosecond shadowgraphic snapshots taken during the nonlinear evolution of the plasma wave in a laser wakefield accelerator with transverse synchronized few-cycle probe pulses. These snapshots can be directly associated with the electron density distribution within the plasma wave and give quantitative information about its size and shape. Our results show that self-injection of electrons into the first plasma-wave period is induced by a lengthening of the first plasma period. Three-dimensional particle-in-cell simulations support our observations.
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Double-stage soft x-ray laser pumped by multiple pulses applied in grazing incidence - Ecker, B.; Aurand, B.; Hochhaus, D. C.; Neumayer, P.; Zielbauer, B.; Oliva, E.; Li, L.; Le, T. T. T.; Jin, Q.; Zhao, H.; Cassou, K.; Daboussi, S.; et al. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2015)
In this paper we report on results obtained with a compact double-stage molybdenum x-ray laser (XRL), operated with a total pump energy of 600 mJ. The two gain regions were pumped using the double-pulse grazing incidence pumping technique, which includes travelling wave excitation for both the seed- and the amplifier-target. In addition, the influence of an additional pre-pulse has been studied. Seeded XRL operation has been demonstrated in both schemes, resulting in XRL pulses with a divergence of 2×2 mrad. The peak brilliance of the amplified XRL of 4×10²³ photons/s/mm²/mrad² in 5×10⁻⁵ relative bandwidth was more than two orders of magnitude larger compared to the original seed pulses. The presented experimental concept provides an alternative approach to the currently more common use of high-order harmonic pulses as a seed source, well suited for applications like laser spectroscopy of highly-charged ions at a storage ring.
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Electron impact ionization cross-sections for few-electron uranium ions - Guerra, M.; St{\"{o}}hlker, T.; Amaro, P.; Machado, J.; Santos, J. P. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2015)
Electron impact ionization cross sections for the U^88+ , U^89+ , U^90+ and U^91+ ions were calculated with the relativistic binary encounter Bethe model (RBEB), the modified RBEB (MRBEB) and the new MRBEB corrected by the ionic factor (MRBR–IF). Our results were compared with the available three sets of experimental data and the most used theoretical results. The MRBEB–IF results are the ones that better agree with the experimental data of the four analysed ions.
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FAIR - Cosmic Matter in the Laboratory - St{\"{o}}cker, H.; St{\"{o}}hlker, T.; Sturm, C. (Journal of Physics: Conference Series, 2015)
To explore cosmic matter in the laboratory {–} this fascinating research prospect becomes available at the Facility for Antiproton and Ion Research, FAIR. The new facility is being constructed within the next five years adjacent to the existing accelerator complex of the GSI Helmholtz Centre for Heavy Ion Research at Darmstadt/Germany, expanding the research goals and technical possibilities substantially. This includes new insights into the dynamics of supernovae depending on the properties of short-lived neutron-rich nuclei which will be investigated with intense rare isotope beams. New insights will be provided into the interior of stars by exploring dense plasmas with intense heavy-ion beams combined with a high-performance laser {–} or into neutron star cores by probing the highest baryon densities in relativistic nucleus{–}nucleus collisions at unprecedented collision rates. To the latter, the properties of hadrons play an important part which will be systematically studied by high precision hadron spectroscopy with antiproton beams at unmatched intensities. The worldwide unique accelerator and experimental facilities of FAIR will open the way for a broad spectrum of unprecedented fore-front research supplying a large variety of experiments in hadron, nuclear, atomic and plasma physics as well as biomedical and material science which will be briefly described in this article. This article is based on the FAIR Green Paper and gives an update of former publications.
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Scattering of twisted relativistic electrons by atoms - Serbo, V.; Ivanov, I. P.; Fritzsche, S.; Seipt, D.; Surzhykov, A. (Physical Review A, 2015)
The Mott scattering of high-energetic twisted electrons by atoms is investigated within the framework of the first Born approximation and Dirac's relativistic equation. Special emphasis is placed on the angular distribution and longitudinal polarization of the scattered electrons. In order to evaluate these angular and polarization properties we consider two experimental setups in which the twisted electron beam collides with either a single well-localized atom or macroscopic atomic target. Detailed relativistic calculations have been performed for both setups and for the electrons with kinetic energy from 10 to 1000 keV. The results of these calculations indicate that the emission pattern and polarization of outgoing electrons differ significantly from the scattering of plane-wave electrons and can be very sensitive to the parameters of the incident twisted beam. In particular, it is shown that the angular- and polarization-sensitive Mott measurements may reveal valuable information about both the transverse and longitudinal components of the linear momentum and the projection of the total angular momentum of twisted electron states. Thus, the Mott scattering emerges as a diagnostic tool for the relativistic vortex beams.
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Towards highest peak intensities for ultra-short MeV-range ion bunches - Busold, S.; Schumacher, D.; Brabetz, C.; Jahn, D.; Kroll, F.; Deppert, O.; Schramm, U.; Cowan, T.; Blazevic, A.; Bagnoud, V.; Roth, M. (Scientific Reports, 2015)
A laser-driven, multi-MeV-range ion beamline has been installed at the GSI Helmholtz center for heavy ion research. The high-power laser PHELIX drives the very short (picosecond) ion acceleration on μm scale, with energies ranging up to 28.4 MeV for protons in a continuous spectrum. The necessary beam shaping behind the source is accomplished by applying magnetic ion lenses like solenoids and quadrupoles and a radiofrequency cavity. Based on the unique beam properties from the laser-driven source, high-current single bunches could be produced and characterized in a recent experiment: At a central energy of 7.8MeV, up to 5×10^8 protons could be re-focused in time to a FWHM bunch length of τ=(462±40) ps via phase focusing. The bunches show a moderate energy spread between 10% and 15% (ΔE/E0 at FWHM) and are available at 6m distance to the source und thus separated from the harsh laser-matter interaction environment. These successful experiments represent the basis for developing novel laser-driven ion beamlines and accessing highest peak intensities for ultra-short MeV-range ion bunches.
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Collinear laser spectroscopy of atomic cadmium: Extraction of nuclear magnetic dipole and electric quadrupole moments - Fr{\"{o}}mmgen, N.; Balabanski, D. L.; Bissell, M. L.; Bieron, J.; Blaum, K.; Cheal, B.; Flanagan, K.; Fritzsche, S.; Geppert, C.; Hammen, M.; Kowalska, M.; Kreim, K.; et al. (European Physical Journal D, 2015)
Hyperfine structure A and B factors of the atomic 5s5p ³P₂ -> 5s6s ³S₁ transition are determined from collinear laser spectroscopy data of ¹⁰⁷−¹²³Cd and ¹¹¹m−¹²³mCd. Nuclear magnetic moments and electric quadrupole moments are extracted using reference dipole moments and calculated electric field gradients, respectively. The hyperfine structure anomaly for isotopes with s₁/₂ and d₅/₂ nuclear ground states and isomeric h11/2 states is evaluated and a linear relationship is observed for all nuclear states except s₁/₂. This corresponds to the Moskowitz-Lombardi rule that was established in the mercury region of the nuclear chart but in the case of cadmium the slope is distinctively smaller than for mercury. In total four atomic and ionic levels were analyzed and all of them exhibit a similar behaviour. The electric field gradient for the atomic 5s5p ³P₂ level is derived from multi-configuration Dirac-Hartree-Fock calculations in order to evaluate the spectroscopic nuclear quadrupole moments. The results are consistent with those obtained in an ionic transition and based on a similar calculation.
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Combined linear polarization and angular distribution measurements of x-rays for precise determination of multipole-mixing in characteristic transitions of high- Z systems - Weber, G.; Br{\"{a}uning, H. and Surzhykov, A. and Brandau, C. and Fritzsche, S. and Geyer, S. and Grisenti, R. E. and Hagmann, S. and Hahn, C. and Hess, R. and Hess, S. and Kozhuharov, C. and K{\"{u}}hnel, M. and M{\"{a}rtin, R. and Petridis, N. and Spillmann, U. and Trotsenko, S. and Winters, D. F. A. and St{\"{o}}hlker, T. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2015)
By applying novel-type position sensitive x-ray detectors as Compton polarimeters we recently performed a study of the linear polarization of Lyman-α₁ radiation following radiative electron capture into initially bare uranium ions. It was found that a model-independent determination of the ratio of the E1 and M2 transition amplitudes, and consequently of the corresponding transition rates, is feasible by combining the linear polarization data with a measurement of the angular distribution of the emitted radiation. In this work a detailed description of the underlying experimental technique for combined measurements of the linear polarization and the angular distribution of characteristic transitions in high-Z ions is presented. Special emphasis is given to the application of two, two-dimensional position-sensitive x-ray detectors for Compton polarimetry of hard x-rays. Moreover, we demonstrate the polarimeter efficiency of such detector systems can be significantly improved if events, where the charge is spread over neighboring segments, are reconstructed to be used in the polarization analysis.
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Crystal optics for precision x-ray spectroscopy on highly charged ions{–}conception and proof - Beyer, H. F.; Gassner, T.; Trassinelli, M.; Hess, R.; Spillmann, U.; Bana{ś}, D.; Blumenhagen, K.-H.; Bosch, F.; Brandau, C.; Chen, W.; Dimopoulou, C.; F{\"{o}}rster, E.; et al. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2015)
The experimental investigation of quantum-electrodydamic contributions to the binding energies of inner shells of highly charged heavy ions requires an accurate spectroscopy in the region of hard x-rays suitable at a limited source strength. For this purpose the focusing compensated asymmetric Laue crystal optics has been developed and a twin-spectrometer assembly has been built and commissioned at the experimental storage ring of the GSI Helmholtzzentrum Darmstadt. We characterize the crystal optics and demonstrate the usefulness of the instrumentation for accurate spectroscopy of both stationary and fast moving x-ray sources. The experimental procedures discussed here may also be applied for other spectroscopic studies where a transition from conventional germanium x-ray detectors to crystal spectrometers seems too demanding because of low source intensity.
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Ground-state excitation of heavy highly-charged ions - Gumberidze, A.; Thorn, D. B.; Fontes, C. J.; Najjari, B.; Zhang, H. L.; Surzhykov, A.; Voitkiv, A.; Fritzsche, S.; Bana{ś}, D.; Beyer, H.; Chen, W.; DuBois, R. D.; et al. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2015)
We have studied the excitation of H-like and He-like uranium (U^91+ and U^90+ ) in relativistic collisions with gaseous targets by observing the subsequent x-ray emission. The experiment was conducted at the ESR storage ring of the GSI accelerator facility in Darmstadt, Germany. The measurements were performed with a newly developed multi-phase target at different collision energies. This enabled us to explore the proton (nucleus) impact excitation as well as the electron impact excitation processes in the relativistic collisions. The large fine-structure splitting in uranium allowed us to unambiguously resolve excitation to different L-shell levels. Moreover, information about the population of different magnetic sublevels has been obtained via an angular differential study of the decay photons associated with the subsequent de-excitation process. The experimental results are compared with calculations performed within the relativistic framework including excitation mechanisms due to both protons (nucleus) and electrons.
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Impact of atmospheric molecular absorption on the temporal and spatial evolution of ultra-short optical pulses - Gebhardt, M.; Gaida, C.; Stutzki, F.; H{\"{a}drich, S. and Jauregui, C. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Express, 2015)
We present a rigorous study on the impact of atmospheric molecular absorption on the linear propagation of ultrashort pulses in the mid-infrared wavelength region. An ultrafast thulium-based fiber laser was employed to experimentally investigate ultrashort-pulse propagation through the atmosphere in a spectral region containing several strong molecular absorption lines. The atmospheric absorption profile causes a significant degradation of the pulse quality in the time domain as well as a distortion of the transverse beam profile in the spatial domain. Numerical simulations carried out in the small signal limit accurately reproduce the experimental observations in the time domain and reveal that the relative loss in peak power after propagation can be more than twice as high as the relative amount of absorbed average power. Although their nature is purely linear (i.e. the intensities considered are sufficiently low) the discussed effects represent significant challenges to performance-scaling of mid-infrared ultrafast lasers operating in spectral regions with molecular absorption bands. Guidelines for an efficient mitigation of the pulse quality degradation and the beam profile distortion are discussed.
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Impact of photodarkening on the mode instability threshold - Otto, H.-J.; Modsching, N.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Express, 2015)
The threshold-like onset of mode instabilities is currently the main limitation for the scaling of the average output power of fiber laser systems with diffraction limited beam quality. In this contribution, the impact of a wavelength shift of the seed signal on the mode instability threshold has been investigated. Against expectations, it is experimentally shown that the highest mode instabilities threshold is reached around 1030 nm and not for the smallest wavelength separation between pump and signal. This finding implies that the quantum defect is not the only source of thermal heating in the fiber. Systematic experiments and simulations have helped in identifying photodarkening as the most likely second heat source in the fiber. It is shown that even a negligible photodarkening-induced power loss can lead to a decrease of the mode instabilities threshold by a factor of two. Consequently, reduction of photodarkening is a promising way to mitigate mode instabilities.
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Laser spectroscopy of the ground-state hyperfine structure in H-like and Li-like bismuth - Vollbrecht, J.; Andelkovic, Z.; Dax, A.; Geithner, W.; Geppert, C.; Gorges, C.; Hammen, M.; Hannen, V.; Kaufmann, S.; K{\"{o}}nig, K.; Litvinov, Y.; Lochmann, M.; et al. (Journal of Physics: Conference Series, 2015)
The LIBELLE experiment performed at the experimental storage ring (ESR) at the GSI Helmholtz Center in Darmstadt aims for the determination of the ground state hyperfine (HFS) transitions and lifetimes in hydrogen-like ( 209 Bi82+ ) and lithium-like ( 209 Bi 0+ ) bismuth. The study of HFS transitions in highly charged ions enables precision tests of QED in extreme electric and magnetic fields otherwise not attainable in laboratory experiments. While the HFS transition in H-like bismuth was already observed in earlier experiments at the ESR, the LIBELLE experiment succeeded for the first time to measure the HFS transition in Li-like bismuth in a laser spectroscopy experiment.
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Nonlinear compression of an ultrashort-pulse thulium-based fiber laser to sub-70 fs in Kagome photonic crystal fiber - Gebhardt, M.; Gaida, C.; H{\"{a}drich, S. and Stutzki, F. and Jauregui, C. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2015)
Nonlinear pulse compression of ultrashort pulses is an established method for reducing the pulse duration and increasing the pulse peak power beyond the intrinsic limits of a given laser architecture. In this proof-of-principle experiment, we demonstrate nonlinear compression of the pulses emitted by a high-repetition-rate thulium-based fiber CPA system. The initial pulse duration of about 400 fs has been shortened to <70 fs with 19.7 μJ of pulse energy, which corresponds to about 200 MW of pulse peak power.
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Quantum Electrodynamical Corrections to Energy Levels of Diatomic Quasimolecules - Artemyev, A. N.; Surzhykov, A. (Physical Review Letters, 2015)
We elaborate an ab initio approach for the evaluation of the one-loop quantum electrodynamical corrections to energy levels of diatomic quasimolecules. The approach accounts for the interaction between an electron and two nuclei in all orders in Zα and can be applied for a wide range of internuclear distances, up to R≈1000 fm. Based on the developed theory, detailed calculations are performed for the self-energy and vacuum-polarization corrections to the energy of the 1σg ground state of the U92+–U91+ dimer that can be produced in slow ion-ion collisions. The calculations predict the remarkable energy shift that arises due to the nonspherical contributions to the electron-nuclei potential taken beyond the standard monopole approximation.
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Search for inhomogeneous phases in fermionic models - Braun, J.; Finkbeiner, S.; Karbstein, F.; Roscher, D. (Physical Review D, 2015)
We revisit the Gross-Neveu model with N fermion flavors in 1+1 dimensions and compute its phase diagram at finite temperature and chemical potential in the large-N limit. To this end, we double the number of fermion degrees of freedom in a specific way which allows us to detect inhomogeneous phases in an efficient manner. We show analytically that this “fermion doubling trick” predicts correctly the position of the boundary between the chirally symmetric phase and the phase with broken chiral symmetry. Most importantly, we find that the emergence of an inhomogeneous ground state is predicted correctly. We critically analyze our approach based on this trick and discuss its applicability to other theories, such as fermionic models in higher dimensions, where it may be used to guide the search for inhomogeneous phases.
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Simultaneous observation of angularly separated laser-driven proton beams accelerated via two different mechanisms - Wagner, F.; Bedacht, S.; Bagnoud, V.; Deppert, O.; Geschwind, S.; Jaeger, R.; Ortner, A.; Tebartz, A.; Zielbauer, B.; Hoffmann, D. H. H.; Roth, M. (Physics of Plasmas, 2015)
We present experimental data showing an angular separation of laser accelerated proton beams. Using flat plastic targets with thicknesses ranging from 200 nm to 1200 nm, a laser intensity of 6×10²⁰ W cm⁻² incident with an angle of 10°, we observe accelerated protons in target normal direction with cutoff energies around 30 MeV independent from the target thickness. For the best match of laser and target conditions, an additional proton signature is detected along the laser axis with a maximum energy of 65 MeV. These different beams can be attributed to two acceleration mechanisms acting simultaneously, i.e., target normal sheath acceleration and acceleration based on relativistic transparency, e.g., laser breakout afterburner, respectively.
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Stimulated photon emission from the vacuum - Karbstein, F.; Shaisultanov, R. (Physical Review D, 2015)
We study the effect of stimulated photon emission from the vacuum in strong space-time-dependent electromagnetic fields. We emphasize the viewpoint that the vacuum subjected to macroscopic electromagnetic fields with at least one nonzero electromagnetic field invariant, as, e.g., attainable by superimposing two laser beams, can represent a source term for outgoing photons. We believe that this view is particularly intuitive and allows for a straightforward and intuitive study of optical signatures of quantum vacuum nonlinearity in realistic experiments involving the collision of high-intensity laser pulses, and exemplify this view for the vacuum subjected to a strong standing electromagnetic wave as generated in the focal spot of two counterpropagating, linearly polarized, high-intensity laser pulses. Focusing on a comparably simple electromagnetic field profile, which should nevertheless capture the essential features of the electromagnetic fields generated in the focal spots of real high-intensity laser beams, we provide estimates for emission characteristics and the numbers of emitted photons attainable with present and near future high-intensity laser facilities.
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An improved value for the hyperfine splitting of hydrogen-like 209 Bi 82+ - Ullmann, J.; Andelkovic, Z.; Dax, A.; Geithner, W.; Geppert, C.; Gorges, C.; Hammen, M.; Hannen, V.; Kaufmann, S.; K{\"{o}}nig, K.; Litvinov, Y.; Lochmann, M.; et al. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2015)
We report an improved measurement of the hyperfine splitting in hydrogen-like bismuth (209 Bi^82+) at the experimental storage ring ESR at GSI by laser spectroscopy on a coasting beam. Accuracy was improved by about an order of magnitude compared to the first observation in 1994. The most important improvement is an in situ high voltage measurement at the electron cooler (EC) platform with an accuracy at the 10 ppm level. Furthermore, the space charge effect of the EC current on the ion velocity was determined with two independent techniques that provided consistent results. The result of lambda₀=243.821(6) nm provides an important reference value for experiments testing bound-state quantum electrodynamics in the strong magnetic field regime by evaluating the specific difference between the splittings in the hydrogen-like and lithium-like ions.
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An optical parametric chirped-pulse amplifier for seeding high repetition rate free-electron lasers - H{\"{o}}ppner, H.; Hage, A.; Tanikawa, T.; Schulz, M.; Riedel, R.; Teubner, U.; Prandolini, M. J.; Faatz, B.; Tavella, F. (New Journal of Physics, 2015)
High repetition rate free-electron lasers (FEL), producing highly intense extreme ultraviolet and x-ray pulses, require new high power tunable femtosecond lasers for FEL seeding and FEL pump-probe experiments. A tunable, 112 W (burst mode) optical parametric chirped-pulse amplifier (OPCPA) is demonstrated with center frequencies ranging from 720–900 nm, pulse energies up to 1.12 mJ and a pulse duration of 30 fs at a repetition rate of 100 kHz. Since the power scalability of this OPCPA is limited by the OPCPA-pump amplifier, we also demonstrate a 6.7–13.7 kW (burst mode) thin-disk OPCPA-pump amplifier, increasing the possible OPCPA output power to many hundreds of watts. Furthermore, third and fourth harmonic generation experiments are performed and the results are used to simulate a seeded FEL with high-gain harmonic generation.
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Coherent combination of two Tm-doped fiber amplifiers - Gaida, C.; Kienel, M.; M{\"{u}}ller, M.; Klenke, A.; Gebhardt, M.; Stutzki, F.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2015)
The efficient coherent combination of two ultrafast Tm-doped fiber amplifiers in the 2-µm wavelength region is demonstrated. The performance of the combined amplifiers is compared to the output characteristics of a single amplifier being limited by the onset of detrimental nonlinear effects. Nearly transform-limited pulses with 830- fs duration, 22-µJ pulse energy, and 25-MW peak power have been achieved with a combining efficiency greater than 90%. Based on this result, it can be expected that 2-µm-ultrafast-fiber-laser systems will enter new performance realms in the near future.
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Comparison study of in vivo dose response to laser-driven versus conventional electron beam - Oppelt, M.; Baumann, M.; Bergmann, R.; Beyreuther, E.; Br{\"{u}}chner, K.; Hartmann, J.; Karsch, L.; Krause, M.; Laschinsky, L.; Le{\"{s}}mann, E.; Nicolai, M.; Reuter, M.; et al. (Radiation and Environmental Biophysics, 2015)
The long-term goal to integrate laser-based particle accelerators into radiotherapy clinics not only requires technological development of high-intensity lasers and new techniques for beam detection and dose delivery, but also characterization of the biological consequences of this new particle beam quality, i.e. ultra-short, ultra-intense pulses. In the present work, we describe successful in vivo experiments with laser-driven electron pulses by utilization of a small tumour model on the mouse ear for the human squamous cell carcinoma model FaDu. The already established in vitro irradiation technology at the laser system JETI was further enhanced for 3D tumour irradiation in vivo in terms of beam transport, beam monitoring, dose delivery and dosimetry in order to precisely apply a prescribed dose to each tumour in full-scale radiobiological experiments. Tumour growth delay was determined after irradiation with doses of 3 and 6 Gy by laser-accelerated electrons. Reference irradiation was performed with continuous electron beams at a clinical linear accelerator in order to both validate the dedicated dosimetry employed for laser-accelerated JETI electrons and above all review the biological results. No significant difference in radiation-induced tumour growth delay was revealed for the two investigated electron beams. These data provide evidence that the ultra-high dose rate generated by laser acceleration does not impact the biological effectiveness of the particles.
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Dielectronic recombination rate coefficients of initially rubidium-like tungsten - Wu, Z.; Zhang, Y.; Fu, Y.; Surzhykov, A.; Fritzsche, S.; Dong, C. (European Physical Journal D, 2015)
Ab initio calculations of dielectronic recombination (DR) rate coefficients of initially rubidium-like W³⁷⁺ ions have been performed for the electron temperatures from 1 eV to 5×10⁴ eV, by using the Flexible Atomic Code based on the relativistic configuration-interaction method. Special attention has been paid to the partial contributions to total DR rate coefficients as associated with the excitation of individual subshells. A detailed comparison of the calculations shows that the excitation from 4p subshell dominates total DR rate coefficients followed by the excitations from 4s and 4d subshells, while the contribution of excitations from 3l(l=s, p, d) subshells becomes important only at high temperatures. Besides, it is found that the electron excitations associated with Δ n=0,1 dominate at low-temperature plasmas, however, the excitations associated with Δ n≥2 become non-negligible at high-temperature ones.
