Peer-Review Publications

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2019

S.-L. Schulz, S. Fritzsche, R. A. Müller, and A. Surzhykov
Modification of multipole transitions by twisted light
Phys. Rev. A, 100 :043416 (October 2019)

Abstract:

A theoretical analysis is presented for the excitation of single many-electron atoms and ions by twisted (or vortex) light. Special emphasis is put on excitations that can proceed via several electric and magnetic multipole channels. We argue that the relative strength of these multipoles is very sensitive to the topological charge and kinematic parameters of the incident light and can be strongly modified with respect to the plane-wave case. Most remarkably, the modification of multipole transitions by twisted radiation can be described by means of a geometrical factor. This factor is independent of the shell structure of a particular target atom and just reflects the properties of the light beam as well as the position of an atom with respect to the vortex axis. An analytical expression for the geometrical factor is derived for Bessel photons and for a realistic experimental situation in which the position of an atom is not well determined. To illustrate the use of the geometrical factor for the analysis of (future) measurements, detailed calculations are presented for the presented for the 3s 3p 3P1 -> 3s 3p 1P1 excitation in neutral Mg.

F. Ozturk, B. Akkus, D. Atanasov, H. Beyer, F. Bosch, D. Boutin, C. Brandau, P. Bühler, R. Cakirli, R. Chen, W. Chen, X. Chen, I. Dillmann, C. Dimopoulou, W. Enders, H. Essel, T. Faestermann, O. Forstner, B. Gao, H. Geissel, R. Gernhäuser, R. Grisenti, A. Gumberidze, S. Hagmann, T. Heftrich, M. Heil, M. Herdrich, P.-M. Hillenbrand, T. Izumikawa, P. Kienle, C. Klaushofer, C. Kleffner, C. Kozhuharov, R. Knöbel, O. Kovalenko, S. Kreim, T. Kühl, C. Lederer-Woods, M. Lestinsky, S. Litvinov, Y. Litvinov, Z. Liu, X. Ma, L. Maier, B. Mei, H. Miura, I. Mukha, A. Najafi, D. Nagae, T. Nishimura, C. Nociforo, F. Nolden, T. Ohtsubo, Y. Oktem, S. Omika, A. Ozawa, N. Petridis, J. Piotrowski, R. Reifarth, J. Rossbach, R. Sanchez, M. Sanjari, C. Scheidenberger, R. Sidhu, H. Simon, U. Spillmann, M. Steck, T. Stöhlker, B. Sun, L. Susam, F. Suzaki, T. Suzuki, S. Torilov, C. Trageser, M. Trassinelli, S. Trotsenko, X. Tu, P. Walker, M. Wang, G. Weber, H. Weick, N. Winckler, D. Winters, P. Woods, T. Yamaguchi, X. Xu, X. Yan, J. Yang, Y. Yuan, Y. Zhang, and X. Zhou
New test of modulated electron capture decay of hydrogen-like ¹⁴²Pm ions: Precision measurement of purely exponential decay
Phys. Lett. B, 797 :134800 (October 2019)

Abstract:

An experiment addressing electron capture (EC) decay of hydrogen-like ¹⁴²Pm⁶⁰⁺ ions has been conducted at the experimental storage ring (ESR) at GSI. The decay appears to be purely exponential and no modulations were observed. Decay times for about 9000 individual EC decays have been measured by applying the single-ion decay spectroscopy method. Both visually and automatically analysed data can be described by a single exponential decay with decay constants of 0.0126(7) s⁻¹ for automatic analysis and 0.0141(7) s⁻¹ for manual analysis. If a modulation superimposed on the exponential decay curve is assumed, the best fit gives a modulation amplitude of merely 0.019(15), which is compatible with zero and by 4.9 standard deviations smaller than in the original observation which had an amplitude of 0.23(4).