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Dynamical Schwinger process in a bifrequent electric field of finite duration: Survey on amplification - Otto, A.; Seipt, D.; Blaschke, D.; Smolyansky, S. A.; K{\"{a}mpfer, B. (Physical Review D, 2015)
The electron-positron pair production due to the dynamical Schwinger process in a slowly oscillating strong electric field is enhanced by the superposition of a rapidly oscillating weaker electric field. A systematic account of the enhancement by the resulting bifrequent field is provided for the residual phase space distribution. The enhancement is explained by a severe reduction of the suppression in both the tunneling and multiphoton regimes.
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Long-term operation of surface high-harmonic generation from relativistic oscillating mirrors using a spooling tape - Bierbach, J.; Yeung, M.; Eckner, E.; Roedel, C.; Kuschel, S.; Zepf, M.; Paulus, G.G. (Optics Express, 2015)
Surface high-harmonic generation in the relativistic regime is demonstrated as a source of extreme ultra-violet (XUV) pulses with extended operation time. Relativistic high-harmonic generation is driven by a frequency-doubled high-power Ti:Sapphire laser focused to a peak intensity of 3·10^19 W/cm2 onto spooling tapes. We demonstrate continuous operation over up to one hour runtime at a repetition rate of 1 Hz. Harmonic spectra ranging from 20 eV to 70 eV (62 nm to 18 nm) were consecutively recorded by an XUV spectrometer. An average XUV pulse energy in the µJ range is measured. With the presented setup, relativistic surface high-harmonic generation becomes a powerful source of coherent XUV pulses that might enable applications in, e.g. attosecond laser physics and the seeding of free-electron lasers, when the laser issues causing 80-% pulse energy fluctuations are overcome.
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Radioactive decays of highly-charged ions - Gao, B. S.; Najafi, M. A.; Atanasov, D. R.; Blaum, K.; Bosch, F.; Brandau, C.; Chen, X. C.; Dillmann, I.; Dimopoulou, C.; Faestermann, T.; Geissel, H.; Gernh{\"{a}user, R. and Hillenbrand, P.-M. and Kovalenko, O. and Kozhuharov, C. and Litvinov, S. A. and Litvinov, Y. A. and Maier, L. and Nolden, F. and Piotrowski, J. and Sanjari, M. S. and Scheidenberger, C. and Spillmann, U. and Steck, M. and St{\"{o}}hlker, T. and Trageser, C. and Tu, X. L. and Weick, H. and Winckler, N. and Xu, H. S. and Yamaguchi, T. and Yan, X. L. and Zhang, Y. H. and Zhou, X. H. (EPJ Web of Conferences, 2015)
Access to stored and cooled highly-charged radionuclides offers unprecedented opportunities to perform high-precision investigations of their decays. Since the few-electron ions, e.g. hydrogen- or helium-like ions, are quantum mechanical systems with clear electronic ground state configurations, the decay studies of such ions are performed under well-defined conditions and allow for addressing fundamental aspects of the decay process. Presented here is a compact review of the relevant experiments conducted at the Experimental Storage Ring ESR of GSI. A particular emphasis is given to the investigations of the two-body beta decay, namely the bound-state β-decay and its time-mirrored counterpart, orbital electron-capture.
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Rayleigh x-ray scattering from many-electron atoms and ions - Surzhykov, A.; Yerokhin, V. A.; St{\"{o}}hlker, T.; Fritzsche, S. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2015)
A theoretical analysis is presented for the elastic Rayleigh scattering of x-rays by many-electron atoms and ions. Special emphasis is placed on the angular distribution and linear polarization of the scattered photons for the case when the incident light is completely (linearly) polarized. Based on second-order perturbation theory and the independent particle approximation, we found that the Rayleigh angular distribution is strongly affected by the charge state and shell structure of the target ions or atoms. This effect can be observed experimentally at modern synchrotron facilities and might provide further insight into the structure of heavy atomic systems.
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Reply to Comment on `Hyperfine-induced modifications to the angular distribution of the Kα₁ x-ray emission' - Wu, Z. W.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2015)
In a recent work, the Kα₁ (1s 2p₃/₂ ¹,³P₁,₂ → 1s² ¹S₀) x-ray emission following the radiative electron capture into initially hydrogen-like ions has been explored for ions with nonzero nuclear spin (I ≠ 0). A rather strong influence upon the angular distribution of the (hyperfine- and fine-structure averaged) Kα₁ radiation was found, especially for isotopes with nuclear spin I=1/2, while this effect are less important for isotopes with nuclear spin I>1/2. Two comments were made by Inal and Benmouna about this work with regard to (i) the incoherent summation of the individual hyperfine components of the 1s2p3/2 1P1→1s21S0 transition and (ii) the treatment of the hyperfine-induced E1-M2 multipole mixing in the 1s 2p3/2 ³P₂ → 1s², ¹S₀ fine-structure component. While we agree with the first comment and here provide updated anisotropy parameters, the hyperfine-induced modification of the Kα₁ emission remains valid and may help in the future to determine the nuclear parameters of radioactive isotopes. We also explain that the hyperfine-induced E1-M2 mixing has already been fully taken into account in our previous work.
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Selective deuterium ion acceleration using the Vulcan petawatt laser - Krygier, A. G.; Morrison, J. T.; Kar, S.; Ahmed, H.; Alejo, A.; Clarke, R.; Fuchs, J.; Green, A.; Jung, D.; Kleinschmidt, A.; Najmudin, Z.; Nakamura, H.; et al. (Physics of Plasmas, 2015)
We report on the successful demonstration of selective acceleration of deuterium ions by target-normal sheath acceleration (TNSA) with a high-energy petawatt laser. TNSA typically produces a multi-species ion beam that originates from the intrinsic hydrocarbon and water vaporcontaminants on the target surface. Using the method first developed by Morrison et al., an ion beam with >99% deuterium ions and peak energy 14 MeV/nucleon is produced with a 200 J, 700 fs, >10²⁰ W/cm² laser pulse by cryogenically freezing heavy water (D₂O) vapor onto the rear surface of the target prior to the shot. Within the range of our detectors (0°-8.5°), we find laser-to-deuterium-ion energy conversion efficiency of 4.3% above 0.7 MeV/nucleon while a conservative estimate of the total beam gives a conversion efficiency of 9.4%.
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Spectroscopy of berylliumlike xenon ions using dielectronic recombination - Bernhardt, D.; Brandau, C.; Harman, Z.; Kozhuharov, C.; B{\"{o}}hm, S.; Bosch, F.; Fritzsche, S.; Jacobi, J.; Kieslich, S.; Knopp, H.; Nolden, F.; Shi, W.; et al. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2015)
Be-like ¹³⁶Xe⁵⁰⁺ ions have been investigated employing the resonant electron-ion collision process of dielectronic recombination (DR) as a spectroscopic tool. The experiments were performed at the experimental storage ring in Darmstadt, Germany, using its electron cooler as a target for free electrons. DR Rydberg resonance series 2s²+e⁻ → 2s 2pj n lj for the associated intra-L-shell transitions 2s² ¹S₀ - 2s 2p₁/₂ ³P₁, 2s² ¹S₀ - 2s 2p₃/₂ ³P₂ and 2s² ¹S₀ - 2s 2p₃/₂ ¹P₁ were observed with high resolution. In addition to these excitations from the ground state we determined resonances associated with excitations 2s 2p₁/₂ ³P₀ to 2p₁/₂ 2p₃/₂ ³P₁ of ions initially in the metastable 2s 2p₁/₂ ³P₀ state. The corresponding excitation energies were determined to be E(¹S₀ → ³P₁)=127.269(46) eV, E(¹S₀ → ³P₂)=469.474(81) eV and E(¹S₀ to ¹P₁)=532.801(16) eV and E(³P₀ to 2p₁/₂ 2p₃/₂ ³P₁)=533.733(22) eV. These excitation energies are compared with previous measurements and with recent state-of-the-art atomic structure calculations.
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Stepwise contraction of the n f Rydberg shells in the 3d photoionization of multiply-charged xenon ions - Schippers, S.; Borovik Jr., A.; Buhr, T.; Hellhund, J.; Holste, K.; Kilcoyne, A. L. D.; Klumpp, S.; Martins, M.; M{\"{u}}ller, A.; Ricz, S.; Fritzsche, S. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2015)
Triple photoionization of Xe³⁺, Xe⁴⁺ and Xe⁵⁺ ions has been studied in the energy range 670-750 eV, including the 3d ionization threshold. The photon-ion merged-beam technique was used at a synchrotron light source to measure the absolute photoionization cross sections. These cross sections exhibit a progressively larger number of sharp resonances as the ion charge state is increased. This clearly visualizes the re-ordering of the ε f continuum into a regular series of (bound) Rydberg orbitals as the ionic core becomes more attractive. The energies and strengths of the resonances are extracted from the experimental data and are further analysed by relativistic atomic-structure calculations.
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The complex ion structure of warm dense carbon measured by spectrally resolved x-ray scattering - Kraus, D.; Vorberger, J.; Helfrich, J.; Gericke, D. O.; Bachmann, B.; Bagnoud, V.; Barbrel, B.; Blazevic, A.; Carroll, D. C.; Cayzac, W.; D{\"{o}}ppner, T.; Fletcher, L. B.; et al. (Physics of Plasmas, 2015)
We present measurements of the complex ion structure of warm dense carbon close to the melting line at pressures around 100 GPa. High-pressure samples were created by laser-driven shock compression of graphite and probed by intense laser-generated x-ray sources with photon energies of 4.75 keV and 4.95 keV. High-efficiency crystal spectrometers allow for spectrally resolving the scattered radiation. Comparing the ratio of elastically and inelastically scattered radiation, we find evidence for a complex bonded liquid that is predicted by ab-initio quantum simulations showing the influence of chemical bonds under these conditions. Using graphite samples of different initial densities we demonstrate the capability of spectrally resolved x-ray scattering to monitor the carbon solid-liquid transition at relatively constant pressure of 150 GPa. Showing first single-pulse scattering spectra from cold graphite of unprecedented quality recorded at the Linac Coherent Light Source, we demonstrate the outstanding possibilities for future high-precision measurements at 4th Generation Light Sources.
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Tunable Subluminal Propagation of Narrow-band X-Ray Pulses - Heeg, K.; Haber, J.; Schumacher, D.; Bocklage, L.; Wille, H.-C.; Schulze, K.; Loetzsch, R.; Uschmann, I.; Paulus, G.; R{\"{u}}ffer, R.; R{\"{o}}hlsberger, R.; Evers, J. (Physical Review Letters, 2015)
Group velocity control is demonstrated for x-ray photons of 14.4 keV energy via a direct measurement of the temporal delay imposed on spectrally narrow x-ray pulses. Subluminal light propagation is achieved by inducing a steep positive linear dispersion in the optical response of Fe57 M{\"{o}}ssbauer nuclei embedded in a thin film planar x-ray cavity. The direct detection of the temporal pulse delay is enabled by generating frequency-tunable spectrally narrow x-ray pulses from broadband pulsed synchrotron radiation. Our theoretical model is in good agreement with the experimental data.
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Ab initio MCDHF calculations of electronnucleus interactions - Bieron, J.; Fischer, C. F.; Fritzsche, S.; Gaigalas, G.; Grant, I. P.; Indelicato, P.; J{\"{o}}nsson, P.; Pyykk{\"{o}}, P. (Physica Scripta, 2015)
We present recent advances in the development of atomic ab initio multiconfiguration Dirac-Hartree-Fock theory, implemented in the GRASP relativistic atomic structure code. For neutral atoms, the deviations of properties calculated within the Dirac-Hartree-Fock (DHF) method (based on independent particle model of an atomic cloud) are usually dominated by electron correlation effects, i.e. the non-central interactions of individual electrons. We present the recent advances in accurate calculations of electron correlation effects in small, medium, and heavy neutral atoms. We describe methods of systematic development of multiconfiguration expansions leading to systematic, controlled improvement of the accuracy of the ab initio calculations. These methods originate from the concept of the complete active space (CAS) model within the DHF theory, which, at least in principle, permits fully relativistic calculations with full account of electron correlation effects. The calculations within the CAS model on currently available computer systems are feasible only for very light systems. For heavier atoms or ions with more than a few electrons, restrictions have to be imposed on the multiconfiguration expansions. We present methods and tools, which are designed to extend the numerical calculations in a controlled manner, where multiconfiguration expansions account for all leading electron correlation effects. We show examples of applications of the GRASP code to calculations of hyperfine structure constants, but the code may be used for calculations of arbitrary bound-state atomic properties. In recent years it has been applied to calculations of atomic and ionic spectra (transition energies and rates), to determinations of nuclear electromagnetic moments, as well as to calculations related to interactions of bound electrons with nuclear electromagnetic moments leading to violations of discrete symmetries.
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Characterization of self-organized crystalline Au nanoparticles embedded in epitaxially grown SrTiO₃ - Bernhardt, H.; Katzer, C.; Undisz, A.; Dr{\"{u}}e, M.; Rettenmayr, M.; Uschmann, I.; Schmidl, F. (Journal of Materials Research, 2015)
Crystalline Au nanoparticles embedded in epitaxially grown SrTiO₃ layers were prepared by an annealing and coating procedure of Au seed layers on SrTiO₃ (STO) substrates. X-ray diffraction and transmission electron microscopy measurements were performed to investigate the size, shape, and deformation of the particles and their crystal orientation. The shape and size of the crystalline Au nanoparticles can be tuned by controlling the Au seed layer thickness and single crystalline elliptically shaped Au nanoparticles have been generated. Furthermore, the orientation of the surrounding SrTiO₃ matrix changes significantly from homoepitaxially grown (001) to secondary (111) and (011) orientations for Au seed layers that are thicker than 4 nm. This is of great interest for modifying the electrical properties of SrTiO₃ layers, whereas the anisotropically shaped crystalline particles are relevant for optical applications, due to localized surface plasmon resonances.
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Exploring new avenues in high repetition rate table-top coherent extreme ultraviolet sources - H{\"{a}drich, S. and Krebs, M. and Hoffmann, A. and Klenke, A. and Rothhardt, J. and Limpert, J. and T{\"{u}}nnermann, A. (Light: Science { &} Applications, 2015)
The process of high harmonic generation (HHG) enables the development of table-top sources of coherent extreme ultraviolet (XUV) light. Although these are now matured sources, they still mostly rely on bulk laser technology that limits the attainable repetition rate to the low kilohertz regime. Moreover, many of the emerging applications of such light sources (e.g., photoelectron spectroscopy and microscopy, coherent diffractive imaging, or frequency metrology in the XUV spectral region) require an increase in the repetition rate. Ideally, these sources are operated with a multi-MHz repetition rate and deliver a high photon flux simultaneously. So far, this regime has been solely addressed using passive enhancement cavities together with low energy and high repetition rate lasers. Here, a novel route with significantly reduced complexity (omitting the requirement of an external actively stabilized resonator) is demonstrated that achieves the previously mentioned demanding parameters. A krypton-filled Kagome photonic crystal fiber is used for efficient nonlinear compression of 9 mJ, 250 fs pulses leading to ,7 mJ, 31 fs pulses at 10.7 MHz repetition rate. The compressed pulses are used for HHG in a gas jet. Particular attention is devoted to achieving phase-matched (transiently) generation yielding .10^13 photons s-1 (.50 mW) at 27.7 eV. This new spatially coherent XUV source improved the photon flux by four orders of magnitude for direct multi-MHZ experiments, thus demonstrating the considerable potential of this source.
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Extreme-field physics in Penning traps - Vogel, M.; Birkl, G.; Ebrahimi, M.; von Lindenfels, D.; Martin, A.; Paulus, G.; Quint, W.; Ringleb, S.; St{\"{o}}hlker, T.; Wiesel, M. (Hyperfine Interactions, 2015)
We present two Penning trap experiments concerned with different aspects of the physics of extreme electromagnetic fields, the ARTEMIS experiment designed for bound-electron magnetic moment measurements in the presence of the extremely strong fields close to the nucleus of highly charged ions, and the HILITE experiment, in which well-defined ion targets are to be subjected to high-intensity laser fields.
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Linear polarization of x-ray transitions due to dielectronic recombination in highly charged ions - J{\"{o}}rg, H.; Hu, Z.; Bekker, H.; Blessenohl, M.; Hollain, D.; Fritzsche, S.; Surzhykov, A.; Crespo L{\'{o}}pez-Urrutia, J.; Tashenov, S. (Physical Review A, 2015)
The linear polarization of x rays produced by dielectronic recombination into highly charged xenon ions was measured at an electron beam ion trap using the Compton polarimetry technique. This opens numerous possibilities for diagnostics of anisotropies of hot plasmas. Moreover, it was observed that the polarization of x rays, following the dielectronic capture populating the [1s 2s2 2p1/2]_1 state, is highly sensitive to the Breit interaction. The experimental results for this transition rule out by 5σ calculations not taking the Breit interaction into account.
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Photon propagation in slowly varying inhomogeneous electromagnetic fields - Karbstein, F.; Shaisultanov, R. (Physical Review D, 2015)
Starting from the Heisenberg-Euler effective Lagrangian, we determine the photon current and photon polarization tensor in inhomogeneous, slowly varying electromagnetic fields. To this end, we consider background field configurations varying in both space and time, paying special attention to the tensor structure. As a main result, we obtain compact analytical expressions for the photon polarization tensor in realistic Gaussian laser pulses, as generated in the focal spots of high-intensity lasers. These expressions are of utmost importance for the investigation of quantum vacuum nonlinearities in realistic high-intensity laser experiments.
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Plasma Undulator Based on Laser Excitation of Wakefields in a Plasma Channel - Rykovanov, S.; Schroeder, C.; Esarey, E.; Geddes, C.; Leemans, W. (Physical Review Letters, 2015)
An undulator is proposed based on the plasma wakefields excited by a laser pulse in a plasma channel. Generation of the undulator fields is achieved by inducing centroid oscillations of the laser pulse in the channel. The period of such an undulator is proportional to the Rayleigh length of the laser pulse and can be submillimeter, while preserving high undulator strength. The electron trajectories in the undulator are examined, expressions for the undulator strength are presented, and the spontaneous radiation is calculated. Multimode and multicolor laser pulses are considered for greater tunability of the undulator period and strength.
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Quantum reflection of photons off spatio-temporal electromagnetic field inhomogeneities - Gies, H.; Karbstein, F.; Seegert, N. (New Journal of Physics, 2015)
We reconsider the recently proposed nonlinear quantum electrodynamics effect of quantum reflection of photons off an inhomogeneous strong-field region. We present new results for strong fields varying both in space and time. While such configurations can give rise to new effects such as frequency mixing, estimated reflection rates based on previous one-dimensional studies are corroborated. On a conceptual level, we critically re-examine the validity regime of the conventional locally-constant-field approximation and identify kinematic configurations which can be treated reliably. Our results further underline the discovery potential of quantum reflection as a new signature of the nonlinearity of the quantum vacuum.
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Relativistic configuration-interaction calculation of Kα transition energies in beryllium-like argon - Yerokhin, V. A.; Surzhykov, A.; Fritzsche, S. (Physica Scripta, 2015)
Relativistic configuration-interaction calculations have been performed for energy levels of the low-lying and core-excited states of beryllium-like argon, Ar14+. These calculations include the one-loop quantum electrodynamics (QED) effects as obtained by two different methods: the screening-potential approach and the model QED operator approach. The calculations are supplemented by a systematic estimation of the uncertainties of the theoretical predictions.
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Towards a fast calculator for the radiation characteristics of radiative recombination and radiative electron capture - Weber, G.; Ding, H.; Herdrich, M. O.; Surzhykov, A. (Journal of Physics: Conference Series, 2015)
The radiative capture of free electrons (radiative recombination) and bound electrons (radiative electron capture) are among the most important charge changing processes for fast, highly-charged ions. While total cross sections can be obtained by an approximate formula with reasonable accuracy, the estimation of angular distributions and polarization properties of the emitted radiation requires a fully relativistic treatment that is numerical expensive. Therefore we recently started the development of a fast calculator for these radiation characteristics. The program is based on a grid of rigorously calculated data points for free- electron capture into bare ions, between which interpolation is performed to obtain radiation characteristics for specific collision systems. Also capture into few-electron systems is taken into account in an approximate way. We present first results from this development.
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Crystallographic investigation of Au nanoparticles embedded in a SrTiO3 thin film for plasmonics applications by means of synchrotron radiation - Pincini, D.; Mazzoli, C.; Bernhardt, H.; Katzer, C.; Schmidl, F.; Uschmann, I.; Detlefs, C. (Journal of Applied Physics, 2015)
Self-organized monocrystalline Au nanoparticles with potential applications in plasmonics are grown in a SrTiO3 matrix by a novel two-step deposition process. The crystalline preferred orientation of these Au nanoparticles is investigated by synchrotron hard x-ray diffraction. Nanoparticles preferentially align with the (111) direction along the substrate normal (001), whereas two in-plane orientations are found with [110]SrTiO3∥[110]Au and [100]SrTiO3∥[110]Au. Additionally, a smaller diffraction signal from nanoparticles with the (001) direction parallel to the substrate normal (001) is observed; once again, two in-plane orientations are found, with [100]SrTiO3∥[100]Au and [100]SrTiO3∥[110]Au. The populations of the two in-plane orientations are found to depend on the thickness of the gold film deposited in the first step of the growth.
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Electron-positron pair creation in low-energy collisions of heavy bare nuclei - Maltsev, I. A.; Shabaev, V. M.; Tupitsyn, I. I.; Bondarev, A. I.; Kozhedub, Y. S.; Plunien, G.; St{\"{o}}hlker, T. (Physical Review A, 2015)
A method for calculations of electron-positron pair-creation probabilities in low-energy heavy-ion collisions is developed. The approach is based on the propagation of all one-electron states via the numerical solving of the time-dependent Dirac equation in the monopole approximation. The electron wave functions are represented as finite sums of basis functions constructed from B-splines using the dual-kinetic-balance technique. The calculations of the created particle numbers and the positron energy spectra are performed for the collisions of bare nuclei at the energies near the Coulomb barrier with the Rutherford trajectory and for different values of the nuclear charge and the impact parameter. To examine the role of the spontaneous pair creation, the collisions with a modified velocity and with a time delay are also considered. The obtained results are compared with the previous calculations and the possibility of observation of the spontaneous pair creation is discussed.
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Gamma-ray emission in near critical density plasmas at laser intensities of 10^21 W/cm2 - Wang, H. Y.; Liu, B.; Yan, X. Q.; Zepf, M. (Physics of Plasmas, 2015)
We study synchrotron radiation emission from laser interaction with near critical density (NCD) plasmas at intensities of 10²¹ W/cm² using three-dimensional particle-in-cell simulations. It is found that the electron dynamics depend on the laser shaping process in NCD plasmas, and thus the angular distribution of the emitted photons changes as the laser pulse evolves in space and time. The final properties of the resulting synchrotron radiation, such as its overall energy, the critical photon energy, and the radiation angular distribution, are strongly affected by the laser polarization and plasma density. By using a 420 TW/50 fs laser pulse at the optimal plasma density ( 1 nc), about 10⁸ photons/0.1% bandwidth are produced at multi-MeV photon energies, providing a route to ultraintense, femtosecond gamma ray pulses.
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Increased isomeric lifetime of hydrogen-like ¹⁹²mOs - Akber, A.; Reed, M. W.; Walker, P. M.; Litvinov, Yu. A.; Lane, G. J.; Kib{\'{e}}di, T.; Blaum, K.; Bosch, F.; Brandau, C.; Carroll, J. J.; Cullen, D. M.; Cullen, I. J.; et al. (Physical Review C, 2015)
An excited metastable nuclear state of Os192 in a hydrogen-like charge state has been studied for the first time. It was populated in projectile fragmentation of a Au197 beam on a Be9 target with the UNILAC-SIS accelerators at GSI. Fragmentation products in the region of interest were passed through the fragment separator and injected into the experimental storage ring (ESR). Cooling of the injected beam particles enabled Schottky mass spectrometry to be performed. Analysis shows the lifetime of the state to be considerably longer than that of the neutral ion [t_neut=8.5(14)s]; this change is attributed to hindrance of internal conversion in hydrogen-like Os192. Calculations have been performed to estimate the lifetime, and the result has been compared with that measured experimentally. There is good agreement between the expected [t_H-like=13.0(24)s] and measured lifetimes (t_rest=15.1+1.5-1.3 s) from the internal decay of Os192m. This provides a test for the reliability of the values obtained from internal conversion coefficient calculations in highly ionized systems and is the first measurement of its kind to be performed using the ESR setup.
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Momentum-resolved study of the saturation intensity in multiple ionization - Wustelt, P.; M{\"{o}}ller, M.; Rathje, T.; Sayler, A. M.; St{\"{o}}hlker, T.; Paulus, G.G. (Physical Review A, 2015)
We present a momentum-resolved study of strong field multiple ionization of ionic targets. Using a deconvolution method we are able to reconstruct the electron momenta from the ion momentum distributions after multiple ionization up to four sequential ionization steps. This technique allows an accurate determination of the saturation intensity as well as of the electron release times during the laser pulse. The measured results are discussed in comparison to typically used models of over-the-barrier ionization and tunnel ionization.
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Narrowband inverse Compton scattering x-ray sources at high laser intensities - Seipt, D.; Rykovanov, S. G.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2015)
Narrowband x- and γ-ray sources based on the inverse Compton scattering of laser pulses suffer from a limitation of the allowed laser intensity due to the onset of nonlinear effects that increase their bandwidth. It has been suggested that laser pulses with a suitable frequency modulation could compensate this ponderomotive broadening and reduce the bandwidth of the spectral lines, which would allow one to operate narrowband Compton sources in the high-intensity regime. In this paper we therefore present the theory of nonlinear Compton scattering in a frequency-modulated intense laser pulse. We systematically derive the optimal frequency modulation of the laser pulse from the scattering matrix element of nonlinear Compton scattering, taking into account the electron spin and recoil. We show that, for some particular scattering angle, an optimized frequency modulation completely cancels the ponderomotive broadening for all harmonics of the backscattered light. We also explore how sensitively this compensation depends on the electron-beam energy spread and emittance, as well as the laser focusing.
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Progress in high-energy-class diode laser pump sources - Crump, P.; Frevert, C.; Bugge, F.; Knigge, S.; Erbert, G.; Trankle, G.; Pietrzak, A.; H{\"{u}}slewede, R.; Zorn, M.; Sebastian, J.; Lotz, J.; Fassbender, W.; et al. (Proceedings of SPIE, 2015)
A new generation of diode-pumped high-energy-class solid-state laser facilities is in development that generate multijoule pulse energies at around 10 Hz. Currently deployed quasi-continuous-wave (QCW) diode lasers deliver average inpulse pump powers of around 300 W per bar. Increased power-per-bar helps to reduce the system size, complexity and cost per Joule and the increased pump brilliance also enables more efficient operation of the solid state laser itself. It has been shown in recent studies, that optimized QCW diode laser bars centered at 940[…]980 nm can operate with an average in-pulse power of >1000 W per bar, triple that of commercial sources. When operated at pulsed condition of 1 ms, 10 Hz, this corresponds to >1 J/bar. We review here the status of these high-energy-class pump sources, showing how the highest powers are enabled by using long resonators (4[…]6 mm) for improved cooling and robustly passivated output facets for high reliability. Results are presented for prototype passively-cooled single bar assemblies and monolithic stacked QCW arrays. We confirm that 1 J/bar is sustained for fast-axis collimated stacks with a bar pitch of 1.7 mm, with narrow lateral far field angle (<12° with 95% power) and spectral width (<12 nm with 95% power). Such stacks are anticipated to enable Joule/bar pump densities to be used near-term in commercial high power diode laser systems. Finally, we briefly summarize the latest status of research into bars with higher efficiencies, including studies into operation at sub-zero temperatures (-70°C), which also enables higher powers and narrower far field and spectra.
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Recent progress in the understanding of mode instabilities - Jauregui, C.; Otto, H.-J.; Modsching, N.; Limpert, J.; T{\"{u}}nnermann, A. (Proceedings of SPIE, 2015)
In this work the latest progress in the understanding of mode instabilities is reviewed. Particular emphasis is put on the recently established influence of photodarkening on the mode instability threshold and its behavior. It is shown, for example, that even degradations of the output power in the order of a few percent can lead to very significant reductions of the mode instability threshold. Moreover, our analysis shows that photodarkening also alters the expected behavior of the mode instability threshold with respect to the signal wavelength and the seed power. Thus photodarkening is revealed as one of the main effects shaping the behavior of the mode instability threshold observed in experiments.
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Self-efficiency improvement and cooling in thulium-doped fibers - Jauregui, C.; Stutzki, F.; Limpert, J.; T{\"{u}}nnermann, A. (Proceedings of SPIE, 2015)
A way to increase the efficiency of Thulium-doped fiber systems and simultaneously prevent the generation of heat by pumping the excited state around 1460 nm has been recently proposed by the authors. In this contribution we show that a Thulium-doped fiber amplifier can lase around 1460nm while simultaneously amplifying signals around 2 μm. Such an operation results in considerably higher amplification efficiencies and in lower operating temperatures without the need for an external pump around 1460 nm.
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Self-Probing Spectroscopy of the SF 6 Molecule: A Study of the Spectral Amplitude and Phase of the High Harmonic Emission - Manschwetus, B.; Lin, N.; Rothhardt, J.; Guichard, R.; Auguste, T.; Camper, A.; Breger, P.; Caillat, J.; G{\'{e}}l{\'{e}}oc, M.; Ruchon, T.; Ta{\"{i}}eb, R.; Carr{\'{e}}, B.; et al. (The Journal of Physical Chemistry A, 2015)
We present characterizations of the attosecond pulse train produced in the high harmonic generation (HHG) from SF6 molecules irradiated by a strong pulsed laser field at 800 nm. At harmonic order 17, we observe a minimum in the amplitude of the emitted spectrum and a corresponding distortion in the phase. Our experimental results are compared to two models: a multicenter interference model focused on the effect of the structure of the SF6 molecule in HHG and a model focused on the interferences between multiple ionization channels in HHG. We find that the experimental results agree very well with the multiple ionization channels model, illustrating that HHG in molecules can be very complex and that it provides insights of the intramolecular electron dynamics during the interaction process.
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Sub-700 fs pulses at 152 W average power from a Tm-doped fiber CPA system - Gaida, C.; Stutzki, F.; Gebhardt, M.; Jansen, F.; Wienke, A.; Zeitner, U. D.; Fuchs, F.; Jauregui, C.; Wandt, D.; Kracht, D.; Limpert, J.; T{\"{u}}nnermann, A. (Proceedings of SPIE, 2015)
Thulium-based fiber lasers potentially provide for the demand of high average-power ultrafast laser systems operating at an emission wavelength around 2 μm. In this work we use a Tm-doped photonic-crystal fiber (PCF) with a mode field diameter of 36 μm enabling high peak powers without the onset of detrimental nonlinear effects. For the first time a Tm-doped PCF amplifier allows for a pump-power limited average output power of 241 W with a slope efficiency above 50%, good beam quality and linear polarization. A record compressed average power of 152 W and a pulse peak power of more than 4 MW at sub-700 fs pulse duration are enabled by dielectric gratings with diffraction efficiencies higher than 98% leading to a total compression efficiency of more than 70%. A further increase of pulse peak power towards the GW-level is planned by employing Tm-doped large-pitch fibers with mode field diameters well above 50 μm. The coherent combination of ultrafast pulses might eventually lead to kW-level average power and multi-GW peak power.
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Total projectile electron loss cross sections of U²⁸⁺ ions in collisions with gaseous targets ranging from hydrogen to krypton - Weber, G.; Herdrich, M.; DuBois, R.; Hillenbrand, P.-M.; Beyer, H.; Bozyk, L.; Gassner, T.; Grisenti, R.; Hagmann, S.; Litvinov, Y.; Nolden, F.; Petridis, N.; et al. (Physical Review ST - Accelerators and Beams, 2015)
Beam lifetimes of stored U^(28+) ions with kinetic energies of 30 and 50 MeV/u, respectively, were measured in the experimental storage ring of the GSI accelerator facility. By using the internal gas target station of the experimental storage ring, it was possible to obtain total projectile electron loss cross sections for collisions with several gaseous targets ranging from hydrogen to krypton from the beam lifetime data. The resulting experimental cross sections are compared to predictions by two theoretical approaches, namely the CTMC method and a combination of the DEPOSIT code and the RICODE program.
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Wavelength dependence of maximal diffraction-limited output power of fiber lasers - Otto, H.-J.; Modsching, N.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Proceedings of SPIE, 2015)
The threshold-like onset of mode instabilities is currently the main limitation for the scaling of the average output power of fiber-laser systems with diffraction limited beam quality. In this contribution wavelength shifting of the seed signal has been experimentally investigated in order to mitigate mode instabilities. Against the expectations, it is experimentally shown that the highest mode instabilities threshold is reached around 1030 nm and not for the smallest wavelength separation between pump and signal wavelength. This finding implies that the quantum defect is not the sole significant source for thermal heating in the fiber.
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Broadband XUV polarimetry of high harmonics from plasma surfaces using multiple Fresnel reflections - Hahn, T.; Bierbach, J.; R{\"{o}}del, C.; Hemmers, D.; Yeung, M.; Dromey, B.; Fuchs, S.; Galestian, A.; Kuschel, S.; Zepf, M.; Paulus, G.; Pretzler, G. (Applied Physics B, 2015)
High-harmonic generation (HHG) by nonlinear interaction of intense laser pulses with gases or plasma surfaces is the most prominent way of creating highly coherent extreme ultraviolet (EUV/XUV) pulses. In the last years, several scientific applications have been found which require the measurement of the polarization of the harmonic radiation. We present a broadband XUV polarimeter based on multiple Fresnel reflections providing an extinction rate of 5–25 for 17–45 nm which is particularly suited for surface harmonics. The device has first been tested at a gas harmonic source providing linearly polarized XUV radiation. In a further experiment using HHG from plasma surfaces, the XUV polarimeter allowed a polarization measurement of high harmonic radiation from plasma surfaces for the first time which reveals a linear polarization state as predicted for our generation parameters. The generation and control of intense polarized XUV pulses - together with the availability of broadband polarizers in the XUV - open the way for a series of new experiments. For instance, dichroism in the XUV, elliptically polarized harmonics from aligned molecules, or the selection rules of relativistic surface harmonics can be studied with the broadband XUV polarimeter.
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Characterization of a double-sided Si(Li) strip Compton polarimeter - Blumenhagen, K.-H.; Gumberidze, A.; M{\"{a}rtin, R. and Schell, N. and Spillmann, U. and Weber, G. and St{\"{o}}hlker, T. (Journal of Physics: Conference Series, 2015)
The response of a double-sided segmented Si(Li) detector system has been investigated. The detector has been irradiated with a collimated, highly linearly polarized beam of 53.2 keV photons from the synchrotron radiation source PETRA III at DESY. The detector was mounted on a platform that could be moved with μm precision thus allowing for a defined beam position on the detector surface. In this paper, the effects of the isolation gaps (gap width = 50 μm) between adjacent segments (strips) were studied, in particular with respect to the effect of charge sharing. The fraction of such charge sharing events increases from about 5% (beam hits center of a strip) to over 50% when the beam is focused just on a gap. The fraction of reconstructed Compton scattering events, which is interesting for Compton polarimetry, amounts to about 3% with the beam impinging at a strip center and 2.8% on average. It can therefore be concluded that events related to charge sharing do not critically degrade the performance of the detector as a Compton polarimter.
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Compton polarimetry using double-sided segmented x-ray detectors - Weber, G.; Blumenhagen, K.-H.; Br{\"{a}uning, H. and Ding, H. and Fritzsche, S. and Hess, S. and M{\"{a}rtin, R. and Spillmann, U. and Surzhykov, A. and Trotsenko, S. and Winters, D. F. A. and Yerokhin, V. A. and St{\"{o}}hlker, T. (Journal of Physics: Conference Series, 2015)
Hard x-ray polarimetry of radiation emitted in collisions of heavy ions, electrons or photons with matter provides detailed information on the collision dynamics as well as of the atomic structure in the presence of extreme field strengths. Moreover, it also opens a route for polarization diagnosis of spin-polarized ion and electron beams which, for example, might be useful in future parity non-conservation studies. Owing to recent progress in the development of highly segmented solid-state detectors, a novel type of polarimeter for the hard x-ray regime has become available. Applied as Compton polarimeters, two-dimensional position-sensitive x-ray detectors now allow for precise and efficient measurements of x-ray linear polarization properties. In this report recent polarimetry studies using such detector systems are reviewed.
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Effect of hydroxyl concentration on Yb3+ luminescence properties in a peraluminous lithium-alumino-silicate glass - Kuhn, S.; Tiegel, M.; Herrmann, A.; K{\"{o}}rner, J.; Seifert, R.; Yue, F.; Kl{\"{o}}pfel, D.; Hein, J.; Kaluza, M.C.; Russel, C. (Optical Materials Express, 2015)
Ytterbium doped lithium-alumino-silicate glasses suitable for diode-pumped laser applications were investigated concerning the hydroxyl quenching of the Yb^(3+) fluorescence. Glasses of the nominal composition 18 mol% Li2O, 22 mol% Al2O3 and 60 mol% SiO2 with variable OH concentrations NOH (between 0.04 and 6.01 ∙ 1019 cm−3) and Yb3+ concentrations NYb (between 0.1 and 9 ∙ 1020 cm−3) were produced and a direct correlation between spontaneous emission decay rate and the product NYb ∙ NOH was observed. The radiative spontaneous emission rate in the glass host is around 1,000 s−1 (radiative lifetime 1.0 ms) and the microparameter for Yb-Yb energy migration, CYb-Yb, was found to be 1.358∙10^−38 cm^6 s−1. It was calculated that on average 17% of the OH groups in the glass contribute to the quenching of the Yb3+ fluorescence. By analysis of the UV edge of the glass it was concluded that melting under inert conditions leads to reduction of iron impurities to Fe2+, which can act as quenching sites for the Yb3+ ions and therefore may additionally reduce the energy storage capability of the laser material.
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Electron-capture-to-continuum cusp in U⁸⁸⁺+N₂ collisions - Hillenbrand, P.-M.; Hagmann, S.; Jakubassa-Amundsen, D. H.; Monti, J. M.; Bana{ś}, D.; Blumenhagen, K.-H.; Brandau, C.; Chen, W.; Fainstein, P. D.; De Filippo, E.; Gumberidze, A.; Guo, D. L.; et al. (Physical Review A, 2015)
For the collision system U88+ -> N2 at a collision energy of 90 MeV/u, the energy distribution of electrons being nonradiatively captured from the target into the projectile continuum has been measured under an angle of 0∘ with respect to the projectile beam axis. This measurement of the electron-capture-to-continuum cusp with the highest effective projectile charge Z_eff,p=88 at a near-relativistic collision velocity of β≈0.41 is shown to be characterized by a strong asymmetry in the cusp shape. By comparing the data to measurements of the radiative-electron-capture-to-continuum cusp for the same collision system, the opposite asymmetry of the cusp is traced back to the varying underlying mechanisms. The experimental results are compared with the two theoretical calculations available for this process, one of them in the semirelativistic impulse approximation and the other in the nonrelativistic continuum-distorted-wave approach. A corresponding fully relativistic treatment may be motivated by the presented experimental data.
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High-energy monoenergetic proton beams from two stage acceleration with a slow laser pulse - Wang, H. Y.; Yan, X. Q.; Zepf, M. (Physical Review ST - Accelerators and Beams, 2015)
We present a new regime to generate high-energy quasimonoenergetic proton beams in a -pulseregime, where the laser group velocity vg<c is reduced by an extended near-critical density plasma. In this regime, for properly matched laser intensity and group velocity, ions initially accelerated by the light sail (LS) mode can be further trapped and reflected by the snowplough potential generated by the laser in the near-critical density plasma. These two acceleration stages are connected by the onset of Rayleigh-Taylor-like (RT) instability. The usual ion energy spectrum broadening by RT instability is controlled and high quality proton beams can be generated. It is shown by multidimensional particle-in-cell simulation that quasimonoenergetic proton beams with energy up to hundreds of MeV can be generated at laser intensities of 10²¹ W/cm².
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Multi-element readout of structured HPGe-detectors for high-resolution x-ray spectroscopy using CUBE-preamplifiers - Krings, T.; Spillmann, U.; Proti{}, D.; Ro{\"{s}}, C.; St{\"{o}}hlker, T.; Weber, G.; Bombelli, L.; Alberti, R.; Frizzi, T. (Journal of Instrumentation, 2015)
Very recently we have shown that CUBE-preamplifiers developed by XGLab s.r.l. can be used for the readout of single elements of thick structured planar HPGe- and Si(Li)-detectors produced by SEMIKON [1]. In this paper we will present the results of a simultaneous multi-element readout of structured detectors using the same preamplifiers for measuring high-energy x-rays (more than 100 keV) with a comparable energy resolution as for the single-element readout. Several high-purity germanium detectors (HPGe-detectors) with different position sensitive structures on one detector contact have been used for the first tests. In addition to that we have modified an existing 16-pixel HPGe-polarimeter from GSI-Darmstadt with the new readout. The detector elements (7 mm × 7 mm each, arranged in a 4 × 4 matrix) are connected to CUBE-preamplifiers used in pulse-reset mode. The technological progress achieved with this detector system resulting in a significant improved energy resolution will contribute a lot to much more precise polarization measurements of x-rays emitted from atom-ion collisions which are part of the physics program of the SPARC collaboration (Stored Particles Atomic Physics Research Collaboration) at GSI and the future FAIR accelerator facility (Facility for Antiproton and Ion Research).
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Multidimensional coherent pulse addition of ultrashort laser pulses - Kienel, M.; M{\"{u}}ller, M.; Klenke, A.; Eidam, T.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2015)
Spatially and temporally separated amplification and subsequent coherent addition of femtosecond pulses is a promising performance-scaling approach for ultrafast laser systems. Herein we demonstrate for the first time the application of this multidimensional scheme in a scalable architecture. Applying actively controlled divided-pulse amplification producing up to four pulse replicas that are amplified in two ytterbium-doped step-index fibers (6 μm core), pulse energies far beyond the damage threshold of the single fiber have been achieved. In this proof-of-principle experiment, high system efficiencies are demonstrated at both high pulse energies (i.e., in case of strong saturation) and high accumulated nonlinear phases.
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Reduction of X-ray generation in high-intensity laser ion acceleration - Aurand, B.; Kuschel, S.; R{\"{o}}del, C.; J{\"{a}ckel, O. and Polz, J. and Elkin, B. and Zhao, H. and Karmakar, A. and Gibbon, P. and Kaluza, M. and K{\"{u}}hl, T. (Applied Physics B, 2015)
In this paper, we report on measurements of bremsstrahlung in laser ion acceleration experiments from ultra-thin, polymer-based target foils. The influence of laser polarization on the generated γ radiation, the maximum achievable proton energy and the total proton number is investigated. A clear benefit in terms of γ radiation reduction by the use of circular polarized light can be observed. At the same time, the total number of accelerated protons was increased.
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Role of charge transfer in heavy-ion-beam{–}plasma interactions at intermediate energies - Ortner, A.; Frank, A.; Blazevic, A.; Roth, M. (Physical Review E, 2015)
In this paper we investigate the influence of the plasma properties on the charge state distribution of a swift heavy ion beam interacting with a plasma. The main finding is that the charge state in plasma can be lower than in cold matter. The charge state distribution is determined by the ionization and recombination rates which are balancing each other out. Both, ionization and recombination rates, as well as atomic excitation and decay rates, depend on the plasma parameters in different ways. These effects have been theoretically studied by Monte Carlo simulations on the example of an argon ion beam at an energy of 4 MeV/u in a carbon plasma. This study covers a plasma parameter space ranging from ion densities from 10¹⁸ to 10²³ cm⁻³ and electron temperatures from 10 to 200 eV.
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A 10-mJ-level compact CPA system based on Yb:KGW for ultrafast optical parametric amplifier pumping - Jo{\ {a}}o, C.; Wagner, F.; K{\"{o}}rner, J.; Hein, J.; Gottschall, T.; Limpert, J.; Bagnoud, V. (Applied Physics B, 2015)
A compact diode-pumped Yb:KGW regenerative amplifier producing 10 Hz, 10-mJ-level picosecond pulses at 1,040 nm is demonstrated. The system is used at the new front end of the PHELIX petawatt laser system to pump an ultrafast optical parametric amplifier for temporal contrast enhancement. Before frequency doubling, a pulse length of ∼1 ps is obtained by using a stretcher/compressor system based on a single large-aperture chirped volume Bragg grating.
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A novel double hohlraum target to create a moderately coupled plasma for ion stopping experiments - Ortner, A.; Faik, S.; Schumacher, D.; Basko, M.; Blazevic, A.; Busold, S.; Bedacht, S.; Cayzac, W.; Frank, A.; Kraus, D.; Rienecker, T.; Schaumann, G.; et al. (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2015)
Abstract We present a new double hohlraum target for the creation of a moderately coupled ( 0.1 < Γ < 1 ) carbon plasma for energy loss and charge state measurements of projectile ions interacting with this plasma. A spherical cavity of 600 μ m in diameter is heated with a 150-J laser pulse ( λ L = 527 nm ) within 1.2 ns to produce a quasi-Planckian X-ray source with a radiation temperature of T r ≈ 100 eV . These X-rays are then used to heat volumetrically two thin carbon foils in a secondary cylindrical hohlraum to a dense plasma state. An axi-symmetric plasma column with a free-electron density of up to 8 × 10^21 cm^- 3 , a temperature of T ≈ 10 eV, and an average ionization degree of Z ≈ 3 is generated. This plasma stays in a dense and an almost uniform state for about 5 ns . Ultimately, such targets are supposed to be used in experiments where a heavy ion beam is launched through the sample plasma, and the ion energy losses as well as the charge distributions are to be measured. The present paper is in a certain sense a symbiotic one, where the theoretical analysis and the experimental results are combined to investigate the basic properties and the prospects of this type of plasma targets.
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Characterization of laser-driven proton beams from near-critical density targets using copper activation - Willingale, L.; Nagel, S.; Thomas, A.; Bellei, C.; Clarke, R.; Dangor, A.; Heathcote, R.; Kaluza, M.; Kamperidis, C.; Kneip, S.; Krushelnick, K.; Lopes, N.; et al. (Journal of Plasma Physics, 2015)
Copper activation was used to characterize high-energy proton beam acceleration from near-critical density plasma targets. An enhancement was observed when decreasing the target density, which is indicative for an increased laser-accelerated hot electron density at the rear target-vacuum boundary. This is due to channel formation and collimation of the hot electrons inside the target. Particle-in-cell simulations support the experimental observations and show the correlation between channel depth and longitudinal electric field strength is directly correlated with the proton acceleration.
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Controlling Fragmentation Reactions of Polyatomic Molecules with Impulsive Laser Alignment - Xie, X.; Doblhoff-Dier, K.; Xu, H.; Roither, S.; Sch{\"{o}}ffler, M.; Kartashov, D.; Erattupuzha, S.; Rathje, T.; Paulus, G.G.; Yamanouchi, K.; Baltuska, A.; Gr{\"{a}fe, S. and Kitzler, M. (Springer Proceedings in Physics, 2015)
We experimentally and theoretically demonstrate channel-selective control over strong-field induced fragmentation of a polyatomic molecule, acetylene, using impulsive laser alignment as the control mechanism.