V. A. Schanz, M. Roth, and V. Bagnoud
Picosecond contrast degradation by surface imperfections in chirped-pulse-amplification stretchers
J. Opt. Soc. Am. A, 36 :1735 (October 2019)

Abstract:

In this paper, we propose a study of the picosecond temporal contrast degradation of ultrashort laser pulses by surface defects in pulse stretchers. In a chirped-pulse-amplification stretcher or compressor, dust and damages on the surface of an optical element lead to a spectral amplitude modulation. Furthermore, surface figure errors of optical elements happening where the pulse is spatially dispersed yield a modulation of the spectral phase. The influence of both amplitude and phase noise effects is numerically investigated using a hybrid ray-tracing method that enables treating separately the influence of noise sources, whether noise occurs in the near field or the far field. We show that the main issue in terms of picosecond contrast degradation is a combined effect of surface pattern distortion in the far field and phase–amplitude coupling caused by spatial frequency filters. Temporal domains can be defined, where the temporal contrast is dominated by different noise effects. The algorithm used in this paper is compared to the cross-correlation trace of a pulse. The conclusions emerging from the presented analysis are universally applicable to known grating stretcher geometries.

J.-P. Schwinkendorf, S. Bohlen, J. Couperus Cabadağ, H. Ding, A. Irman, S. Karsch, A. Köhler, J. Krämer, T. Kurz, S. Kuschel, J. Osterhoff, L. Schaper, D. Schinkel, U. Schramm, O. Zarini, and R. D'Arcy
Charge calibration of DRZ scintillation phosphor screens
J. Instrum., 14 :P09025 (September 2019)

Abstract:

As a basic diagnostic tool, scintillation screens are employed in particle accelerators to detect charged particles. In extension to the recent revision on the calibration of scintillation screens commonly applied in the context of plasma acceleration [T. Kurz et al., Rev. Sci. Instrum. 89 (2018) 093303], herewe present the charge calibration of three DRZ screens (Std, Plus, High), which promise to offer similar spatial resolution to other screen types whilst reaching higher conversion efficiencies. The calibration was performed at the Electron Linac for beams with high Brilliance and low Emittance (ELBE) at the Helmholtz-Zentrum Dresden-Rossendorf, which delivers picosecond-long beams of up to 40 MeV energy. Compared to the most sensitive screen, Kodak BioMAX MS, of the aforementioned recent investigation by Kurz et al., the sample with highest yield in this campaign, DRZ High, revealed a 30% increase in light yield. The detection threshold with these screens was found to be below 10 pC/mm(2). For higher charge-densities (several nC/mm(2)) saturation effects were observed. In contrast to the recent reported work, the DRZ screens were more robust, demonstrating higher durability under the same high level of charge deposition.

I. A. Maltsev, V. M. Shabaev, R. V. Popov, Y. S. Kozhedub, G. Plunien, X. Ma, T. Stöhlker, and D. A. Tumakov
How to Observe the Vacuum Decay in Low-Energy Heavy-Ion Collisions
Phys. Rev. Lett., 123 :113401 (September 2019)

Abstract:

In slow collisions of two bare nuclei with the total charge larger than the critical value Zcr≈173, the initially neutral vacuum can spontaneously decay into the charged vacuum and two positrons. The detection of the spontaneous emission of positrons would be direct evidence of this fundamental phenomenon. However, the spontaneously produced particles are indistinguishable from the dynamical background in the positron spectra. We show that the vacuum decay can nevertheless be observed via impact-sensitive measurements of pair-production probabilities. The possibility of such an observation is demonstrated using numerical calculations of pair production in low-energy collisions of heavy nuclei.

W. Paufler, B. Böning, and S. Fritzsche
Strong-Field Ionization with Few-Cycle Bessel Pulses: Interplay Between Orbital Angular Momentum and Carrier Envelope Phase
Quantum Collisions and Confinement of Atomic and Molecular Species, and Photons. Springer Proceedings in Physics, vol 23 (September 2019)

J. Fellinger, A. S. Mayer, G. Winkler, W. Grosinger, G.-W. Truong, S. Droste, C. Li, C. M. Heyl, I. Hartl, and O. H. Heckl
Tunable dual-comb from an all-polarization-maintaining single-cavity dual-color Yb:fiber laser
Opt. Express, 27 :28062 (September 2019)

Abstract:

We demonstrate dual-comb generation from an all-polarization-maintaining dual-color ytterbium (Yb) fiber laser. Two pulse trains with center wavelengths at 1030 nm and 1060 nm respectively are generated within the same laser cavity with a repetition rate around 77 MHz. Dual-color operation is induced using a tunable mechanical spectral filter, which cuts the gain spectrum into two spectral regions that can be independently mode-locked. Spectral overlap of the two pulse trains is achieved outside the laser cavity by amplifying the 1030-nm pulses and broadening them in a nonlinear fiber. Spatially overlapping the two arms on a simple photodiode then generates a down-converted radio frequency comb. The difference in repetition rates between the two pulse trains and hence the line spacing of the down-converted comb can easily be tuned in this setup. This feature allows for a flexible adjustment of the tradeoff between non-aliasing bandwidth vs. measurement time in spectroscopy applications. Furthermore, we show that by fine-tuning the center-wavelengths of the two pulse trains, we are able to shift the down-converted frequency comb along the radio-frequency axis. The usability of this dual-comb setup is demonstrated by measuring the transmission of two different etalons while the laser is completely free-running.

H. Kang, S. Chen, Y. Wang, W. Chu, J. Yao, J. Chen, X. Liu, Y. Cheng, and Z. Xu
Wavelength-dependent nonsequential double ionization of magnesium by intense femtosecond laser pulses
Phys. Rev. A, 100 :033403 (September 2019)

Abstract:

We report on a systematic investigation of wavelength scaling strong-field double ionization of Mg in intense laser fields. A significant decrease of nonsequential double ionization (NSDI) yield with increasing wavelength from 800-2000 nm is observed. Our data is well reproduced by a three-dimensional Monte Carlo simulation considering recollision impact excitation cross section. We demonstrate that the NSDI of Mg mainly occurs via the first ionic excited state Mg+*(3p(2)P(3/2)(,)(1/2)) pumped by returning electron impact process. The recollision impact direct ionization pathway plays a minor role here. The wavelength dependence of the NSDI ratio is due to the recollision energy-dependent excitation cross section as well as the electron wave packet diffusion effects, both sensitively depending on the wavelength. Our work represents a step towards strong-field double ionization experiments on Mg in the long wavelength limit and sheds light on the NSDI mechanism of alkaline-earth metal atoms.

M. Schuster, V. Ludwig, B. Akstaller, M. Seifert, A. Wolf, T. Michel, P. Neumayer, S. Funk, and G. Anton
A fast alignment method for grating-based X-ray phase-contrast imaging systems
J. Instrum., 14 :P08003 (August 2019)

Abstract:

The alignment of a grating-based X-ray phase-contrast interferometer is an iterative process that requires numerous steps of changing the distances and angles between the gratings. For each alignment step an image is acquired to evaluate the detected intensity signature in order to optimize the observed moire pattern. Thus, a large number of images has to be taken for the alignment procedure. This is not feasible within reasonable time at X-ray sources like X-ray backlighters where the time between two X-ray shots is on the scale of hours. Here, we report on the development of a stable and transportable setup ready-to-use for grating-based X-ray phase-contrast imaging. A comprehensive set of reference images taken at a continuous beam serves as a look-up table which enables the grating alignment within very few alignment steps. Since this method features a fast, reliable and predictable alignment, it is also beneficial for grating-based X-ray phase-contrast imaging systems at common X-ray sources.

V. A. Zaytsev, A. V. Volotka, D. Yu, S. Fritzsche, X. Ma, H. Hu, and V. M. Shabaev
Ab initio QED Treatment of the Two-Photon Annihilation of Positrons with Bound Electrons
Phys. Rev. Lett., 123 :093401 (August 2019)

Abstract:

The process of a positron—bound-electron annihilation with simultaneous emission of two photons is investigated theoretically. A fully relativistic formalism based on an ab initio QED description of the process is worked out. The developed approach is applied to evaluate the annihilation of a positron with K-shell electrons of a silver atom, for which a strong contradiction between theory and experiment was previously stated. The results obtained here resolve this longstanding disagreement and, moreover, demonstrate a sizable difference with approaches so far used for calculations of the positron—bound-electron annihilation process, namely, Lee’s and the impulse approximations.