Electron-ion collision spectroscopy: Lithium-like xenon ions - Bernhardt, D.; Brandau, C.; Harman, Z.; Kozhuharov, C.; B{\"{o}}hm, S.; Bosch, F.; Fritzsche, S.; Jacobi, J.; Kieslich, S.; Knopp, H.; Nolden, F.; Shi, W.; et al. (Physical Review A, 2015)
The resonant process of dielectronic recombination (DR) has been applied as a spectroscopic tool to investigate intra-L-shell excitations 2s−2pj in Li-like 136Xe51+. The experiments were carried out at the electron cooler of the Experimental Storage Ring of the GSI-Helmholtzzentrum f{\"{u}}r Schwerionenforschung, Darmstadt, Germany. The observed center-of-mass energy range (0–505 eV) covers all resonances associated with the 2s+e−→(2p1/2nlj)J and (2p3/2nlj)J DR processes. Energies and strengths of isolated 2p1/2n and 2p3/2n DR-resonance groups were obtained for principal quantum numbers n up to 43 and 36, respectively. The 2s−2p1/2 and 2s−2p3/2 excitation energies were deduced to be 119.816(42) eV and 492.174(52) eV. The excitation energies are compared with previous measurements of other groups and with recent QED calculations. In addition, the experimental spectra and extracted resonance strengths are compared with multiconfiguration Dirac-Fock calculations. Measurements and theory are found to be in good agreement with each other.
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Electronic Pre-determination of Ethylene Fragmentation Dynamics - Kitzler, M.; Xie, X.; Roither, S.; L{\"{o}}tstedt, E.; Sch{\"{o}}ffler, M.; Kartashov, D.; Paulus, G.G.; Iwasaki, A.; Baltuska, A.; Yamanouchi, K. (Springer Proceedings in Physics, 2015)
We demonstrate, using ethylene, that controlling lower-valence ionization and field-driven excitation dynamics with ultrashort, intense laser pulses allows steering fragmentation reactions of polyatomic molecules along a certain pathway towards a specific set of fragment ions.

Interaction of twisted light with many-electron atoms and ions - Surzhykov, A.; Seipt, D.; Serbo, V. G.; Fritzsche, S. (Physical Review A, 2015)
The excitation of many-electron atoms and ions by twisted light has been studied within the framework of the density-matrix theory and Dirac's relativistic equation. Special attention is paid to the magnetic sublevel population of excited atomic states as described by means of the alignment parameters. General expressions for the alignment of the excited states are obtained under the assumption that the photon beam, prepared as a coherent superposition of two twisted Bessel states, irradiates a macroscopic target. We demonstrate that for this case the population of excited atoms can be sensitive to both the transverse momentum and the (projection of the) total angular momentum of the incident radiation. While the expressions are general and can be employed to describe the photoexcitation of any atom, independent on its shell structure and number of electrons, we performed calculations for the 3s→3p transition in sodium. These calculations indicate that the “twistedness” of incoming radiation can lead to a measurable change in the alignment of the excited P3/22 state as well as the angular distribution of the subsequent fluorescence emission.
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Laser-driven ion acceleration with hollow laser beams - Brabetz, C.; Busold, S.; Cowan, T.; Deppert, O.; Jahn, D.; Kester, O.; Roth, M.; Schumacher, D.; Bagnoud, V. (Physics of Plasmas, 2015)
The laser-driven acceleration of protons from thin foils irradiated by hollow high-intensity laser beams in the regime of target normal sheath acceleration (TNSA) is reported for the first time. The use of hollow beams aims at reducing the initial emission solid angle of the TNSA source, due to a flattening of the electron sheath at the target rear side. The experiments were conducted at the PHELIX laser facility at the GSI Helmholtzzentrum f{\"{u}}r Schwerionenforschung GmbH with laser intensities in the range from 10^18 W cm^−2 to 10^20 W cm^−2 . We observed an average reduction of the half opening angle by (3.07±0.42)° or (13.2±2.0)% when the targets have a thickness between 12 μm and 14 μm. In addition, the highest proton energies were achieved with the hollow laser beam in comparison to the typical Gaussian focal spot.
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Lifting shell structures in the dynamically assisted Schwinger effect in periodic fields - Otto, A.; Seipt, D.; Blaschke, D.; K{\"{a}mpfer, B. and Smolyansky, S. (Physics Letters B, 2015)
Abstract The dynamically assisted pair creation (Schwinger effect) is considered for the superposition of two periodic electric fields acting in a finite time interval. We find a strong enhancement by orders of magnitude caused by a weak field with a frequency being a multitude of the strong-field frequency. The strong low-frequency field leads to shell structures which are lifted by the weaker high-frequency field. The resonance type amplification refers to a new, monotonously increasing mode, often hidden in some strong oscillatory transient background, which disappears during the smoothly switching off the background fields, thus leaving a pronounced residual shell structure in phase space.
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Pre-chirp managed nonlinear amplification in fibers delivering 100 W, 60 fs pulses - Liu, W.; Schimpf, D. N.; Eidam, T.; Limpert, J.; T{\"{u}}nnermann, A.; K{\"{a}rtner, F. X. and Chang, G. (Optics Letters, 2015)
We demonstrate a pre-chirp managed Yb-doped fiber laser system that outputs 75 MHz, 130 W spectrally broadened pulses, which are compressed by a diffraction-grating pair to 60 fs with average powers as high as 100 W. Fine tuning the pulse chirp prior to amplification leads to high-quality compressed pulses. Detailed experiments and numerical simulation reveal that the optimum pre-chirp group-delay dispersion increases from negative to positive with increasing output power for rod-type high-power fiber amplifiers. The resulting laser parameters are suitable for extreme nonlinear optics applications such as frequency conversion in femtosecond enhancement cavities.
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Quantum Path Interference and Multiple Electron Scattering in Soft X-Ray High-Order Harmonic Generation - Seres, J.; Seres, E.; Landgraf, B.; Aurand, B.; Kuehl, T.; Spielmann, C. (Photonics, 2015)
High-order harmonic generation is an important mechanism to generate coherent radiation in the few–100-eV spectral range with ultrashort laser pulses. Moreover, a closer inspection of the measured spectra provides unique information about the underlying physics and allows deriving guidelines for improvements. The long-range modulation of the spectral envelope is linked to phase matching, and we will show how to improve it with a double-pulse excitation scheme. Additionally, the spectrum contains only every fourth harmonic, which can be well explained by the quantum interference of multiple scattered electrons, and two dominant electron trajectories were selected by X-ray parametric interaction.
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Relativistically prolonged lifetime of the 2s2p 3P0 level of zero nuclear-spin beryllium-like ions - Fritzsche, S.; Surzhykov, A.; Volotka, A. (New Journal of Physics, 2015)
The E1M1 transition rate of the 2s2p ³P₀→ 2s² ¹S₀ line in beryllium-like ions has been calculated within the framework of relativistic second-order perturbation theory. Both multiconfiguration and quantum-electrodynamical computations have been carried out independently to better understand and test for all major electron–electron correlation contributions in the representation of the initial, intermediate and final states. By comparing the results from these methods, which agree well for all ions along the beryllium isoelectronic sequence, the lifetime of the metastable 2s2p 3P0 level is found to be longer by about 2–3 orders of magnitude for all medium and heavy elements than was estimated previously. This makes the 3P0 level of beryllium-like ions to one of the longest living (low-lying) electronic excitations of a tightly bound system with potential applications for atomic clocks and in astro physics and plasma physics.
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Tm-based fiber-laser system with more than 200 MW peak power - Stutzki, F.; Gaida, C.; Gebhardt, M.; Jansen, F.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2015)
Tm-based fiber-laser systems are an attractive concept for the development of high-performance laser sources in the spectral region around 2 μm wavelength. Here we present a system delivering a pulse-peak power higher than 200 MW in combination with 24 W average power and 120 μJ pulse energy. Key components enabling this performance level are a Tm-doped large-pitch fiber with a mode-field diameter of 65 μm, highly efficient dielectric gratings, and a Tm-based fiber oscillator operating in the stretched-pulse regime.
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22 GW peak-power fiber chirped-pulse-amplification system - Klenke, A.; H{\"{a}drich, S. and Eidam, T. and Rothhardt, J. and Kienel, M. and Demmler, S. and Gottschall, T. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2014)
In this Letter, we report on a femtosecond fiber chirped-pulse-amplification system based on the coherent combination of the output of four ytterbium-doped large-pitch fibers. Each single channel delivers a peak power of about 6.2 GW after compression. The combined system emits 200 fs long pulses with a pulse energy of 5.7 mJ at 230 W of average power together with an excellent beam quality. The resulting peak power is 22 GW, which to the best of our knowledge is the highest value directly emitted from any fiber-based laser system.
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Bright Subcycle Extreme Ultraviolet Bursts from a Single Dense Relativistic Electron Sheet - Ma, W.; Bin, J.; Wang, H.; Yeung, M.; Kreuzer, C.; Streeter, M.; Foster, P.; Cousens, S.; Kiefer, D.; Dromey, B.; Yan, X.; Meyer-ter-Vehn, J.; et al. (Physical Review Letters, 2014)
Double-foil targets separated by a low density plasma and irradiated by a petawatt-class laser are shown to be a copious source of coherent broadband radiation. Simulations show that a dense sheet of relativistic electrons is formed during the interaction of the laser with the tenuous plasma between the two foils. The coherent motion of the electron sheet as it transits the second foil results in strong broadband emission in the extreme ultraviolet, consistent with our experimental observations.
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Energy deposition dynamics of femtosecond pulses in water - Minardi, S.; Mili{\'{a}}n, C.; Majus, D.; Gopal, A.; Tamo{{s}}auskas, G.; Couairon, A.; Pertsch, T.; Dubietis, A. (Applied Physics Letters, 2014)
We exploit inverse Raman scattering and solvated electron absorption to perform a quantitative characterization of the energy loss and ionization dynamics in water with tightly focused near-infrared femtosecond pulses. A comparison between experimental data and numerical simulations suggests that the ionization energy of water is 8 eV, rather than the commonly used value of 6.5 eV. We also introduce an equation for the Raman gain valid for ultra-short pulses that validates our experimental procedure.
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Method for the production of highly charged ions with polarized nuclei and zero total electron angular momentum - Bondarevskaya, A.; Mistonova, E. A.; Lyashchenko, K. N.; Andreev, O. Yu.; Surzhykov, A.; Labzowsky, L. N.; Plunien, G.; Liesen, D.; Bosch, F.; St{\"{o}}hlker, T. (Physical Review A, 2014)
The capture of polarized electrons by H-like ions provides a method for polarizing the nuclei of He-like heavy ions with zero total electron angular momentum in storage rings for high-energy ions. A detailed analysis for Eu-151 ions with nuclear spin I=5/2 predicts a nuclear polarization degree of about 47% already after one passage through a target containing 100% polarized electrons. Almost 50% of the polarized He-like ions are predicted to be in states with zero total electron angular momentum. Such ions were recently considered as the most promising candidates in experiments at storage rings for the search for violations of the fundamental symmetries and for a nuclear and an electron electric dipole moment.
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Pre-plasma formation in experiments using petawatt lasers - Wagner, F.; Bedacht, S.; Ortner, A.; Roth, M.; Tauschwitz, A.; Zielbauer, B.; Bagnoud, V. (Optics Express, 2014)
We used time-resolved shadowgraphy to characterize the pre-plasma formation in solid-target interaction experiments with micrometer-scale accuracy. We performed quantitative measurements of the plasma density for amplified spontaneous emission (ASE) levels ranging from 2 x 10^-7 to 10^-10 backed with 2-dimensional hydrodynamic simulations. We find that ASE levels above 10^-9 are able to create a significant pre-plasma plume that features a plasma canal driving a self-focusing of the laser beam. For ASE levels of 1010, no ASE pre-plasma could be detected.
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Real-time and Sub-wavelength Ultrafast Coherent Diffraction Imaging in the Extreme Ultraviolet - Z{\"{u}}rch, M.; Rothhardt, J.; H{\"{a}drich, S. and Demmler, S. and Krebs, M. and Limpert, J. and T{\"{u}}nnermann, A. and Guggenmos, A. and Kleineberg, U. and Spielmann, C. (Scientific Reports, 2014)
Coherent Diffraction Imaging is a technique to study matter with nanometer-scale spatial resolution based on coherent illumination of the sample with hard X-ray, soft X-ray or extreme ultraviolet light delivered from synchrotrons or more recently X-ray Free-Electron Lasers. This robust technique simultaneously allows quantitative amplitude and phase contrast imaging. Laser-driven high harmonic generation XUV-sources allow table-top realizations. However, the low conversion efficiency of lab-based sources imposes either a large scale laser system or long exposure times, preventing many applications. Here we present a lensless imaging experiment combining a high numerical aperture (NA = 0.8) setup with a high average power fibre laser driven high harmonic source. The high flux and narrow-band harmonic line at 33.2 nm enables either sub-wavelength spatial resolution close to the Abbe limit (Δr = 0.8λ) for long exposure time, or sub-70 nm imaging in less than one second. The unprecedented high spatial resolution, compactness of the setup together with the real-time capability paves the way for a plethora of applications in fundamental and life sciences.
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Relativistic calculations of double K-shell-photoionization cross sections for neutral medium-Z atoms - Yerokhin, V. A.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2014)
Fully relativistic calculations are presented for the double K-shell photoionization cross section for several neutral medium-Z atoms, from magnesium (Z=10) up to silver (Z=47). The calculations take into account all multipoles of the absorbed photon as well as the retardation of the electron-electron interaction. The approach is based on the partial-wave representation of the Dirac continuum states and uses the Green's-function technique to represent the full Dirac spectrum of intermediate states. The method is strictly gauge invariant, which is used as an independent cross-check of the computational procedure. The calculated ratios of the double-to-single K-shell ionization cross sections are compared with the experimental data and with previous computations.
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Relativistic calculations of the isotope shifts in highly charged Li-like ions - Zubova, N. A.; Kozhedub, Y. S.; Shabaev, V. M.; Tupitsyn, I. I.; Volotka, A. V.; Plunien, G.; Brandau, C.; St{\"{o}}hlker, T. (Physical Review A, 2014)
Relativistic calculations of the isotope shifts of energy levels in highly charged Li-like ions are performed. The nuclear recoil (mass shift) contributions are calculated by merging the perturbative and large-scale configuration-interaction Dirac-Fock-Sturm (CI-DFS) methods. The nuclear size (field shift) contributions are evaluated by the CI-DFS method including the electron-correlation, Breit, and QED corrections. The nuclear deformation and nuclear polarization corrections to the isotope shifts in Li-like neodymium, thorium, and uranium are also considered. The results of the calculations are compared with the theoretical values obtained with other methods.
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Relativistic theory for radiative forward electron emission in heavy ion-atom encounters - Jakubassa-Amundsen, D.; M{\"{u}}ller, R.; Surzhykov, A.; Yerokhin, V. (The European Physical Journal D, 2014)
The forward electron emission with simultaneous photon production during the scattering of relativistic, highly stripped projectiles from light target atoms is calculated within the Dirac theory. The method of calculation is a simplification of the impulse approximation and is based on the relation of the cross section for radiative capture to continuum of loosely bound electrons to the frame-transformed electron bremsstrahlung cross section. It is demonstrated that such an approximation is well justified in a large region of energies and photon emission angles, with the exception of the extreme forward and backward emission and the soft-photon energy limit. The cusp spectrum and the corresponding angular distribution are compared to recent experimental data for the collision system 90.38 MeV/amu U88+ + N2.
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Ultrafast fluorescence spectroscopy for axial resolution of flurorophore distributions - Gr{\"{a}fe, M. and Hoffmann, A. and Spielmann, C. (Applied Physics B, 2014)
A new method for determining the fluorophore distribution along the propagation axis of an ultrashort optical pulse is presented. The axial resolution is obtained by temporal gating of the backward emitted fluorescence via optical parametric amplification, and we demonstrated a resolution in the order of a few 100 μm. With this approach, sampling of the fluorophore concentration of thin layers without using optics with a large numerical aperture will be possible, such as investigating the human retina via time-resolved fluorescence measurements. Additionally, we verified the gain is orders of magnitude higher for coherent seeding, making optical parametric gating very interesting for discriminating between coherently and incoherently scattered light for other multimodal imaging applications.
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2 kW average power from a pulsed Yb-doped rod-type fiber amplifier - Otto, H.-J.; Stutzki, F.; Modsching, N.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2014)
This Letter reports on a fiber-laser system that, employing a 1 m long rod-type photonic-crystal fiber as its main-amplifier, emits a record average output power of 2 kW, by amplifying stretched ps-pulses. A further increase of the output power was only limited by the available laser-diode pump power. The energy of the pulses is 100 μJ, corresponding to MW-level peak powers extracted directly from the fiber of the main amplifier. The corresponding M2 at the maximum output power is <3, due to the onset of mode instabilities. The Letter covers the influence of this effect on the evolution of the beam quality with the output power. The numerical results show that the M2 value settles at around 3, even if the output average power is further increased.
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A pulsed supersonic gas jet target for precision spectroscopy at the HITRAP facility at GSI - Tiedemann, D.; Stiebing, K. E.; Winters, D. F. A.; Quint, W.; Varentsov, V.; Warczak, A.; Malarz, A.; St{\"{o}}hlker, T. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2014)
A pulsed supersonic gas jet target for experiments at the HITRAP facility at the GSI Helmholtzzentrum f{\"{u}}r Schwerionenforschung in Darmstadt has been designed and built as a multi-purpose installation for key experiments on fundamental atomic physics in strong fields. This setup is currently installed at the Institut f{\"{u}}r Kernphysik of Goethe-University, Frankfurt am Main (IKF), in order to explore its operation prior to its installation at the HITRAP facility. Design and performance of the target are described. The measured target densities of 5.9×10^12 atoms/cm3 for helium and 8.1×10^12 atoms/cm³ for argon at the stagnation pressure of 30 bar match the required values. The target-beam diameter of 0.9 mm and the pulsed operation mode (jet built-up-time ≤15 ms) are well suited for the use at HITRAP.
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Determination of small level splittings in highly charged ions via angle-resolved measurements of characteristic x rays - Wu, Z. W.; Kabachnik, N. M.; Surzhykov, A.; Dong, C. Z.; Fritzsche, S. (Physical Review A, 2014)
The angular distribution and the photon-photon angular correlation have been investigated for the x-ray emission from two-step radiative cascades that proceed via overlapping intermediate resonances. In particular, density matrix theory is applied in order to explore how the splitting of these intermediate levels affects the subsequent x-ray emission and whether measurements of photon angular distributions may help reveal level crossings in highly charged ions, if analyzed along isoelectronic sequences. Detailed computations within the multiconfiguration Dirac-Fock method were performed especially for the two-step 1s2p2J_i=1/2,3/2→1s2s2pJ=1/2,3/2+γ1→1s22sJ_f=1/2+γ1+γ2 cascade of lithiumlike ions, for which a level crossing of the two 1s2s2pJ=1/2,3/2 intermediate resonances occurs in the range 74≤Z≤79. For this cascade, a remarkably strong depolarization effect, associated with the finite lifetime of these intermediate levels, is found for the angular distribution and the photon-photon correlation function for all level splittings Δω≲0.2a.u.≈5.4 eV. We therefore suggest that accurate angle-resolved measurements of the x-ray emission may serve also as a tool for determining small level splittings in excited highly charged ions.
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Diagnostics for studies of novel laser ion acceleration mechanisms - Senje, L.; Yeung, M.; Aurand, B.; Kuschel, S.; R{\"{o}}del, C.; Wagner, F.; Li, K.; Dromey, B.; Bagnoud, V.; Neumayer, P.; Roth, M.; Wahlstr{\"{o}}m, C.-G.; et al. (Review of Scientific Instruments, 2014)
Diagnostic for investigating and distinguishing different laser ion acceleration mechanisms has been developed and successfully tested. An ion separation wide angle spectrometer can simultaneously investigate three important aspects of the laser plasma interaction: (1) acquire angularly resolved energy spectra for two ion species, (2) obtain ion energy spectra for multiple species, separated according to their charge to mass ratio, along selected axes, and (3) collect laser radiation reflected from and transmitted through the target and propagating in the same direction as the ion beam. Thus, the presented diagnostic constitutes a highly adaptable tool for accurately studying novel acceleration mechanisms in terms of their angular energy distribution, conversion efficiency, and plasma density evolution.
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Identification of X-ray spectra in the Na-like to O-like rubidium ions in the range of 3.8–7.3 {{A}} - Denis-Petit, D.; Comet, M.; Bonnet, T.; Hannachi, F.; Gobet, F.; Tarisien, M.; Versteegen, M.; Gosselin, G.; M{\'{e}}ot, V.; Morel, P.; Pain, J.-C.; Gilleron, F.; et al. (Journal of Quantitative Spectroscopy and Radiative Transfer, 2014)
Abstract The X-rays emitted by a rubidium plasma source created by the PHELIX laser at an intensity of about 6×1014 W/cm2 were studied. The lines have been measured using Bragg crystals in the wavelength range between 3.8 and 7.3 {{A}} and identified by means of a numerical method developed to describe highly charged rubidium ions in LTE plasma. The experimental plasma temperature, density and charge state distributions have been estimated using non-LTE codes such as CHIVAS and AVERROES. The LTE plasma temperature and density used in the calculations are those allowing to reproduce the calculated NLTE charge state distribution. In order to optimize the use of computational resources, a criterion is established to select the configurations contributing most to the spectra among all those obtained in detailed level accounting based on the MCDF code. Seventy Rb X-rays have been identified among which forty-nine are reported for the first time. The capabilities of our method are demonstrated by the good agreement of our identifications with previously published data when available.
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Image plate characterization and absolute calibration to low kilo-electron-volt electrons - Busold, S.; Philipp, K.; Otten, A.; Roth, M. (Review of Scientific Instruments, 2014)
We report on the characterization of an image plate and its absolute calibration to electrons in the low keV energy range (1–30 keV). In our case, an Agfa MD4.0 without protection layer was used in combination with a Fuji FLA7000 scanner. The calibration data are compared to other published data and a consistent picture of the sensitivity of image plates to electrons is obtained, which suggests a validity of the obtained calibration up to 100 keV.
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Quasi-monoenergetic femtosecond photon sources from Thomson Scattering using laser plasma accelerators and plasma channels - Rykovanov, S. G.; Geddes, C. G. R.; Vay, J.-L.; Schroeder, C. B.; Esarey, E.; Leemans, W. P. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2014)
Narrow bandwidth, high energy photon sources can be generated by Thomson scattering of laser light from energetic electrons, and detailed control of the interaction is needed to produce high quality sources. We present analytic calculations of the energy-angular spectra and photon yield that parametrize the influences of the electron and laser beam parameters to allow source design. These calculations, combined with numerical simulations, are applied to evaluate sources using conventional scattering in vacuum and methods for improving the source via laser waveguides or plasma channels. We show that the photon flux can be greatly increased by using a plasma channel to guide the laser during the interaction. Conversely, we show that to produce a given number of photons, the required laser energy can be reduced by an order of magnitude through the use of a plasma channel. In addition, we show that a plasma can be used as a compact beam dump, in which the electron beam is decelerated in a short distance, thereby greatly reducing radiation shielding. Realistic experimental errors such as transverse jitter are quantitatively shown to be tolerable. Examples of designs relevant to nuclear resonance fluorescence and photofission are provided.
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Ultrahigh Brilliance Multi-MeV γ-Ray Beams from Nonlinear Relativistic Thomson Scattering - Sarri, G.; Corvan, D.; Schumaker, W.; Cole, J.; Di Piazza, A.; Ahmed, H.; Harvey, C.; Keitel, C.; Krushelnick, K.; Mangles, S.; Najmudin, Z.; Symes, D.; et al. (Physical Review Letters, 2014)
We report on the generation of a narrow divergence (θγ<2.5 mrad), multi-MeV (Emax≈18 MeV) and ultrahigh peak brilliance (>1.8×10^20 photons s^−1 mm^−2 mrad^−2 0.1% BW) γ-ray beam from the scattering of an ultrarelativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude a0≈2). The spectrum of the generated γ-ray beam is measured, with MeV resolution, seamlessly from 6 to 18 MeV, giving clear evidence of the onset of nonlinear relativistic Thomson scattering. To the best of our knowledge, this photon source has the highest peak brilliance in the multi-MeV regime ever reported in the literature.
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A concept for multiterawatt fibre lasers based on coherent pulse stacking in passive cavities - Breitkopf, S.; Eidam, T.; Klenke, A.; Grafenstein, L. v.; Carstens, H.; Holzberger, S.; Fill, E.; Schreiber, T.; Krausz, F.; T{\"{u}}nnermann, A.; Pupeza, I.; Limpert, J. (Light: Science { &} Applications, 2014)
Since the advent of femtosecond lasers, performance improvements have constantly impacted on existing applications and enabled novel applications. However, one performance feature bearing the potential of a quantum leap for high-field applications is still not available: the simultaneous emission of extremely high peak and average powers. Emerging applications such as laser particle acceleration require exactly this performance regime and, therefore, challenge laser technology at large. On the one hand, canonical bulk systems can provide pulse peak powers in the multi-terawatt to petawatt range, while on the other hand, advanced solid-state-laser concepts such as the thin disk, slab or fibre are well known for their high efficiency and their ability to emit high average powers in the kilowatt range with excellent beam quality. In this contribution, a compact laser system capable of simultaneously providing high peak and average powers with high wall-plug efficiency is proposed and analysed. The concept is based on the temporal coherent combination (pulse stacking) of a pulse train emitted from a high-repetition-rate femtosecond laser system in a passive enhancement cavity. Thus, the pulse energy is increased at the cost of the repetition rate while almost preserving the average power. The concept relies on a fast switching element for dumping the enhanced pulse out of the cavity. The switch constitutes the key challenge of our proposal. Addressing this challenge could, for the first time, allow the highly efficient dumping of joule-class pulses at megawatt average power levels and lead to unprecedented laser parameters.
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Designing advanced very-large-mode-area fibers for power scaling of fiber-laser systems - Stutzki, F.; Jansen, F.; Otto, H.-J.; Jauregui, C.; Limpert, J.; T{\"{u}}nnermann, A. (Optica, 2014)
Fiber lasers are a highly regarded solid-state laser concept due to their high efficiency, beam quality, and easy thermal management. Unfortunately, the performance of high-power fiber-laser systems is challenged by the onset of detrimental nonlinear effects. Their impact can be reduced dramatically by employing fibers with larger mode-field areas. Even though this is an efficient way to mitigate nonlinear effects, maintaining effective single-mode operation, and with it high beam quality, becomes increasingly difficult as the core is enlarged. In this paper the demands and challenges for the design of a very-large-mode-area (VLMA) fiber are discussed. The benefits of using higher-order mode delocalization as the working principle of active double-clad VLMA fibers are described. Finally, a new low-symmetry large-pitch fiber, which is expected to improve the performance of state-of-the-art fiber-laser systems by increasing higher-order mode delocalization, is proposed and thoroughly analyzed.
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Electron-loss-to-continuum cusp in U⁸⁸⁺ + N₂ collisions - Hillenbrand, P.-M.; Hagmann, S.; Voitkiv, A. B.; Najjari, B.; Bana{ś}, D.; Blumenhagen, K.-H.; Brandau, C.; Chen, W.; De Filippo, E.; Gumberidze, A.; Guo, D. L.; Kozhuharov, C.; et al. (Physical Review A, 2014)
The electron loss to the continuum has been studied for the collision system U88+ + N2→U89+ + [N2]∗ + e− at the low-relativistic projectile energy of 90 MeV/u. Using a magnetic electron spectrometer, the energy distribution of cusp electrons emitted at an angle of 0∘ with respect to the projectile beam was measured in coincidence with the up-charged projectile. At the experimental collision energy ionization of the berylliumlike U88+ projectile proceeds predominantly from the L shell, but a contribution from the K shell could also be identified experimentally. The measurement is shown to be in accordance with fully relativistic Dirac calculations applying first-order perturbation theory. Furthermore, the underlying continuum electron distribution in the projectile frame is illustrated.
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Low-energy electron rescattering in laser-induced ionization - Becker, W.; Goreslavski, S. P.; Milo{{s}}evi{}, D. B.; Paulus, G.G. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2014)
The low-energy structure (LES) in the energy spectrum of above-threshold ionization of rare-gas atoms is reinvestigated from three different points of view. First, the role of forward rescattering in the completely classical simple-man model (SMM) is considered. Then, the corresponding classical electronic trajectories are retrieved in the quantum-mechanical ionization amplitude derived in the strong-field approximation augmented to allow for rescattering. Third, classical trajectories in the presence of both the laser field and the Coulomb field are scrutinized in order to see how they are related to the LES. It is concluded that the LES is already rooted in the SMM. The Coulomb field enhances the structure so that it can successfully compete with other contributions and become visible in the total spectrum.
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Relativistic calculations of x-ray emission following a Xe-Bi 83+ collision - Kozhedub, Y. S.; Shabaev, V. M.; Tupitsyn, I. I.; Gumberidze, A.; Hagmann, S.; Plunien, G.; St{\"{o}}hlker, T. (Physical Review A, 2014)
We study the x-ray emission following the collision of a Bi83+ ion with a neutral Xe atom at the projectile energy 70 MeV/u. The collisional and post-collisional processes are treated separately. The probabilities of various many-electron processes at the collision are calculated within a relativistic independent electron model using the coupled-channel approach with atomiclike Dirac-Fock-Sturm orbitals. The analysis of the post-collisional processes resulting in the x-ray emission is based on the fluorescence yields, the radiation, and Auger decay rates, and allows one to derive intensities of the x-ray emission and compare them with experimental data. A reasonable agreement between the theoretical results and the recent experimental data is observed. The role of the relativistic effects is investigated.
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Ultra-high contrast frontend for high peak power fs-lasers at 1030 nm - Liebetrau, H.; Hornung, M.; Seidel, A.; Hellwing, M.; Kessler, A.; Keppler, S.; Schorcht, F.; Hein, J.; Kaluza, M.C. (Optics Express, 2014)
We present the results from a new frontend within a double-chirped pulse amplification architecture (DCPA) utilizing crossed-polarized wave generation (XPW) for generating ultra-high contrast, 150 μJ-level, femtosecond seed pulses at 1030 nm. These pulses are used in the high energy class diode-pumped laser system Polaris at the Helmholtz Institute in Jena. Within this frontend, laser pulses from a 75 MHz oscillator-pulse train are extracted at a repetition rate of 1 Hz, temporally stretched, amplified and then recompressed reaching a pulse energy of 2 mJ, a bandwidth of 12 nm and 112 fs pulse duration at a center wavelength of 1030 nm. These pulses are temporally filtered via XPW in a holographic-cut BaF2 crystal, resulting in 150 μJ pulse energy with an efficiency of 13 %. Due to this non-linear filtering, the relative intensity of the amplified spontaneous emission preceding the main pulse is suppressed to 2×10^−13. This is, to the best of our knowledge, the lowest value achieved in a high peak power laser system operating at 1030 nm center wavelength.
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53 W average power few-cycle fiber laser system generating soft x rays up to the water window - Rothhardt, J.; H{\"{a}drich, S. and Klenke, A. and Demmler, S. and Hoffmann, A. and Gotschall, T. and Eidam, T. and Krebs, M. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2014)
We report on a few-cycle laser system delivering sub-8-fs pulses with 353 µJ pulse energy and 25 GW of peak power at up to 150 kHz repetition rate. The corresponding average output power is as high as 53 W, which represents the highest average power obtained from any few-cycle laser architecture so far. The combination of both high average and high peak power provides unique opportunities for applications. We demonstrate high harmonic generation up to the water window and record-high photon flux in the soft x-ray spectral region. This tabletop source of high-photon flux soft x rays will, for example, enable coherent diffractive imaging with sub-10-nm resolution in the near future.
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Bent crystal spectrometer for both frequency and wavenumber resolved x-ray scattering at a seeded free-electron laser - Zastrau, U.; Fletcher, L.; F{\"{o}}rster, E.; Galtier, E.; Gamboa, E.; Glenzer, S.; Heimann, P.; Marschner, H.; Nagler, B.; Schropp, A.; Wehrhan, O.; Lee, H. (Review of Scientific Instruments, 2014)
We present a cylindrically curved GaAs x-ray spectrometer with energy resolution ΔE/E = 1.1 × 10^−4 and wave-number resolution of Δk/k = 3 × 10^−3, allowing plasmon scattering at the resolution limits of the Linac Coherent Light Source (LCLS) x-ray free-electron laser. It spans scattering wavenumbers of 3.6 to 5.2/{{A}} in 100 separate bins, with only 0.34% wavenumber blurring. The dispersion of 0.418 eV/13.5 μm agrees with predictions within 1.3%. The reflection homogeneity over the entire wavenumber range was measured and used to normalize the amplitude of scattering spectra. The proposed spectrometer is superior to a mosaic highly annealed pyrolytic graphite spectrometer when the energy resolution needs to be comparable to the LCLS seeded bandwidth of 1 eV and a significant range of wavenumbers must be covered in one exposure.
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Characterisation of deuterium spectra from laser driven multi-species sources by employing differentially filtered image plate detectors in Thomson spectrometers - Alejo, A.; Kar, S.; Ahmed, H.; Krygier, A. G.; Doria, D.; Clarke, R.; Fernandez, J.; Freeman, R. R.; Fuchs, J.; Green, A.; Green, J. S.; Jung, D.; et al. (Review of Scientific Instruments, 2014)
A novel method for characterising the full spectrum of deuteron ions emitted by laser driven multi-species ion sources is discussed. The procedure is based on using differential filtering over the detector of a Thompson parabola ion spectrometer, which enables discrimination of deuterium ions from heavier ion species with the same charge-to-mass ratio (such as C^(6+), O^(8+), etc.). Commonly used Fuji Image plates were used as detectors in the spectrometer, whose absolute response to deuterium ions over a wide range of energies was calibrated by using slotted CR-39 nuclear track detectors. A typical deuterium ion spectrum diagnosed in a recent experimental campaign is presented, which was produced from a thin deuterated plastic foil target irradiated by a high power laser.
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High photon flux table-top coherent extreme-ultraviolet source - H{\"{a}drich, S. and Klenke, A. and Rothhardt, J. and Krebs, M. and Hoffmann, A. and Pronin, O. and Pervak, V. and Limpert, J. and T{\"{u}}nnermann, A. (Nature Photonics, 2014)
High harmonic generation (HHG) enables extreme-ultraviolet radiation with table-top set-ups. Its exceptional properties, such as coherence and (sub)-femtosecond pulse durations, have led to a diversity of applications. Some of these require a high photon flux and megahertz repetition rates, for example, to avoid space charge effects in photoelectron spectroscopy. To date, this has only been achieved with enhancement cavities. Here, we establish a novel route towards powerful HHG sources. By achieving phase-matched HHG of a megahertz fibre laser we generate a broad plateau (25 eV – 40 eV) of strong harmonics, each containing more than 1 × 10^12 photons s–1, which constitutes an increase by more than one order of magnitude in that wavelength range. The strongest harmonic (H25, 30 eV) has an average power of 143 μW (3 × 10^13 photons s–1). This concept will greatly advance and facilitate applications in photoelectron or coincidence spectroscopy, coherent diffractive imaging or (multidimensional) surface science.
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Integrated reflectivity measurements of hydrogen phthalate crystals for high-resolution soft x-ray spectroscopy - Zastrau, U.; F{\"{o}}rster, E. (Journal of Instrumentation, 2014)
The integrated x-ray reflectivity of Potassium Hydrogen Phthalate (KAP) and Rubidium Hydrogen Phthalate (RAP) crystals is studied at a photon energy of (1740±14) eV using a double-crystal setup. The absolute measured reflectivities are in < 5% agreement with the values predicted by the dynamic diffraction theory for perfect crystals when absorption is included. Within 4% experimental error margins, specimen that were exposed to ambient conditions over many years show identical reflectivity as specimen that were cleaved just before the measurement. No differences are observed between cleaving off a 10 μm surface layer and splitting the entire crystal bulk of 2 mm thickness. We conclude that at 1.7 keV photon energy the penetration depth of 1 μm is large compared to a potentially deteriorated surface layer of a few 10 nm.
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LambdaMSbar (nf=2) from a momentum space analysis of the quark-antiquark static potential - Karbstein, F.; Peters, A.; Wagner, M. (Journal of High Energy Physics, 2014)
We determine LambdaMSbar (nf=2) by fitting perturbative expressions for the quark-antiquark static potential to lattice results for QCD with nf=2 dynamical quark flavors. To this end we use the perturbative static potential at the presently best known accuracy, i.e. up to O(α_s^4), in momentum space. The lattice potential is computed on a fine lattice with a≈0.042fm in position space. To allow for a comparison and matching of both results, the lattice potential is transformed into momentum space by means of a discrete Fourier transform. The value of LambdaMSbar (nf=2) is extracted in momentum space. All sources of statistical and systematic errors are discussed. The uncertainty in the value of LambdaMSbar (nf=2) is found to be smaller than that obtained in a recent position space analysis of the static potential based on the same lattice data.
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Observation of Coherence in the Time-Reversed Relativistic Photoelectric Effect - Tashenov, S.; Bana{ś}, D.; Beyer, H.; Brandau, C.; Fritzsche, S.; Gumberidze, A.; Hagmann, S.; Hillenbrand, P.-M.; J{\"{o}}rg, H.; Kojouharov, I.; Kozhuharov, Ch.; Lestinsky, M.; et al. (Physical Review Letters, 2014)
The photoelectric effect has been studied in the regime of hard x rays and strong Coulomb fields via its time-reversed process of radiative recombination (RR). In the experiment, the relativistic electrons recombined into the 2p3/2 excited state of hydrogenlike uranium ions, and both the RR x rays and the subsequently emitted characteristic x rays were detected in coincidence. This allowed us to observe the coherence between the magnetic substates in a highly charged ion and to identify the contribution of the spin-orbit interaction to the RR process.
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Observation of the hyperfine transition in lithium-like bismuth 209Bi80+ : Towards a test of QED in strong magnetic fields - Lochmann, M.; J{\"{o}}hren, R.; Geppert, C.; Andelkovic, Z.; Anielski, D.; Botermann, B.; Bussmann, M.; Dax, A.; Fr{\"{o}}mmgen, N.; Hammen, M.; Hannen, V.; K{\"{u}}hl, T.; et al. (Physical Review A, 2014)
We performed a laser spectroscopic determination of the 2s hyperfine splitting (HFS) of Li-like 209Bi80+ and repeated the measurement of the 1s HFS of H-like 209Bi82+. Both ion species were subsequently stored in the Experimental Storage Ring at the GSI Helmholtzzentrum f{\"{u}}r Schwerionenforschung Darmstadt and cooled with an electron cooler at a velocity of ≈0.71c. Pulsed laser excitation of the M1 hyperfine transition was performed in anticollinear and collinear geometry for Bi82+ and Bi80+, respectively, and observed by fluorescence detection. We obtain ΔE(1s)=5086.3(11)meV for Bi82+, different from the literature value, and ΔE(2s)=797.50(18)meV for Bi80+. These values provide experimental evidence that a specific difference between the two splitting energies can be used to test QED calculations in the strongest static magnetic fields available in the laboratory independent of nuclear structure effects. The experimental result is in excellent agreement with the theoretical prediction and confirms the sum of the Dirac term and the relativistic interelectronic-interaction correction at a level of 0.5%, confirming the importance of accounting for the Breit interaction.
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Performance Scaling of Ultrafast Laser Systems by Coherent Addition of Femtosecond Pulses - Limpert, J.; Klenke, A.; Kienel, M.; Breitkopf, S.; Eidam, T.; H{\"{a}drich, S. and Jauregui, C. and T{\"{u}}nnermann, A. (IEEE Journal of Selected Topics in Quantum Electronics, 2014)
In the last decades, ultrafast lasers and amplifiers have achieved an extraordinary power increase and have enabled a plethora of scientific, medical or industrial applications. However, especially in recent years, it has become more and more challenging to keep up with this pace since intrinsic physical limitations are becoming difficult to avoid. A promising way to get around this problem is the technique of spatially and/or temporally separated amplification and subsequent coherent addition of ultrashort pulses. It turns out that fiber amplifiers are perfect candidates for this concept due to their outstanding average-power capability and their simple single-pass setups, which can be easily parallelized. Herein we provide an overview of the most important experimental implementations of this concept and recent results. We discuss the ability of these approaches to generate laser parameters that, only a few years ago, seemed impossible to achieve.
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Polarization of atomic bremsstrahlung in coincidence studies - M{\"{u}}ller, R. A.; Yerokhin, V. A.; Surzhykov, A. (Physical Review A, 2014)
We present a theoretical study of bremsstrahlung produced by high-energy electrons scattered by heavy atomic targets. Considering coincident observation of the emitted photons and the scattered electrons, we pay special attention to the polarization degree and direction of the outgoing light. To investigate these properties of atomic bremsstrahlung, we apply the density matrix approach and solutions of the Dirac equation. Detailed calculations are performed for initial electron energies ranging from 100 to 500 keV and different fixed electron scattering angles. The results of these calculations are compared with predictions obtained under the assumption that the scattered electrons remain unobserved. This comparison reveals that both the degree and the direction of linear polarization of bremsstrahlung are very sensitive to the direction of the scattered electron.
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Realizing a laser-driven electron source applicable for radiobiological tumor irradiation - Nicolai, M.; S{\"{a}vert, A. and Reuter, M. and Schnell, M. and Polz, J. and J{\"{a}ckel, O. and Karsch, L. and Sch{\"{u}}rer, M. and Oppelt, M. and Pawelke, J. and Kaluza, M.C. (Applied Physics B, 2014)
Laser-accelerated electron pulses have been used to irradiate human tumors grown on mice’s ears during radiobiological experiments. These experiments have been carried out with the JETI laser system at the Institute of Optics and Quantum Electronics in Jena, Germany. To treat a total of more than 50 mice, a stable and reliable operation of the laser-electron accelerator with a dose rate exceeding 1 Gy/min was necessary. To achieve this, a sufficient number of electrons at energies in excess of 5 MeV had to be generated. The irradiation time for a single mouse was a few minutes. Furthermore, the particle pulses’ parameters needed to remain achievable for a time period of several weeks. Due to the online detection of the radiation dose, the unavoidable shot-to-shot fluctuations, currently still typical for laser-based particle accelerators, could be compensated. The results demonstrate that particle pulses generated with laser-based accelerators have the potential to be a future alternative for conventional particle accelerators used for the irradiation of tumors.
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Spatio-temporal coherence of free-electron laser radiation in the extreme ultraviolet determined by a Michelson interferometer - Hilbert, V.; R{\"{o}}del, C.; Brenner, G.; D{\"{o}}ppner, T.; D{\"{u}}sterer, S.; Dziarzhytski, S.; Fletcher, L.; F{\"{o}}rster, E.; Glenzer, S. H.; Harmand, M.; Hartley, N. J.; Kazak, L.; et al. (Applied Physics Letters, 2014)
A key feature of extreme ultraviolet (XUV) radiation from free-electron lasers (FELs) is its spatial and temporal coherence. We measured the spatio-temporal coherence properties of monochromatized FEL pulses at 13.5 nm using a Michelson interferometer. A temporal coherence time of (59±8) fs has been determined, which is in good agreement with the spectral bandwidth given by the monochromator. Moreover, the spatial coherence in vertical direction amounts to about 15% of the beam diameter and about 12% in horizontal direction. The feasibility of measuring spatio-temporal coherence properties of XUV FEL radiation using interferometric techniques advances machine operation and experimental studies significantly.
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Test of Time Dilation Using Stored Li+ Ions as Clocks at Relativistic Speed - Botermann, B.; Bing, D.; Geppert, C.; Gwinner, G.; H{\"{a}nsch, T. and Huber, G. and Karpuk, S. and Krieger, A. and K{\"{u}}hl, T. and N{\"{o}}rtersh{\"{a}user, W. and Novotny, C. and Reinhardt, S. and Sanchez, R. and Schwalm, D. and St{\"{o}}hlker, T. and Wolf, A. and Saathoff, G. (Physical Review Letters, 2014)
We present the concluding result from an Ives-Stilwell-type time dilation experiment using Li+7 ions confined at a velocity of β=v/c=0.338 in the storage ring ESR at Darmstadt. A Λ-type three-level system within the hyperfine structure of the Li+7S13→P23 line is driven by two laser beams aligned parallel and antiparallel relative to the ion beam. The lasers’ Doppler shifted frequencies required for resonance are measured with an accuracy of <4×10−9 using optical-optical double resonance spectroscopy. This allows us to verify the special relativity relation between the time dilation factor γ and the velocity β, γ√(1- β^2)=1 to within ±2.3×10−9 at this velocity. The result, which is singled out by a high boost velocity β, is also interpreted within Lorentz invariance violating test theories.
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152 W average power Tm-doped fiber CPA system - Stutzki, F.; Gaida, C.; Gebhardt, M.; Jansen, F.; Wienke, A.; Zeitner, U.; Fuchs, F.; Jauregui, C.; Wandt, D.; Kracht, D.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2014)
A high-power thulium (Tm)-doped fiber chirped-pulse amplification system emitting a record compressed average output power of 152 W and 4 MW peak power is demonstrated. This result is enabled by utilizing Tm-doped photonic crystal fibers with mode-field diameters of 35 μm, which mitigate detrimental nonlinearities, exhibit slope efficiencies of more than 50%, and allow for reaching a pump-power-limited average output power of 241 W. The high-compression efficiency has been achieved by using multilayer dielectric gratings with diffraction efficiencies higher than 98%.
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Fluorescence and thermal stress properties of Yb 3+ -doped alumino silicate glasses for ultra high peak power laser applications - Tiegel, M.; Herrmann, A.; Kuhn, S.; Russel, C.; K{\"{o}}rner, J.; Kl{\"{o}}pfel, D.; Seifert, R.; Hein, J.; Kaluza, M.C. (Laser Physics Letters, 2014)
Various alumino silicate glasses (network modifier ions: Li^+ , Mg^2+ , Zn^2+ and/or La^^3+ ) doped with 1 × 10^20 Yb^3+ cm^−3 (about 0.2 mol% Yb_2 O_3 ) were prepared. The glasses were studied with respect to their thermo-mechanical and fluorescence properties. Huge differences are found for the coefficients of thermal expansion which determine the thermal shock resistance of the material and hence are required for ultra-high power laser applications. Here, zinc and magnesium alumino silicate glasses show the lowest values. The fluorescence lifetimes of the glasses increase with decreasing average atomic weight of the glass composition (685–1020 µ s). All glasses show broad and smooth emission spectra with little variations due to compositional changes. Mixed lithium zinc or lithium magnesium alumino silicate glasses could be promising new laser materials especially with respect to ultra-high peak power systems or applications with high repetition rates.