F. Karbstein, A. Blinne, H. Gies, and M. Zepf
Boosting Quantum Vacuum Signatures by Coherent Harmonic Focusing
Phys. Rev. Lett., 123 :091802 (August 2019)

Abstract:

We show that coherent harmonic focusing provides an efficient mechanism to boost all-optical signatures of quantum vacuum nonlinearity in the collision of high-intensity laser fields, thereby offering a promising route to their first experimental detection. Assuming two laser pulses of given parameters at our disposal, we demonstrate a substantial increase of the number of signal photons measurable in experiments where one of the pulses undergoes coherent harmonic focusing before it collides with the fundamental-frequency pulse. Imposing a quantitative criterion to discern the signal photons from the background of the driving laser photons and accounting for the finite purity of polarization filtering, we find that signal photons arising from inelastic scattering processes constitute a promising signature. By contrast, quasielastic contributions which are conventionally assumed to form the most prospective signal remain background dominated. Our findings may result in a paradigm shift concerning which photonic signatures of quantum vacuum nonlinearity are accessible in experiment.

N. A. Zubova, I. S. Anisimova, M. Y. Kaygorodov, Y. S. Kozhedub, A. V. Malyshev, V. M. Shabaev, I. I. Tupitsyn, G. Plunien, C. Brandau, and T. Stöhlker
Isotope shifts of the 1s²2s2p (J)–1s²2s²$ transition energies in Be-like thorium and uranium
J. Phys. B, 52 :185001 (August 2019)

Abstract:

Precise calculations of the isotope shifts in berylliumlike thorium and uranium ions are presented. The main contributions to the field and mass shifts are calculated within the framework of the Dirac–Coulomb–Breit Hamiltonian employing the configuration-interaction Dirac–Fock–Sturm method. These calculations include the relativistic, electron–electron correlation, and Breit-interaction effects. The QED, nuclear deformation, and nuclear polarization corrections are also evaluated.

S. Rothe, J. Sundberg, J. Welander, K. Chrysalidis, T. D. Goodacre, V. Fedosseev, S. Fiotakis, O. Forstner, R. Heinke, K. Johnston, T. Kron, U. Köster, Y. Liu, B. Marsh, A. Ringvall-Moberg, R. E. Rossel, C. Seiffert, D. Studer, K. Wendt, and D. Hanstorp
Laser photodetachment of radioactive ¹²⁸I⁻
J. Phys. G: Nucl. Part. Phys., 44 :104003 (August 2019)

Abstract:

The first experimental investigation of the electron affinity (EA) of a radioactive isotope has been conducted at the CERN-ISOLDE radioactive ion beam facility. The EA of the radioactive iodine isotope ¹²⁸I (t 1/2 = 25 min) was determined to be 3.059 052(38) eV. The experiment was conducted using the newly developed Gothenburg ANion Detector for Affinity measurements by Laser PHotodetachment (GANDALPH) apparatus, connected to a CERN-ISOLDE experimental beamline. ¹²⁸I was produced in fission induced by 1.4 GeV protons striking a thorium/tantalum foil target and then extracted as singly charged negative ions at a beam energy of 20 keV. Laser photodetachment of the fast ion beam was performed in a collinear geometry inside the GANDALPH chamber. Neutral atoms produced in the photodetachment process were detected by allowing them to impinge on a glass surface, creating secondary electrons which were then detected using a channel electron multiplier. The photon energy of the laser was tuned across the threshold of the photodetachment process and the detachment threshold data were fitted to a Wigner law function in order to extract the EA. This first successful demonstration of photodetachment at an isotope separator on line facility opens up the opportunity for future studies of the fundamental properties of negatively charged radioactive isotopes such as the EA of astatine and polonium.

C. Danson, C. Haefner, J. Bromage, T. Butcher, J.-C. Chanteloup, E. Chowdhury, A. Galvanauskas, L. Gizzi, J. Hein, and D. Hillier
Petawatt and exawatt class lasers worldwide
HPLaser, 7 :e54 (August 2019)

Abstract:

In the 2015 review paper ‘Petawatt Class Lasers Worldwide’ a comprehensive overview of the current status of high-power facilities of was presented. This was largely based on facility specifications, with some description of their uses, for instance in fundamental ultra-high-intensity interactions, secondary source generation, and inertial confinement fusion (ICF). With the 2018 Nobel Prize in Physics being awarded to Professors Donna Strickland and Gerard Mourou for the development of the technique of chirped pulse amplification (CPA), which made these lasers possible, we celebrate by providing a comprehensive update of the current status of ultra-high-power lasers and demonstrate how the technology has developed. We are now in the era of multi-petawatt facilities coming online, with 100 PW lasers being proposed and even under construction. In addition to this there is a pull towards development of industrial and multi-disciplinary applications, which demands much higher repetition rates, delivering high-average powers with higher efficiencies and the use of alternative wavelengths: mid-IR facilities. So apart from a comprehensive update of the current global status, we want to look at what technologies are to be deployed to get to these new regimes, and some of the critical issues facing their development.