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Laser photon merging in an electromagnetic field inhomogeneity - Gies, H.; Karbstein, F.; Shaisultanov, R. (Physical Review D, 2014)
We study the effect of laser photon merging, or equivalently high harmonic generation, in the quantum vacuum subject to inhomogeneous electromagnetic fields. Such a process is facilitated by the effective nonlinear couplings arising from charged particle-antiparticle fluctuations in the quantum vacuum subject to strong electromagnetic fields. We derive explicit results for general kinematic and polarization configurations involving optical photons. Concentrating on merged photons in reflected channels which are preferable in experiments for reasons of noise suppression, we demonstrate that photon merging is typically dominated by the competing nonlinear process of quantum reflection, though appropriate polarization and signal filtering could specifically search for the merging process. As a byproduct, we devise a novel systematic expansion of the photon polarization tensor in plane wave fields.
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Off-axis low-energy structures in above-threshold ionization - M{\"{o}}ller, M.; Meyer, F.; Sayler, A. M.; Paulus, G.G.; Kling, M. F.; Schmidt, B. E.; Becker, W.; Milo{{s}}evi{}, D. B. (Physical Review A, 2014)
The velocity map of the above-threshold ionization electron spectrum at long laser wavelength exhibits a characteristic structure normal to the laser polarization, which has the appearance of a trident or a three-pronged fork. The forklike structure vanishes for few-cycle laser pulses. It is explained in terms of the classical-electron-trajectories model of strong-field ionization augmented so as to allow for rescattering. The analysis reveals its relation to the so-called low-energy structure, which was recently observed for very small transverse momenta.
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Phase structure of many-flavor QED₃ - Braun, J.; Gies, H.; Janssen, L.; Roscher, D. (Physical Review D, 2014)
We analyze the many-flavor phase diagram of quantum electrodynamics (QED) in 2+1 (Euclidean) space-time dimensions. We compute the critical flavor number above which the theory is in the quasiconformal massless phase. For this, we study the renormalization group fixed-point structure in the space of gauge interactions and pointlike fermionic self-interactions, the latter of which are induced dynamically by fermion-photon interactions. We find that a reliable estimate of the critical flavor number crucially relies on a careful treatment of the Fierz ambiguity in the fermionic sector. Using a Fierz-complete basis, our results indicate that the phase transition towards a chirally broken phase occurring at small flavor numbers could be separated from the quasiconformal phase at larger flavor numbers, allowing for an intermediate phase which is dominated by fluctuations in a vector channel. If these interactions approach criticality, the intermediate phase could be characterized by a Lorentz-breaking vector condensate.
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Radiative-electron-capture-to-continuum cusp in U⁸⁸⁺+N₂ collisions and the high-energy endpoint of electron-nucleus bremsstrahlung - Hillenbrand, P.-M.; Hagmann, S.; Atanasov, D.; Bana{ś}, D.; Blumenhagen, K.-H.; Brandau, C.; Chen, W.; De Filippo, E.; Gumberidze, A.; Guo, D. L.; Jakubassa-Amundsen, D. H.; Kovtun, O.; et al. (Physical Review A, 2014)
The radiative electron capture of a target electron into the projectile continuum has been studied for the collision system U88+ + N2 → U88+ + [N+2]∗ + e− + γ at 90 MeV/u. Using a magnetic electron spectrometer, the energy distribution of cusp electrons emitted under an angle of 0∘ with respect to the projectile beam and with a velocity close to the projectile velocity has been measured in coincidence with the emitted photons under various observation angles. The experimental results provide a stringent test for the corresponding process in inverse kinematics, namely, the theory of electron-nucleus bremsstrahlung at the high-energy endpoint. For comparison this process is calculated using fully relativistic Dirac wave functions and using semirelativistic Sommerfeld-Maue wave functions.
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Relativistic configuration-interaction calculation of Kα transition energies in berylliumlike iron - Yerokhin, V. A.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2014)
We perform relativistic configuration-interaction calculations of the energy levels of the low-lying and core-excited states of berylliumlike iron, Fe^(22+). The results include the QED contributions calculated by two different methods, the model QED operator approach and the screening-potential approach. The uncertainties of theoretical energies are estimated systematically. The predicted wavelengths of the Kα transitions in berylliumlike iron improve previous theoretical results and compare favorably with the experimental data.
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Temporal contrast control at the PHELIX petawatt laser facility by means of tunable sub-picosecond optical parametric amplification - Wagner, F.; Jo{\ {a}}o, C.; Fils, J.; Gottschall, T.; Hein, J.; K{\"{o}}rner, J.; Limpert, J.; Roth, M.; St{\"{o}}hlker, T.; Bagnoud, V. (Applied Physics B, 2014)
We report on the development of a preamplifier module for temporal contrast enhancement and control at petawatt-class lasers. The module is based on an ultrafast optical parametric amplifier (uOPA), which produces temporally clean pulses at the 60 μJ level for seeding a chirped pulse amplification (CPA) system, namely the petawatt facility PHELIX. The amplifier module allows for gain reduction in the following amplifiers, resulting in an attenuation of amplified spontaneous emission (ASE) by more than 4 orders of magnitude. Since the ASE of a CPA system linearly depends on the seeding energy, we were able to demonstrate a continuous variation of the temporal contrast by tuning the gain of the uOPA.
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Absorption of twisted light by hydrogenlike atoms - Scholz-Marggraf, H. M.; Fritzsche, S.; Serbo, V. G.; Afanasev, A.; Surzhykov, A. (Physical Review A, 2014)
Theoretical analysis of the excitation of low-Z hydrogenlike atoms by incident twisted light is presented. Emphasis is placed on the cross sections that describe transitions between particular atomic substates. Simple expressions for these partial cross sections are derived using the nonrelativistic first-order perturbation theory and the momentum representation of the photon wave functions. Based on the developed approach, calculations have been performed for the 1s→2p and 2p→3d transitions induced in the course of interaction of twisted (Bessel) light with a macroscopic hydrogen target. Results of these calculations, supported by an analytical analysis, clearly indicate that the sublevel population of residual atoms following absorption of twisted photons differs much from what is expected for the standard plane-wave case; this effect can be easily observed experimentally by measuring the linear polarization of the subsequent fluorescent emission.
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Equilibration dynamics and conductivity of warm dense hydrogen - Zastrau, U.; Sperling, P.; Becker, A.; Bornath, T.; Bredow, R.; D{\"{o}}ppner, T.; Dziarzhytski, S.; Fennel, T.; Fletcher, L. B.; F{\"{o}}rster, E.; Fortmann, C.; Glenzer, S. H.; et al. (Physical Review E, 2014)
We investigate subpicosecond dynamics of warm dense hydrogen at the XUV free-electron laser facility (FLASH) at DESY (Hamburg). Ultrafast impulsive electron heating is initiated by a ≤300-fs short x-ray burst of 92 eV photon energy. A second pulse probes the sample via x-ray scattering at jitter-free variable time delay. We show that the initial molecular structure dissociates within (0.9±0.2) ps, allowing us to infer the energy transfer rate between electrons and ions. We evaluate Saha and Thomas-Fermi ionization models in radiation hydrodynamics simulations, predicting plasma parameters that are subsequently used to calculate the static structure factor. A conductivity model for partially ionized plasma is validated by two-temperature density-functional theory coupled to molecular dynamic simulations and agrees with the experimental data. Our results provide important insights and the needed experimental data on transport properties of dense plasmas.
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High precision measurement of undulator polarization in the regime of hard x-rays - Marx, B.; Schulze, K. S.; Uschmann, I.; K{\"{a}mpfer, T. and Wehrhan, O. and Wille, H. C. and Schlage, K. and R{\"{o}}hlsberger, R. and Weckert, E. and F{\"{o}}rster, E. and St{\"{o}}hlker, T. and Paulus, G.G. (Applied Physics Letters, 2014)
We have measured the polarization purity of undulator radiation at 12.9 keV, with hitherto unachievable precision. We could measure a polarization purity of 1.8 × 10−4 by using a silicon channel-cut crystal with six Bragg reflections at 45° as analyzer.
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Ray-tracing simulations of spherical Johann diffraction spectrometer for in-beam X-ray experiments - Jagodzinski, P.; Pajek, M.; Bana{ś}, D.; Beyer, H.; Trassinelli, M.; St{\"{o}}hlker, T. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2014)
The results of the Monte-Carlo ray-tracing simulations for a Johann-type Bragg spectrometer with spherically curved-crystal designed to detect the X-rays from a fast-moving source are reported. These calculations were performed to optimize the X-ray spectrometer to be used at the gas-target installed at ion storage ring for high-resolution X-ray experiments. In particular, the two-dimensional distributions of detected photons were studied using the Monte-Carlo method both for the stationary and moving X-ray sources, taking into account a detailed description of X-ray source and X-ray diffraction on the crystal as well as a role of the Doppler effect for in-beam experiments. The origin of the asymmetry of observed X-ray profiles was discussed in detail and the procedure to derive a precise (sub-eV) X-ray transition energy for such asymmetric profiles was proposed. The results are important for the investigations of View the MathML source1s2p P2/3→1s2s S1/3 intrashell transition in excited He-like uranium ions in in-beam X-ray experiments.
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Spectroscopic characterization of Yb3+-doped laser materials at cryogenic temperatures - K{\"{o}}rner, J.; Jambunathan, V.; Hein, J.; Seifert, R.; Loeser, M.; Siebold, M.; Schramm, U.; Sikocinski, P.; Lucianetti, A.; Mocek, T.; Kaluza, M.C. (Applied Physics B, 2014)
We present measurements of the absorption and emission cross-sections for Yb:YAG, Yb:LuAG and Yb:CaF_2 as a function of temperature between 80 and 340 K. The cross-sections are determined by the combination of the McCumber relation and the Fuchtbauer–Ladenburg (FL) equation to achieve reliable results in spectral regions of high and low absorption. The experimental setup used for the fluorescence measurements minimizes re-absorption effects due to the measurement from small sample volume, providing nearly undisturbed raw data for the FL approach. The retrieved cross-sections together with the spectral characteristics of the tested materials provide important information for the design of energy efficient, high-power laser amplifiers.
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Structured x-ray beams from twisted electrons by inverse Compton scattering of laser light - Seipt, D.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2014)
The inverse Compton scattering of laser light on high-energetic twisted electrons is investigated with the aim to construct spatially structured x-ray beams. In particular, we analyze how the properties of the twisted electrons, such as the topological charge and aperture angle of the electron Bessel beam, affect the energy and angular distribution of scattered x rays. We show that with suitably chosen initial twisted electron states one can synthesize tailor-made x-ray beam profiles with a well-defined spatial structure, in a way not possible with ordinary plane-wave electron beams.
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Thermal properties of borate crystals for high power optical parametric chirped-pulse amplification - Riedel, R.; Rothhardt, J.; Beil, K.; Gronloh, B.; Klenke, A.; H{\"{o}}ppner, H.; Schulz, M.; Teubner, U.; Kr{\"{a}nkel, C. and Limpert, J. and T{\"{u}}nnermann, A. and Prandolini, M. and Tavella, F. (Optics Express, 2014)
The potential of borate crystals, BBO, LBO and BiBO, for high average power scaling of optical parametric chirped-pulse amplifiers is investigated. Up-to-date measurements of the absorption coefficients at 515 nm and the thermal conductivities are presented. The measured absorption coefficients are a factor of 10–100 lower than reported by the literature for BBO and LBO. For BBO, a large variation of the absorption coefficients was found between crystals from different manufacturers. The linear and nonlinear absorption coefficients at 515 nm as well as thermal conductivities were determined for the first time for BiBO. Further, different crystal cooling methods are presented. In addition, the limits to power scaling of OPCPAs are discussed.
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Characterization of a broadband interferometric autocorrelator for visible light with ultrashort blue laser pulses - Z{\"{u}}rch, M.; Hoffmann, A.; Gr{\"{a}fe, M. and Landgraf, B. and Riediger, M. and Spielmann, Ch. (Optics Communications, 2014)
We present a compact interferometric autocorrelator that allows the characterization of ultrashort laser pulses in the visible light domain (370–740nm). The presented device uses a GaN photodiode with corresponding two-photon absorption. Different GaN and AlGaN photodiodes were characterized for this purpose. Despite AlGaN diodes have a better matched bandgap for this application, we have found that only the GaN diodes show sufficient nonlinear behavior. Using the autocorrelator we were able to characterize ultrashort frequency doubled Ti:Sapphire laser pulses with a pulse duration down to 18fs in the second harmonic having just a few hundred nanojoules of pulse energy. The broadband behavior and extension towards the UV along with the need for only low energetic pulses are the novelties of this device.
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Coherent beam combination of Yb:YAG single-crystal rod amplifiers - Kienel, M.; M{\"{u}}ller, M.; Demmler, S.; Rothhardt, J.; Klenke, A.; Eidam, T.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2014)
Coherent combination of ultrashort laser pulses emitted from spatially separated amplifiers is a promising power-scaling technique for ultrafast laser systems. It has been successfully applied to fiber amplifiers, since guidance of the signal provides the advantage of an excellent beam quality and straightforward superposition of beams as compared to bulk-type amplifier implementations. Herein we demonstrate, for the first time to our knowledge, a two-channel combining scheme employing Yb:YAG single-crystal rod amplifiers as an energy booster in a fiber chirped-pulse amplification system. In this proof-of-principle experiment, combined and compressed pulses with a duration of 695 fs and an energy of 3 mJ (3.7 GW of peak power) are obtained. The combining efficiency is as high as 94% and the beam quality of the combined output is characterized by a measured M2-value of 1.2.
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Coherent combination of spectrally broadened femtosecond pulses for nonlinear compression - Klenke, A.; H{\"{a}drich, S. and Kienel, M. and Eidam, T. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2014)
The coherent combination of ultrashort pulses has recently been established as a technique to overcome the limitations of laser amplifiers regarding pulse peak-power, pulse energy, and average power. Similar limitations also occur in nonlinear compression setups. In a proof-of-principle experiment, we show that the techniques developed for the combination of amplifiers can be adapted to nonlinear compression. We create two spatially separated pulse replica that undergo self-phase modulation in independent optical fibers and are recombined afterwards. Using this technique we demonstrate operation above the self-focusing threshold of a single pulse. Furthermore, we prove that the recombined pulses can be temporally compressed. This experiment paves the way for higher energy or average power operation of various nonlinear compression setups.
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Collimated proton acceleration in light sail regime with a tailored pinhole target - Wang, H. Y.; Yan, X. Q.; Zepf, M. (Physics of Plasmas, 2014)
A scheme for producing collimated protons from laser interactions with a diamond-like-carbon + pinhole target is proposed. The process is based on radiation pressure acceleration in the multi-species light-sail regime [B. Qiao et al., Phys. Rev. Lett. 105, 155002 (2010); T. P. Yu et al., Phys. Rev. Lett. 105, 065002 (2010)]. Particle-in-cell simulations demonstrate that transverse quasistatic electric field at TV/m level can be generated in the pinhole. The transverse electric field suppresses the transverse expansion of protons effectively, resulting in a higher density and more collimated proton beam compared with a single foil target. The dependence of the proton beam divergence on the parameters of the pinhole is also investigated.
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Design of a compact spectrometer for high-flux MeV gamma-ray beams - Corvan, D. J.; Sarri, G.; Zepf, M. (Review of Scientific Instruments, 2014)
A novel design for a compact gamma-ray spectrometer is presented. The proposed system allows for spectroscopy of high-flux multi-MeV gamma-ray beams with MeV energy resolution in a compact design. In its basic configuration, the spectrometer exploits conversion of gamma-rays into electrons via Compton scattering in a low-Z material. The scattered electron population is then spectrally resolved using a magnetic spectrometer. The detector is shown to be effective for gamma-ray energies between 3 and 20 MeV. The main properties of the spectrometer are confirmed by Monte Carlo simulations.
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Development of a Joule-class Yb:YAG amplifier and its implementation in a CPA system generating 1 TW pulses - Wandt, C.; Klingebiel, S.; Keppler, S.; Hornung, M.; Loeser, M.; Siebold, M.; Skrobol, C.; Kessel, A.; Trushin, S.; Major, Z.; Hein, J.; Kaluza, M.C.; et al. (Laser { &} Photonics Reviews, 2014)
In this paper the development and implementation of a novel amplifier setup as an additional stage for the CPA pump laser of the Petawatt Field Synthesizer, currently developed at the Max-Planck-Institute of Quantum Optics, is presented. This amplifier design comprises 20 relay-imaged passes through the active medium which are arranged in rotational symmetry. As the gain material, an in-house-developed Yb:YAG active-mirror is used. With this setup, stretched 4 ns seed pulses are amplified to output energies exceeding 1 J with repetition rates of up to 2 Hz. Furthermore, a spectral bandwidth of 3.5 nm (FWHM) is maintained during amplification and the compression of the pulses down to their Fourier-limit of 740 fs is achieved. To the best of our knowledge, this is the first demonstration of 1 TW pulses generated via CPA in diode-pumped Yb:YAG.
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Enhancing the Macroscopic Yield of Narrow-Band High-Order Harmonic Generation by Fano Resonances - Rothhardt, J.; H{\"{a}drich, S. and Demmler, S. and Krebs, M. and Fritzsche, S. and Limpert, J. and T{\"{u}}nnermann, A. (Physical Review Letters, 2014)
Resonances in the photoabsorption spectrum of the generating medium can modify the spectrum of high-order harmonics. In particular, window-type Fano resonances can reduce photoabsorption within a narrow spectral region and, consequently, lead to an enhanced emission of high-order harmonics in absorption-limited generation conditions. For high harmonic generation in argon it is shown that the 3s3p6np1P1 window resonances (n=4, 5, 6) give rise to enhanced photon yield. In particular, the 3s3p64p1P1 resonance at 26.6 eV allows a relative enhancement up to a factor of 30 in a 100 meV bandwidth compared to the characteristic photon emission of the neighboring harmonic order. This enhanced, spectrally isolated, and coherent photon emission line has a relative energy bandwidth of only ΔE/E=3×10^−3. Therefore, it might be very useful for applications such as precision spectroscopy or coherent diffractive imaging. The presented mechanism can be employed for tailoring and controlling the high harmonic emission of manifold target materials.
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Evidence for a glassy state in strongly driven carbon - Brown, C. R. D.; Gericke, D. O.; Cammarata, M.; Cho, B. I.; D{\"{o}}ppner, T.; Engelhorn, K.; F{\"{o}}rster, E.; Fortmann, C.; Fritz, D.; Galtier, E.; Glenzer, S. H.; Harmand, M.; et al. (Scientific Reports, 2014)
Here, we report results of an experiment creating a transient, highly correlated carbon state using a combination of optical and x-ray lasers. Scattered x-rays reveal a highly ordered state with an electrostatic energy significantly exceeding the thermal energy of the ions. Strong Coulomb forces are predicted to induce nucleation into a crystalline ion structure within a few picoseconds. However, we observe no evidence of such phase transition after several tens of picoseconds but strong indications for an over-correlated fluid state. The experiment suggests a much slower nucleation and points to an intermediate glassy state where the ions are frozen close to their original positions in the fluid.
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High precision elimination of angular chirp in CPA laser systems with large stretching factors or high bandwidth - B{\"{o}}defeld, R.; Hornung, M.; Hein, J.; Kaluza, M.C. (Applied Physics B, 2014)
We present a novel and highly sensitive method to determine the residual angular dispersion of high-power laser pulses after stretching, amplification, and re-compression of the pulses in a chirped-pulse amplification laser system. This method is based on the intentional deflection of a part of the the spectrum within the compressor and aligning the centers of gravity of the two resulting and separated foci with largest possible spectral separation in the far field. Using this technique, we were able to reduce the residual angular dispersion on pulses to less than 0.05 μrad/nm in the vertical plane and less than 0.03 μrad/nm in the horizontal plane, respectively. With this method, it is possible to minimize the deviation of the actual peak intensity for the focused laser pulses to less than 2% of its theoretical limit.
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Strong-field control of the dissociative ionization of N 2 O with near-single-cycle pulses - K{\"{u}}bel, M.; Alnaser, A. S.; Bergues, B.; Pischke, T.; Schmidt, J.; Deng, Y.; Jendrzejewski, C.; Ullrich, J.; Paulus, G.G.; Azzeer, A. M.; Kleineberg, U.; Moshammer, R.; et al. (New Journal of Physics, 2014)
The dissociative ionization of N 2 O by near-single-cycle laser pulses is studied using phase-tagged ion–ion coincidence momentum imaging. Carrier–envelope phase (CEP) dependences are observed in the absolute ion yields and the emission direction of nearly all ionization and dissociation pathways of the triatomic molecule. We find that laser-field-driven electron recollision has a significant impact on the dissociative ionization dynamics and results in pronounced CEP modulations in the dication yields, which are observed in the product ion yields after dissociation. The results indicate that the directional emission of coincident ##IMG## [http://ej.iop.org/images/1367-2630/16/6/065017/njp494655ieqn1.gif] N^+ and ##IMG## [http://ej.iop.org/images/1367-2630/16/6/065017/njp494655ieqn2.gif] NO^+ ions in the denitrogenation of the dication can be explained by selective ionization of oriented molecules. The deoxygenation of the dication with the formation of coincident ##IMG## [http://ej.iop.org/images/1367-2630/16/6/065017/njp494655ieqn3.gif] N₂^+ + ##IMG## [http://ej.iop.org/images/1367-2630/16/6/065017/njp494655ieqn4.gif] O^+ ions exhibits an additional shift in its CEP dependence, suggesting that this channel is further influenced by laser interaction with the dissociating dication. The experimental results demonstrate how few-femtosecond dynamics can drive and steer molecular reactions taking place on (much) longer time scales.
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The all-diode-pumped laser system POLARIS – an experimentalist’s tool generating ultra-high contrast pulses with high energy - Hornung, M.; Liebetrau, H.; Seidel, A.; Keppler, S.; Kessler, A.; K{\"{o}}rner, J.; Hellwing, M.; Schorcht, F.; Kl{\"{o}}pfel, D.; Arunachalam, A. K.; Becker, G. A.; S{\"{a}vert, A. and Polz, J. and Hein, J. and Kaluza, M.C. (High Power Laser Science and Engineering, 2014)
The development, the underlying technology and the current status of the fully diode-pumped solid-state laser system POLARIS is reviewed. Currently, the POLARIS system delivers 4 J energy, 144 fs long laser pulses with an ultra-high temporal contrast of 5×10^12 for the ASE, which is achieved using a so-called double chirped-pulse amplification scheme and cross-polarized wave generation pulse cleaning. By tightly focusing, the peak intensity exceeds 3.5×10^20 W cm^−2. These parameters predestine POLARIS as a scientific tool well suited for sophisticated experiments, as exemplified by presenting measurements of accelerated proton energies. Recently, an additional amplifier has been added to the laser chain. In the ramp-up phase, pulses from this amplifier are not yet compressed and have not yet reached the anticipated energy. Nevertheless, an output energy of 16.6 J has been achieved so far.
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Amplification of ultra-short light pulses by ion collective modes in plasmas - Frank, A.; Fuchs, J.; Lancia, L.; Lehmann, G.; Marqu{\`{e}}s, J.-R.; Mourou, G.; Riconda, C.; Spatschek, K.; Toncian, T.; Vassura, L.; Weber, S. (The European Physical Journal Special Topics, 2014)
The use of plasmas provides a way to overcome the damage threshold of classical solid-state based optical materials which is the main limitation encountered in producing extreme power laser pulses. In particular one can use plasmas to directly amplify ultra-short laser pulses to very high intensities. Multi-dimensional kinetic simulations and first proof-of-principle experiments show the feasibility of using plasma instabilities involving ion waves, such as stimulated Brillouin backscattering, in a controlled way to transfer energy from a long pump pulse to a short seed pulse and thereby increase the intensity of the latter. Plasma parametric amplification, and the use of plasma mirrors for focusing, is part of the newly developping domain of plasma optics, which eventually will pave the way to Exawatt lasers.
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BBO-sapphire sandwich structure for frequency conversion of high power lasers - Rothhardt, C.; Rothhardt, J.; Klenke, A.; Peschel, T.; Eberhardt, R.; Limpert, J.; T{\"{u}}nnermann, A. (Optical Materials Express, 2014)
We report on successful joining of a beta barium borate crystal by plasma-activated direct bonding. Based on this technology, a sandwich structure consisting of a beta barium borate crystal, joined with two sapphire heat spreaders has been fabricated. Due to the high thermal conductivity of sapphire, the sandwich structure possesses superior thermal properties compared to the single crystal. Simulations based on the finite element method indicate a significant reduction of thermal gradients and the resulting mechanical stresses. A proof of principle experiment demonstrates the high power capability of the fabricated structure. A pulsed fiber laser emitting up to 253 W average power has been frequency doubled with both a single BBO crystal and the fabricated sandwich structure. The bonded stack showed better heat dissipation and less thermo-optical beam distortion than the single crystal. The work demonstrates the huge potential of optical sandwich structures with enhanced functionality. In particular, frequency conversion at average powers in the kW range with excellent beam quality will be feasible in future.
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Full characterization of the amplified spontaneous emission from a diode-pumped high-power laser system - Keppler, S.; Hornung, M.; B{\"{o}}defeld, R.; S{\"{a}vert, A. and Liebetrau, H. and Hein, J. and Kaluza, M.C. (Optics Express, 2014)
We present the first complete temporal and spatial characterization of the amplified spontaneous emission (ASE) of laser radiation generated by a diode-pumped high-power laser system. The ASE of the different amplifiers was measured independently from the main pulse and was characterized within a time window of 10ms {ł}eq t {ł}eq 10ms and an accuracy of up to 15fs around the main pulse. Furthermore, the focusability and the energy of the ASE from each amplifier was measured after recompression. Using our analysis method, the laser components, which need to be optimized for a further improvement of the laser contrast, can be identified. This will be essential for laser-matter interaction experiments requiring a minimized ASE intensity or fluence.
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Instability-free ion acceleration by two laser pulses - Zhou, M.; Zhao, S.; Wang, H.; Lin, C.; Lu, H.; Lu, Y.; Tajima, T.; He, X.; Chen, C.; Gu, Y.; Yan, X. (The European Physical Journal Special Topics, 2014)
We demonstrate the instability-free ion acceleration regime by introducing laser control with two parallel circularly polarized laser pulses at an intensity of I = 6.8 × 10^21 W/cm2, normally incident on a hydrogen foil. The special structure of the equivalent wave front of those two pulses, which contains Gaussian peaks in both sides and a concavity in the centre (2D), can suppress the transverse instabilities and hole boring effects to constrain a high density ion clump in the centre of the foil, leading to an acceleration over a long distance and gain above 1 GeV/u for the ion bunches.
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Metal vapor target for precise studies of ion-atom collisions - Chen, W.; Vorobyev, G.; Guo, D.; Herfurth, F.; Hillenbrand, P.-M.; Spillmann, U.; Trotsenko, S.; Gumberidze, A.; St{\"{o}}hlker, T. (Review of Scientific Instruments, 2014)
Although different ion-atom collisions have been studied in various contexts, precise values of cross-sections for many atomic processes were seldom obtained. One of the main uncertainties originates from the value of target densities. In this paper, we describe a unique method to measure a target density precisely with a combination of physical vapor deposition and inductively coupled plasma optical emission spectrometry. This method is preliminarily applied to a charge transfer cross-section measurement in collisions between highly charged ions and magnesium vapor. The final relative uncertainty of the target density is less than 2.5%. This enables the precise studies of atomic processes in ion-atom collisions, even though in the trial test the deduction of precise capture cross-sections was limited by other systematic errors.
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Monte-Carlo simulations of the radiative recombination of ions with electrons in cold magnetized plasma - Bana{ś}, D.; Jagodzinski, P.; Pajek, M.; Gumberidze, A.; Surzhykov, A.; St{\"{o}}hlker, T. (Physica Scripta, 2014)
We report here on the results of Monte-Carlo simulations of the radiative recombination of highly charged ions with low-energy electrons in the presence of a guiding magnetic field. The simulations are based on a semi-classical geometrical model, recently proposed by our group, which has been developed in order to explain systematic discrepancies, the so-called effect’, of the radiative recombination rates measured in the guiding magnetic field of electron coolers with respect to theoretical calculations. With the simulations, we demonstrate that the enhancement of radiative recombination rates in the magnetic field could be caused by ’ collisions with the impact parameter in the μm range and the impact parameter cut-off value depending on the strength of the guiding B -field in magnetized plasma. In this paper, the methodology of the simulations, the obtained B -field dependence of the radiative recombination enhancement and the observed impact parameter cut-off will be discussed.
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Radiative capture of twisted electrons by bare ions - Matula, O.; Hayrapetyan, A. G.; Serbo, V. G.; Surzhykov, A.; Fritzsche, S. (New Journal of Physics, 2014)
Recent advances in the production of twisted electron beams with a subnanometer spot size offer unique opportunities to explore the role of orbital angular momentum in basic atomic processes. In the present work, we address one of these processes: radiative recombination of twisted electrons with bare ions. On the basis of the density matrix formalism and the non-relativistic Schr{\"{o}}dinger theory, analytical expressions are derived for the angular distribution and the linear polarization of photons emitted due to the capture of twisted electrons into the ground state of (hydrogen-like) ions. We show that these angular and polarization distributions are sensitive to both the transverse momentum and the topological charge of the electron beam. To observe in particular the value of this charge, we propose an experiment that makes use of the coherent superposition of two twisted beams.
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Scaling the mode instability threshold with multicore fibers - Otto, H.-J.; Klenke, A.; Jauregui, C.; Stutzki, F.; Limpert, J.; T{\"{u}}nnermann, A. (Optics Letters, 2014)
Mode instabilities (MIs) have quickly become the most limiting effect for the average power scaling of nearly diffraction-limited beams from state-of-the-art fiber laser systems. In this work it is shown that, by using an advanced multicore photonic crystal fiber design, the threshold power of MIs can be increased linearly with the number of cores. An average output power of 536 W, corresponding to 4 times the threshold power of a single core, is demonstrated.
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Spectral broadening and compression of sub-millijoule laser pulses in hollow-core fibers filled with sulfur hexafluoride - Hoffmann, A.; Z{\"{u}}rch, M.; Gr{\"{a}fe, M. and Spielmann, C. (Optics Express, 2014)
Spectral broadening in gas-filled hollow-core fibers is discussed for sulfur hexafluoride, a molecular gas with Raman activity. Experimental results for compressed pulses are presented for input pulses longer than the Raman period and shorter than the dephasing time at a central wavelength of 800 nm and 400 nm, respectively. For both wavelengths we compress the pulses by a factor of three and maintain a good pulse quality. The obtained results are of interest for compressing pulses generated with Yb doped lasers.
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Vortex algebra by multiply cascaded four-wave mixing of femtosecond optical beams - Hansinger, P.; Maleshkov, G.; Garanovich, I. L.; Skryabin, D. V.; Neshev, D. N.; Dreischuh, A.; Paulus, G.G. (Optics Express, 2014)
Experiments performed with different vortex pump beams show for the first time the algebra of the vortex topological charge cascade, that evolves in the process of nonlinear wave mixing of optical vortex beams in Kerr media due to competition of four-wave mixing with self-and cross-phase modulation. This leads to the coherent generation of complex singular beams within a spectral bandwidth larger than 200nm. Our experimental results are in good agreement with frequency-domain numerical calculations that describe the newly generated spectral satellites.
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Asymmetric photoelectron momentum distributions due to quantum interference in strong-field ionization by a few-cycle pulse - Shvetsov-Shilovski, N. I.; R{\"{a}s{\"{a}nen, E. and Paulus, G.G. and Madsen, L. B. (Physical Review A, 2014)
We calculate the left-right asymmetry of the photoelectron momentum distributions generated in a hydrogen atom exposed to an intense few-cycle laser pulse as a function of both the carrier-envelope phase and the laser intensity. We present results of the numerical solution of the three-dimensional time-dependent Schrodingere equation, semiclassical simulations accounting for both laser and Coulomb fields, and the strong-field approximation. We predict pronounced oscillations of the asymmetry parameter as a function of the intensity for a particular range of the carrier-envelope phase. In order to reveal the mechanism underlying these oscillations, we investigate in detail the electron momentum distributions in the one-dimensional case. We show that quantum interference among a large set of both bound and continuum field-free states is responsible for the oscillatory behavior of the left-right asymmetry.
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Determination of the polarization state of x rays with the help of anomalous transmission - Schulze, K. S.; Marx, B.; Uschmann, I.; F{\"{o}}rster, E.; St{\"{o}}hlker, T.; Paulus, G.G. (Applied Physics Letters, 2014)
Besides intensity and direction, the polarization of an electromagnetic wave provides characteristic information on the crossed medium. Here, we present two methods for the determination of the polarization state of x rays by polarizers based on anomalous transmission (Borrmann effect). Using a polarizer-analyzer setup, we have measured a polarization purity of less than 1.5 × 10^−5, three orders of magnitude better than obtained in earlier work. Using the analyzer crystal in multiple-beam case with slightly detuned azimuth, we show how the first three Stokes parameters can be determined with a single angular scan. Thus, polarization analyzers based on anomalous transmission make it possible to detect changes of the polarization in a range from degrees down to arcseconds.
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Double lepton pair production with electron capture in relativistic heavy-ion collisions - Artemyev, A.; Serbo, V.; Surzhykov, A. (The European Physical Journal C, 2014)
We present a theoretical study of a double lepton-pair production in ultra-relativistic collision between two bare ions. Special emphasis is placed to processes in which creation of (at least one) e+e− pair is accompanied by the capture of an electron into a bound ionic state. To evaluate the probability and cross section of these processes we employ two approaches based on (i) the first-order perturbation theory and multipole expansion of Dirac wavefunctions, and (ii) the equivalent photon approximation. With the help of such approaches, detailed calculations are made for the creation of two bound–free e+e− pairs as well as of bound–free e+e− and free–free μ+μ− pairs in collisions of bare lead ions, Pb 82+ . The results of the calculations indicate that observation of the double lepton processes may become feasible at the LHC facility.
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Electron polarimetry with bremsstrahlung - Tashenov, S.; B{\"{a}ck, T. and Barday, R. and Cederwall, B. and Enders, J. and Khaplanov, A. and Fritzsche, Y. and Surzhykov, A. and Yerokhin, V. A. and Jakubassa-Amundsen, D. (Journal of Physics: Conference Series, 2014)
Due to the spin-orbit interaction, the electron scattering from the nucleus is sensitive to the spin orientation of that electron. This is used for polarimetry of electron beams in the Mott method. The spin-orbit interaction was also observed in bremsstrahlung. In this article we analyze its potential for polarimetry as an alternative to the Mott method. It can simultaneously measure all three electron polarization components. It should work in the energy range of 50 keV up to several MeV and can be applied at beam intensities higher than 100 nA. It needs a thin heavy element target, two or four x-ray detectors and one x-ray linear polarimeter.
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Electron-Ion Equilibration in Ultrafast Heated Graphite - White, T. G.; Hartley, N. J.; Borm, B.; Crowley, B. J. B.; Harris, J. W. O.; Hochhaus, D. C.; Kaempfer, T.; Li, K.; Neumayer, P.; Pattison, L. K.; Pfeifer, F.; Richardson, S.; et al. (Physical Review Letters, 2014)
We have employed fast electrons produced by intense laser illumination to isochorically heat thermal electrons in solid density carbon to temperatures of ∼10 000 K. Using time-resolved x-ray diffraction, the temperature evolution of the lattice ions is obtained through the Debye-Waller effect, and this directly relates to the electron-ion equilibration rate. This is shown to be considerably lower than predicted from ideal plasma models. We attribute this to strong ion coupling screening the electron-ion interaction.
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Electronic Predetermination of Ethylene Fragmentation Dynamics - Xie, X.; Roither, S.; Sch{\"{o}}ffler, M.; L{\"{o}}tstedt, E.; Kartashov, D.; Zhang, L.; Paulus, G.; Iwasaki, A.; Baltuska, A.; Yamanouchi, K.; Kitzler, M. (Physical Review X, 2014)
We experimentally investigate the dependence of the fragmentation behavior of the ethylene dication on the intensity and duration of the laser pulses that initiate the fragmentation dynamics by strong-field double ionization. Using coincidence momentum imaging for the detection of ionic fragments, we disentangle the different contributions of ionization from lower-valence orbitals and field-driven excitation dynamics to the population of certain dissociative excited ionic states that are connected to one of several possible fragmentation pathways towards a given set of fragment ions. We find that the excitation probability to a particular excited state and therewith the outcome of the fragmentation reaction strongly depend on the parameters of the laser pulse. This, in turn, opens up new possibilities for controlling the outcome of fragmentation reactions of polyatomic molecules in that it may allow one to selectively enhance or suppress individual fragmentation channels, which was not possible in previous attempts of controlling fragmentation processes of polyatomic molecules with strong laser fields.
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Hot electron production in laser solid interactions with a controlled pre-pulse - Culfa, O.; Tallents, G. J.; Wagenaars, E.; Ridgers, C. P.; Dance, R. J.; Rossall, A. K.; Gray, R. J.; McKenna, P.; Brown, C. D. R.; James, S. F.; Hoarty, D. J.; Booth, N.; et al. (Physics of Plasmas, 2014)
Hot electron generation plays an important role in the fast ignition approach to inertial confinement fusion (ICF) and other applications with ultra-intense lasers. Hot electrons of temperature up to 10–20 MeV have been produced by high contrast picosecond duration laser pulses focussed to intensities of ∼10^20 W cm^(−2) with a deliberate pre-pulse on solid targets using the Vulcan Petawatt Laser facility. We present measurements of the number and temperature of hot electrons obtained using an electron spectrometer. The results are correlated to the density scale length of the plasma produced by a controlled pre-pulse measured using an optical probe diagnostic. 1D simulations predict electron temperature variations with plasma density scale length in agreement with the experiment at shorter plasma scale lengths ( <7.5μ<7.5μ<7.5μ m), but with the experimental temperatures (13–17 MeV) dropping below the simulation values (20–25 MeV) at longer scale lengths. The experimental results show that longer interaction plasmas produced by pre-pulses enable significantly greater number of hot electrons to be produced.
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Inelastic Raman scattering of light by hydrogenlike ions - Jahrsetz, T.; Fritzsche, S.; Surzhykov, A. (Physical Review A, 2014)
The inelastic Raman scattering of light by hydrogenlike ions has been studied by means of second-order perturbation theory and the relativistic Coulomb Green's-function approach. In particular, we investigate the total and angle-differential Raman cross sections as well as the magnetic sublevel population of the residual (excited) ions. Detailed calculations are performed for the inelastic scattering of photons by neutral hydrogen as well as hydrogenlike xenon and uranium ions, accompanied by the 1s1/2→2s1/2, 1s1/2→2p1/2, and 1s1/2→2p3/2 transitions. Moreover, we discuss how the Raman scattering is affected by relativistic and resonance effects as well as the higher-multipole contributions to the electron-photon interaction.
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Interaction of Relativistic Electron-Vortex Beams with Few-Cycle Laser Pulses - Hayrapetyan, A.; Matula, O.; Aiello, A.; Surzhykov, A.; Fritzsche, S. (Physical Review Letters, 2014)
We study the interaction of relativistic electron-vortex beams (EVBs) with laser light. Exact analytical solutions for this problem are obtained by employing the Dirac-Volkov wave functions to describe the (monoenergetic) distribution of the electrons in vortex beams with well-defined orbital angular momentum. Our new solutions explicitly show that the orbital angular momentum components of the laser field couple to the total angular momentum of the electrons. When the field is switched off, it is shown that the laser-driven EVB coincides with the field-free EVB as reported by Bliokh et al. [Phys. Rev. Lett. 107, 174802 (2011)]. Moreover, we calculate the probability density for finding an electron in the beam profile and demonstrate that the center of the beam is shifted with respect to the center of the field-free EVB.
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Mapping spin-orbit activated interchannel coupling - Dachraoui, H.; Laan, d.; M{\"{u}}ller, N.; Milde, T.; Porer, M.; Manzke, R.; Huber, R.; Fritzsche, S.; Heinzmann, U. (Europhysics Letters, 2014)
Recent advances in the generation of femtosecond extreme ultraviolet pulses have opened up the possibility to study final-state wave functions in photoemission experiments. Here, we investigate, for the first time using femtosecond time-resolved core-level spectroscopy, the feasibility of observing the buildup of a state correlation in a direct time domain. Giant changes in the ratio of photoemission cross-sections of spin-orbit split core states, the branching ratio, are identified. Multi-configuration Dirac-Fock calculations show that the origin of the branching ratio deviation is due to strong core-valence interactions. The possibility to tune this interaction by charge transfer offers intriguing opportunities to study correlation effects in solid and molecular systems in the future.
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Multielectron effects in strong-field dissociative ionization of molecules - Gong, X.; Kunitski, M.; Betsch, K. J.; Song, Q.; Schmidt, L. Ph. H.; Jahnke, T.; Kling, N.; Herrwerth, O.; Bergues, B.; Senftleben, A.; Ullrich, J.; Moshammer, R.; et al. (Physical Review A, 2014)
We study triple-ionization-induced, spatially asymmetric dissociation of N_2 using angular streaking in an elliptically polarized laser pulse in conjunction with few-cycle pump-probe experiments. The kinetic-energyrelease dependent directional asymmetry in the ion sum-momentum distribution reflects the internuclear distance dependence of the fragmentation mechanism. Our results show that for 5-35 fs near-infrared laser pulses with intensities reaching 10^(15) W/cm^(2), charge exchange between nuclei plays aminor role in the triple ionization of N_2. We demonstrate that angular streaking provides a powerful tool for probing multielectron effects in strong-field dissociative ionization of small molecules.
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Observations of strong ion-ion correlations in dense plasmasa) - Ma, T.; Fletcher, L.; Pak, A.; Chapman, D. A.; Falcone, R. W.; Fortmann, C.; Galtier, E.; Gericke, D. O.; Gregori, G.; Hastings, J.; Landen, O. L.; Le Pape, S.; et al. (Physics of Plasmas, 2014)
Using simultaneous spectrally, angularly, and temporally resolved x-ray scattering, we measure the pronounced ion-ion correlation peak in a strongly coupled plasma. Laser-driven shock-compressed aluminum at ∼3× solid density is probed with high-energy photons at 17.9 keV created by molybdenum He-α emission in a laser-driven plasma source. The measured elastic scattering feature shows a well-pronounced correlation peak at a wave vector of k=4 {{A}}^(−1). The magnitude of this correlation peak cannot be described by standard plasma theories employing a linear screened Coulomb potential. Advanced models, including a strong short-range repulsion due to the inner structure of the aluminum ions are however in good agreement with the scattering data. These studies have demonstrated a new highly accurate diagnostic technique to directly measure the state of compression and the ion-ion correlations. We have since applied this new method in single-shot wave-number resolved S(k) measurements to characterize the physical properties of dense plasmas.
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Quark-antiquark static energy from a restricted Fourier transform - Karbstein, F. (Journal of High Energy Physics, 2014)
We provide a fully analytical determination of the perturbative quark-antiquark static energy in position space as defined by a restricted Fourier transformation from momentum to position space. Such a determination is complicated by the fact that the static energy genuinely decomposes into a strictly perturbative part (made up of contributions ∼α_s^n, with n∈ℕ) which is conventionally evaluated in momentum space, and a so-called ultrasoft part (including terms ∼α_s^n+mln^m(α_s), with n≥3 and m∈ℕ) which, conversely, is naturally evaluated in position space. Our approach facilitates the explicit determination of the static energy in position space at the accuracy with which the perturbative potential in momentum space is known, i.e., presently up to order α_s^4.
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Selective Control over Fragmentation Reactions in Polyatomic Molecules Using Impulsive Laser Alignment - Xie, X.; Doblhoff-Dier, K.; Xu, H.; Roither, S.; Sch{\"{o}}ffler, M.; Kartashov, D.; Erattupuzha, S.; Rathje, T.; Paulus, G.; Yamanouchi, K.; Baltuska, A.; Gr{\"{a}fe, S. and Kitzler, M. (Physical Review Letters, 2014)
We investigate the possibility of using molecular alignment for controlling the relative probability of individual reaction pathways in polyatomic molecules initiated by electronic processes on the few-femtosecond time scale. Using acetylene as an example, it is shown that aligning the molecular axis with respect to the polarization direction of the ionizing laser pulse does not only allow us to enhance or suppress the overall fragmentation yield of a certain fragmentation channel but, more importantly, to determine the relative probability of individual reaction pathways starting from the same parent molecular ion. We show that the achieved control over dissociation or isomerization pathways along specific nuclear degrees of freedom is based on a controlled population of associated excited dissociative electronic states in the molecular ion due to relatively enhanced ionization contributions from inner valence orbitals.
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Three-dimensional thermal simulations of thin solid carbon foils for charge stripping of high current uranium ion beams at a proposed new heavy-ion linac at GSI - Tahir, N.; Kim, V.; Schlitt, B.; Barth, W.; Groening, L.; Lomonosov, I.; Piriz, A.; St{\"{o}}hlker, T.; Vormann, H. (Physical Review Special Topics - Accelerators and Beams, 2014)
This paper presents an extensive numerical study of heating of thin solid carbon foils by 1.4 MeV/u uranium ion beams to explore the possibility of using such a target as a charge stripper at the proposed new Gesellschaft f{\"{u}}r Schwerionenforschung high energy heavy–ion linac. These simulations have been carried out using a sophisticated 3D computer code that accounts for physical phenomena that are important in this problem. A variety of beam and target parameters have been considered. The results suggest that within the considered parameter range, the target will be severely damaged by the beam. Thus, a carbon foil stripper does not seem to be a reliable option for the future Gesellschaft f{\"{u}}r Schwerionenforschung high energy heavy–ion linac, in particular, at FAIR design beam intensities.
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16.6 J chirped femtosecond laser pulses from a diode-pumped Yb:CaF2 amplifier - Kessler, A.; Hornung, M.; Keppler, S.; Schorcht, F.; Hellwing, M.; Liebetrau, H.; K{\"{o}}rner, J.; S{\"{a}vert, A. and Siebold, M. and Schnepp, M. and Hein, J. and Kaluza, M.C. (Optics Letters, 2014)
We report the amplification of laser pulses at a center wavelength of 1034 nm to an energy of 16.6 J from a fully diode-pumped amplifier using Yb:CaF2 as the active medium. Pumped by a total optical power of 300 kW from high-power laser diodes, a gain factor of g=6.1 was achieved in a nine-pass amplifier configuration agreeing with numerical simulations. A measured spectral bandwidth of 10 nm full width at half-maximum promises a bandwidth-limited compression of the pulses down to a duration of 150 fs. These are, to our knowledge, the most energetic laser pulses achieved from a diode-pumped chirped-pulse amplifier so far.
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Absorption-aided x-ray emission tomography of planar targets - Stambulchik, E.; Kroupp, E.; Maron, Y.; Zastrau, U.; Uschmann, I.; Paulus, G.G. (Physics of Plasmas, 2014)
Suggested is a tomography-like method for studying properties of solid-density plasmas with cylindrical symmetry, such as formed in the interaction of high-power lasers with planar targets. The method is based on simultaneous observation of the target x-ray fluorescence at different angles. It can be applied for validation of existing hypotheses and lately for reconstruction of the plasma properties with three-dimensional resolution. The latter becomes straightforward if the resonance x-ray self-absorption is negligible. The utility of the method is demonstrated by examples.
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Absorption-limited and phase-matched high harmonic generation in the tight focusing regime - Rothhardt, J.; Krebs, M.; H{\"{a}drich, S. and Demmler, S. and Limpert, J. and T{\"{u}}nnermann, A. (New Journal of Physics, 2014)
High harmonic generation (HHG) at a high repetition rate requires tight focusing of the moderate peak power driving pulses. So far the conversion efficiencies that have been achieved in this regime are orders of magnitude behind the values that have been demonstrated with loose focusing of high energy (high peak power) lasers. In this contribution, we discuss the scaling laws for the main physical quantities of HHG and in particular analyze the limiting effects: dephasing, absorption and plasma defocusing. It turns out that phase-matched and absorption-limited HHG can be achieved even for very small focal spot sizes using a target gas provided with an adequately high density. Experimentally, we investigate HHG in a gas jet of argon, krypton and xenon. By analyzing the pressure dependence we are able to disentangle the dephasing and absorption effects and prove that the generated high order harmonics are phase-matched and absorption-limited. The obtained conversion efficiency is as high as 8 × 10^−6 for the 17th harmonic generated in xenon and 1.4 × 10^−6 for the 27th harmonic generated in argon. Our findings pave the way for highly efficient harmonic generation at megahertz repetition rates.
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Commissioning of a compact laser-based proton beam line for high intensity bunches around 10 MeV - Busold, S.; Schumacher, D.; Deppert, O.; Brabetz, C.; Kroll, F.; Blazevic, A.; Bagnoud, V.; Roth, M. (Physical Review ST - Accelerators and Beams, 2014)
We report on the first results of experiments with a new laser-based proton beam line at the GSI accelerator facility in Darmstadt. It delivers high current bunches at proton energies around 9.6 MeV, containing more than 10^9 particles in less than 10 ns and with tunable energy spread down to 2.7% (ΔE/E0 at FWHM). A target normal sheath acceleration stage serves as a proton source and a pulsed solenoid provides for beam collimation and energy selection. Finally a synchronous radio frequency (rf) field is applied via a rf cavity for energy compression at a synchronous phase of −90 deg. The proton bunch is characterized at the end of the very compact beam line, only 3 m behind the laser matter interaction point, which defines the particle source.
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Dependence of Laser-Driven Coherent Synchrotron Emission Efficiency on Pulse Ellipticity and Implications for Polarization Gating - Yeung, M.; Dromey, B.; Cousens, S.; Dzelzainis, T.; Kiefer, D.; Schreiber, J.; Bin, J.; Ma, W.; Kreuzer, C.; Meyer-ter-Vehn, J.; Streeter, M.; Foster, P.; et al. (Physical Review Letters, 2014)
The polarization dependence of laser-driven coherent synchrotron emission transmitted through thin foils is investigated experimentally. The harmonic generation process is seen to be almost completely suppressed for circular polarization opening up the possibility of producing isolated attosecond pulses via polarization gating. Particle-in-cell simulations suggest that current laser pulses are capable of generating isolated attosecond pulses with high pulse energies.
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Enhanced radiation pressure-assisted acceleration by temporally tuned counter-propagating pulses - Aurand, B.; Kuschel, S.; J{\"{a}ckel, O. and R{\"{o}}del, C. and Zhao, H. and Herzer, S. and Paz, A. and Bierbach, J. and Polz, J. and Elkin, B. and Karmakar, A. and Gibbon, P. and Kaluza, M.C. and K{\"{u}}hl, T. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2014)
Within the last decade, laser-ion acceleration has become a field of broad interest. The possibility to generate short proton- or heavy ion bunches with an energy of a few tens of MeV by table-top laser systems could open new opportunities for medical or technical applications. Nevertheless, today's laser-acceleration schemes lead mainly to a temperature-like energy distribution of the accelerated ions, a big disadvantage compared to mono-energetic beams from conventional accelerators. Recent results [1] of laser-ion acceleration using radiation-pressure appear promising to overcome this drawback. In this paper, we demonstrate the influence of a second counter-propagating laser pulse interacting with a nm-thick target, creating a well defined pre-plasma.
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Modified Thomson spectrometer design for high energy, multi-species ion sources - Gwynne, D.; Kar, S.; Doria, D.; Ahmed, H.; Cerchez, M.; Fernandez, J.; Gray, R. J.; Green, J. S.; Hanton, F.; MacLellan, D. A.; McKenna, P.; Najmudin, Z.; et al. (Review of Scientific Instruments, 2014)
A modification to the standard Thomson parabola spectrometer is discussed, which is designed to measure high energy (tens of MeV/nucleon), broad bandwidth spectra of multi-species ions accelerated by intense laser plasma interactions. It is proposed to implement a pair of extended, trapezoidal shaped electric plates, which will not only resolve ion traces at high energies, but will also retain the lower energy part of the spectrum. While a longer (along the axis of the undeflected ion beam direction) electric plate design provides effective charge state separation at the high energy end of the spectrum, the proposed new trapezoidal shape will enable the low energy ions to reach the detector, which would have been clipped or blocked by simply extending the rectangular plates to enhance the electrostatic deflection.
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Non-sequential double ionization of Ar: from the single- to the many-cycle regime - K{\"{u}}bel, M.; Betsch, K. J.; Kling, N. G.; Alnaser, A. S.; Schmidt, J.; Kleineberg, U.; Deng, Y.; Ben-Itzhak, I.; Paulus, G.G.; Pfeifer, T.; Ullrich, J.; Moshammer, R.; et al. (New Journal of Physics, 2014)
The transition from the near-single to the multi-cycle regime in non-sequential double ionization of argon is investigated experimentally. Argon atoms are exposed to intense laser pulses with a center wavelength around 790 nm and the momenta of electrons and ions generated in the double ionization process are measured in coincidence using a reaction microscope. The duration of the near transform-limited pulses is varied from 4 to 30 fs. We observe an abrupt collapse of the cross-shaped two-electron momentum distribution [17] in the few-cycle regime. The transition to longer pulses is further accompanied by a strong increase in the fraction of anti-correlated to correlated electrons.
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Pair production in rotating electric fields - Blinne, A.; Gies, H. (Physical Review D, 2014)
We explore Schwinger pair production in rotating time-dependent electric fields using the real-time Dirac-Heisenberg-Wigner formalism. We determine the time evolution of the Wigner function as well as asymptotic particle distributions neglecting backreactions on the electric field. Whereas qualitative features can be understood in terms of effective Keldysh parameters, the field rotation leaves characteristic imprints in the momentum distribution that can be interpreted in terms of interference and multiphoton effects. These phenomena may seed characteristic features of QED cascades created in the antinodes of a high-intensity standing wave laser field.
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Parametric amplification of attosecond pulse trains at 11nm - Seres, J.; Seres, E.; Landgraf, B.; Ecker, B.; Aurand, B.; Hoffmann, A.; Winkler, G.; Namba, S.; Kuehl, T.; Spielmann, C. (Scientific Reports, 2014)
We report the first experimental demonstration of the parametric amplification of attosecond pulse trains at around 11 nm. The helium amplifier is driven by intense laser pulses and seeded by high-order harmonics pulses generated in a neon gas jet. Our measurements suggest that amplification takes place only if the seed pulse-trains are perfectly synchronized in time with the driving laser field in the amplifier. Varying the delay, we estimate the durations of the individual extreme ultraviolet pulses within the train to be on the order of 0.2 fs. Our results demonstrate that strong-field parametric amplification can be a suitable tool to amplify weak attosecond pulses from non-destructive pump-probe experiments and it is an important step towards designing amplifiers for realization of energetic XUV pulses with sub-femtosecond duration using compact lasers fitting in university laboratories.
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Parity-nonconservation effect in the dielectronic recombination of polarized electrons with heavy He-like ions - Zaytsev, V. A.; Maiorova, A. V.; Shabaev, V. M.; Volotka, A. V.; Tashenov, S.; Plunien, G.; St{\"{o}}hlker, T. (Physical Review A, 2014)
We investigate the parity nonconservation (PNC) effect in the dielectronic recombination of a polarized electron with a heavy He-like ion into doubly excited [(1s2p1/2)_0 nκ]_1/2 and [(1s2s)_0 nκ]_1/2 states of a Li-like ion. We determine the nuclear charge number Z for which these opposite-parity levels are almost crossing and therefore the PNC effect will be significantly enhanced. Calculations are performed for quantum numbers n≥4 and κ=±1.
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Power scaling of supercontinuum seeded megahertz-repetition rate optical parametric chirped pulse amplifiers - Riedel, R.; Stephanides, A.; Prandolini, M. J.; Gronloh, B.; Jungbluth, B.; Mans, T.; Tavella, F. (Optics Letters, 2014)
Optical parametric chirped-pulse amplifiers with high average power are possible with novel high-power Yb:YAG amplifiers with kW-level output powers. We demonstrate a compact wavelength-tunable sub-30-fs amplifier with 11.4 W average power with 20.7% pump-to-signal conversion efficiency. For parametric amplification, a beta-barium borate crystal is pumped by a 140 W, 1 ps Yb:YAG InnoSlab amplifier at 3.25 MHz repetition rate. The broadband seed is generated via supercontinuum generation in a YAG crystal.
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Resolving Ultrafast Heating of Dense Cryogenic Hydrogen - Zastrau, U.; Sperling, P.; Harmand, M.; Becker, A.; Bornath, T.; Bredow, R.; Dziarzhytski, S.; Fennel, T.; Fletcher, L.; F{\"{o}}rster, E.; G{\"{o}}de, S.; Gregori, G.; et al. (Physical Review Letters, 2014)
We report on the dynamics of ultrafast heating in cryogenic hydrogen initiated by a ≲300 fs, 92 eV free electron laser x-ray burst. The rise of the x-ray scattering amplitude from a second x-ray pulse probes the transition from dense cryogenic molecular hydrogen to a nearly uncorrelated plasmalike structure, indicating an electron-ion equilibration time of ∼0.9 ps. The rise time agrees with radiation hydrodynamics simulations based on a conductivity model for partially ionized plasma that is validated by two-temperature density-functional theory.
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Shaping laser accelerated ions for future applications – The LIGHT collaboration - Busold, S.; Almomani, A.; Bagnoud, V.; Barth, W.; Bedacht, S.; Blazevic, A.; Boine-Frankenheim, O.; Brabetz, C.; Burris-Mog, T.; Cowan, T.; Deppert, O.; Droba, M.; et al. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2014)
Abstract The generation of intense ion beams from high-intensity laser-generated plasmas has been the focus of research for the last decade. In the LIGHT collaboration the expertise of heavy ion accelerator scientists and laser and plasma physicists has been combined to investigate the prospect of merging these ion beams with conventional accelerator technology and exploring the possibilities of future applications. We report about the goals and first results of the LIGHT collaboration to generate, handle and transport laser driven ion beams. This effort constitutes an important step in research for next generation accelerator technologies.
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SPARC collaboration: new strategy for storage ring physics at FAIR - St{\"{o}}hlker, T.; Litvinov, Y.; Br{\"{a}uning-Demian, A. and Lestinsky, M. and Herfurth, F. and Maier, R and Prasuhn, D. and Schuch, R. and Steck, M. (Hyperfine Interactions, 2014)
SPARC collaboration at FAIR pursues the worldwide unique research program by utilizing storage ring and trapping facilities for highly-charged heavy ions. The main focus is laid on the exploration of the physics at strong, ultra-short electromagnetic fields including the fundamental interactions between electrons and heavy nuclei as well as on the experiments at the border between nuclear and atomic physics. Very recently SPARC worked out a realization scheme for experiments with highly-charged heavy-ions at relativistic energies in the High-Energy Storage Ring HESR and at very low-energies at the CRYRING coupled to the present ESR. Both facilities provide unprecedented physics opportunities already at the very early stage of FAIR operation. The installation of CRYRING, dedicated Low-energy Storage Ring (LSR) for FLAIR, may even enable a much earlier realisation of the physics program of FLAIR with slow anti-protons.
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Spin dynamics in relativistic ionization with highly charged ions in super-strong laser fields - Klaiber, M.; Yakaboylu, E.; M{\"{u}}ller, C.; Bauke, H.; Paulus, G.G.; Hatsagortsyan, K. Z. (Journal of Physics B: Atomic, Molecular and Optical Physics, 2014)
Spin dynamics and induced spin effects in above-threshold ionization of hydrogenlike highly charged ions in super-strong laser fields are investigated. Spin-resolved ionization rates in the tunnelling regime are calculated by employing two versions of a relativistic Coulomb-corrected strong-field approximation (SFA). An intuitive simpleman model is developed which explains the derived scaling laws for spin flip and spin asymmetry effects. The intuitive model as well as our ab initio numerical simulations support the analytical results for the spin effects obtained in the dressed SFA where the impact of the laser field on the electron spin evolution in the bound state is taken into account. In contrast, the standard SFA is shown to fail in reproducing spin effects in ionization even at a qualitative level. The anticipated spin-effects are expected to be measurable with modern laser techniques combined with an ion storage facility.
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Structure and fluorescence properties of ternary aluminosilicate glasses doped with samarium and europium - Herrmann, A.; Kuhn, S.; Tiegel, M.; Russel, C.; K{\"{o}}rner, J.; Kl{\"{o}}pfel, D.; Hein, J.; Kaluza, M. (Journal of Materials Chemistry C, 2014)
Various ternary aluminosilicate glasses with the molar compositions 20 Al2O3-60 SiO2-20 R2O (R = Li or Na), 20 Al2O3-60 SiO2-20 RO (R = Mg, Ca or Zn) and 23.1 Al2O3-69.2 SiO2-7.7 R2O3 (R = Y or La) doped with 1 [times] 1020 Sm3+ cm-3 or 1 [times] 1020 Eu3+ cm-3 (about 0.2 mol% Sm2O3 or Eu2O3) were prepared. The glasses were studied with respect to their molecular structure, and their thermo-mechanical and fluorescence properties. All glasses show relatively broad fluorescence excitation and only a weak effect of the glass composition on the emission spectra is observed. Although the glasses should be structurally very similar, huge differences are found in the coefficients of thermal expansion and the glass transition temperatures. The fluorescence lifetime increases steadily with decreasing mean atomic weight and decreasing refractive index of the glasses, which may be explained by local field effects. The only exception from this rule is the zinc aluminosilicate glass, which shows a relatively high fluorescence lifetime. The highest fluorescence lifetime is found for the lithium aluminosilicate glass. The lowest coefficients of thermal expansion are found for zinc- and magnesium aluminosilicate glasses. A low coefficient of thermal expansion is a prerequisite for a high thermal shock resistance of the material and hence favorable for high-power laser applications.
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Detailed analysis of hollow ions spectra from dense matter pumped by X-ray emission of relativistic laser plasma - Hansen, S. B.; Colgan, J.; Faenov, A. Ya.; Abdallah, J.; Pikuz, S. A.; Skobelev, I. Yu.; Wagenaars, E.; Booth, N.; Culfa, O.; Dance, R. J.; Tallents, G. J.; Evans, R. G.; et al. (Physics of Plasmas, 2014)
X-ray emission from hollow ions offers new diagnostic opportunities for dense, strongly coupled plasma. We present extended modeling of the x-ray emission spectrum reported by Colgan et al. [Phys. Rev. Lett. 110, 125001 (2013)] based on two collisional-radiative codes: the hybrid-structure Spectroscopic Collisional-Radiative Atomic Model (SCRAM) and the mixed-unresolved transition arrays (MUTA) ATOMIC model. We show that both accuracy and completeness in the modeled energy level structure are critical for reliable diagnostics, investigate how emission changes with different treatments of ionization potential depression, and discuss two approaches to handling the extensive structure required for hollow-ion models with many multiply excited configurations.
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Effective mass signatures in multiphoton pair production - Kohlf{\"{u}}rst, C.; Gies, H.; Alkofer, R. (Physical Review Letters, 2014)
Electron-positron pair production in oscillating electric fields is investigated in the nonperturbative threshold regime. Accurate numerical solutions of quantum kinetic theory for corresponding observables are presented and analyzed in terms of a proposed model for an effective mass of electrons and positrons acquired within the given strong electric field. Although this effective mass cannot provide an exact description of the collective interaction of a charged particle with the strong field, physical observables are identified which carry direct and sensitive signatures of the effective mass.
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Energy scaling of femtosecond amplifiers using actively controlled divided-pulse amplification - Kienel, M.; Klenke, A.; Eidam, T.; H{\"{a}drich, S. and Limpert, J. and T{\"{u}}nnermann, A. (Optics Letters, 2014)
Divided-pulse amplification is a promising method for the energy scaling of femtosecond laser amplifiers, where pulses are temporally split prior to amplification and coherently recombined afterwards. We present a method that uses an actively stabilized setup with separated stages for splitting and combining. The additional degrees of freedom can be employed to mitigate the limitations originating from saturation of the amplifier that cannot be compensated in passive double-pass configurations using just one common stage for pulse splitting and combining. In a first proof-of-principle experiment, actively controlled divided pulses are applied in a fiber chirped-pulse amplification system resulting in combined and compressed pulses with an energy of 1.25 mJ and a peak power of 2.9 GW.
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High-harmonic generation and parametric amplification in the soft X-rays from extended electron trajectories - Seres, J.; Seres, E.; Landgraf, B.; Ecker, B.; Aurand, B.; Kuehl, T.; Spielmann, C. (Scientific Reports, 2014)
We report, for the first time, the generation of high-order harmonics in a spectral range between 200 eV and 1 keV with an unusual spectral property: only every 4th (4i + 1, i∈ℵ) harmonic line appears, whereas the usual high-harmonic spectra consist of every odd (2i + 1) harmonic. We attribute this unique property to the quantum path interference of two extended electron trajectories that experience multiple re-scattering. In the well-established theory, electrons emitted via tunnel ionisation are accelerated by a laser field, return to the ion and recombine. The acceleration typically lasts for less than one optical cycle, and the electrons radiate in the extreme ultraviolet range at recombination. In contrast, for extended trajectories, electrons are accelerated over two or more optical cycles. Here, we demonstrate that two sets of trajectories dominate and provide substantial contributions to the generated soft X-ray radiation because they fulfil the resonance condition for X-ray parametric amplification.
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Hyperfine-induced modifications to the angular distribution of the Kα1x-ray emission - Wu, Z. W.; Surzhykov, A.; Fritzsche, S. (Physical Review A, 2014)
The angular distribution of the Kα_1 (1s2p_3/2^1,3P_1,2→1s^21S_0) x-ray emission following the radiative electron capture into initially hydrogenlike ions with nonzero nuclear spin has been studied within the density matrix theory and the multiconfiguration Dirac-Fock method. Emphasis is placed especially upon the hyperfine interaction and how this interaction of the magnetic moment of the nucleus with those of the electrons affects the angular properties of the Kα_1 radiation. Calculations were performed for selected isotopes of heliumlike Sn48+, Xe52+, and Tl79+ ions. A quite sizable contribution of the hyperfine interaction upon the Kα_1 angular emission is found for isotopes with nuclear spin I = 1/2, while its effect is suppressed for (most) isotopes with nuclear spin I > 1/2. We therefore suggest that accurate measurements of the Kα_1 angular distribution at ion storage rings can be utilized as a tool for determining the nuclear parameters of rare stable and radioactive isotopes with I ≥ 1/2.
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Interaction between one-dimensional dark spatial solitons and semi-infinite dark stripes - Hansinger, P.; Maleshkov, G.; Gorunski, N.; Dimitrov, N.; Dreischuh, A.; Paulus, G.G. (Optics Communications, 2014)
In this work we numerically study the evolution and interaction of one-dimensional (1-D) dark spatial solitons and semi-infinite dark stripes (SIDSs) in a local self-defocusing Kerr nonlinear medium. The experimental results in the linear regime of propagation confirm that the SIDS bending and fusion with the infinite 1-D dark beam modeled for negative nonlinearity is due to the opposite phase semi-helicities of SID beam ends. Results for several interaction scenaria show that bending ends of the semi-infinite dark stripes splice to the 1-D dark beam to form structures resembling waveguide couplers/branchers. Well pronounced modulational stability of 1-D dark spatial solitons under strong symmetric background beam modulation from decaying SIDSs is predicted.
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Laser-assisted Compton scattering of x-ray photons - Seipt, D.; K{\"{a}mpfer, B. (Physical Review A, 2014)
The Compton scattering of x-ray photons, assisted by a short intense optical laser pulse, is discussed. The differential scattering cross section reveals the interesting feature that the main Klein-Nishina line is accompanied by a series of side lines forming a broad plateau where up to O(10^(3)) laser photons participate simultaneously in a single scattering event. An analytic formula for the width of the plateau is given. Due to the nonlinear mixing of x-ray and laser photons a frequency-dependent rotation of the polarization of the final-state x-ray photons relative to the scattering plane emerges. A consistent description of the scattering process with short laser pulses requires to work with x-ray pulses. An experimental investigation can be accomplished, e.g., at LCLS or the European XFEL, in the near future.
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Role of proton dynamics in efficient photoionization of hydrocarbon molecules - Xie, X.; Roither, S.; Sch{\"{o}}ffler, M.; Xu, H.; Bubin, S.; L{\"{o}}tstedt, E.; Erattuphuza, S.; Iwasaki, A.; Kartashov, D.; Varga, K.; Paulus, G.G.; Baltuska, A.; et al. (Physical Review A, 2014)
We experimentally investigate the ionization mechanism behind the formation of remarkably high charge states observed in the laser-pulse-induced fragmentation of different hydrocarbon molecules by Roither et al. [Phys. Rev. Lett. 106, 163001 (2011)], who suggested enhanced ionization occurring at multiple C-H bonds as the underlying ionization mechanism. Using multiparticle coincidence momentum imaging we measure the yield of multiply charged fragmenting ethylene and acetylene molecules at several intensities and pulse durations ranging from the few-cycle regime to 25 fs. We observe, at constant intensity, a strong increase of the proton energy with increasing laser pulse duration. It is shown that this is caused by a strong increase in the yield of highly charged parent molecular ions with pulse duration. Based on experimental evidence we explain this increase by the necessary population of precursor states in the parent ion that feature fast C-H stretch dynamics to the critical internuclear distance, where efficient ionization via enhanced ionization takes place. For increasing pulse duration these precursor ionic states are more efficiently populated, which leads in turn to a higher enhanced-ionization probability for longer pulses. Our work provides experimental evidence for the existence of a multiple-bond version of enhanced ionization in polyatomic molecules.
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The Feynman tools for quantum information processing: Design and implementation - Fritzsche, S. (Computer Physics Communications, 2014)
The Feynman tools have been re-designed with the goal to establish and implement a high-level (computer) language that is capable to deal with the physics of finite, n -qubit systems, from frequently required computations to mathematically advanced tasks in quantum information processing. In particular, emphasis has been placed to introduce a small but powerful set of keystring-driven commands in order to support both, symbolic and numerical computations. Though the current design is implemented again within the framework of Maple, it is general and flexible enough to be utilized and combined with other languages and computational environments. The present implementation facilitates a large number of computational tasks, including the definition, manipulation and parametrization of quantum states, the evaluation of quantum measures and quantum operations, the evolution of quantum noise in discrete models, quantum measurements and state estimation, and several others. The design is based on a few high-level commands, with a syntax close to the mathematical notation and its use in the literature, and which can be generalized quite readily in order to solve computational tasks at even higher degree of complexity. In this work, I present and discuss the (re-design of the) Feynman tools and make major parts of the code available for public use. Moreover, a few selected examples are shown and demonstrate possible application of this toolbox. The Feynman tools are provided as Maple library and can hence be used on all platforms on which this computer-algebra system is accessible.
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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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