F. Aumayr, K. Ueda, E. Sokell, S. Schippers, H. Sadeghpour, F. Merkt, T. F. Gallagher, F. B. Dunning, P. Scheier, O. Echt, T. Kirchner, S. Fritzsche, A. Surzhykov, X. Ma, R. Rivarola, O. Fojon, L. Tribedi, E. Lamour, J. R. C. Lopez-Urrutia, Y. A. Litvinov, V. Shabaev, H. Cederquist, H. Zettergren, M. Schleberger, R. A. Wilhelm, T. Azuma, P. Boduch, H. T. Schmidt, and T. Stöhlker
Roadmap on photonic, electronic and atomic collision physics: III. Heavy particles: with zero to relativistic speeds
J. Phys. B, 52 :171003 (August 2019)

Abstract:

We publish three Roadmaps on photonic, electronic and atomic collision physics in order to celebrate the 60th anniversary of the ICPEAC conference. Roadmap III focusses on heavy particles: with zero to relativistic speeds. Modern theoretical and experimental approaches provide detailed insight into the wide range of many-body interactions involving projectiles and targets of varying complexity ranging from simple atoms, through molecules and clusters, complex biomolecules and nanoparticles to surfaces and crystals. These developments have been driven by technological progress and future developments will expand the horizon of the systems that can be studied. This Roadmap aims at looking back along the road, explaining the evolution of the field, and looking forward, collecting nineteen contributions from leading scientists in the field.

A. A. Peshkov, S. Fritzsche, and A. Surzhykov
Scattering of twisted light from a crystal
Phys. Scripta, 94 :105402 (August 2019)

Abstract:

Recent years have seen significant progress in the generation and application of twisted beams carrying orbital angular momentum. Here we study the elastic scattering of twisted Bessel light from a crystal and compare our predictions with the results for incident plane-wave radiation. Based on form-factor approximation our numerical calculations of the differential scattering cross sections have been carried out for a crystal of lithium at x-ray energies. It is shown that the use of twisted light can lead to a measurable change in the scattering cross section for the nanocrystals approaching a few nm in size.

F. Karbstein, and E. A. Mosman
X-ray photon scattering at a focused high-intensity laser pulse
Phys. Rev. D, 100 :033002 (August 2019)

Abstract:

We study x-ray photon scattering in the head-on collision of an XFEL pulse and a focused high-intensity laser pulse, described as paraxial Laguerre-Gaussian beam of arbitrary mode composition. For adequately chosen relative orientations of the polarization vectors of the colliding laser fields, this gives rise to a vacuum birefringence effect manifesting itself in polarization flipped signal photons. Throughout this article the XFEL is assumed to be mildly focused to a waist larger than that of the high-intensity laser beam. As previously demonstrated for the special case of a fundamental paraxial Gaussian beam, this scenario is generically accompanied by a scattering phenomenon of x-ray energy signal photons outside the forward cone of the XFEL beam, potentially assisting the detection of the effect in experiment. Here, we study the fate of the x-ray scattering signal under exemplary deformations of the transverse focus profile of the high-intensity pump.

S. Fritzsche
A fresh computational approach to atomic structures, processes and cascades
Comput. Phys. Commun., 240 :1 (July 2019)

Abstract:

Electronic structure computations of atoms and ions have a long tradition in physics with applications in basic research, spectroscopy, life sciences and technology. Various theoretical methods (and codes) have therefore been developed to account for the many-particle structure of atoms, from simple semi-empirical estimates to accurate predictions of selected data, and up to highly advanced time-independent and time-dependent numerical techniques. — Here, I present a fresh concept and implementation of (relativistic) atomic structure theory that supports the computation of interaction amplitudes, properties as well as a large number of excitation and decay processes for open-shell atoms and ions across the whole periodic table. This implementation will facilitate also studies on atomic cascades, responses as well as the time-evolution of atoms and ions. It is based on Julia, a new programming language for scientific computing, and provides an easy-to-use but powerful platform to extent atomic theory towards new applications.

W. Paufler, B. Böning, and S. Fritzsche
Coherence control in high-order harmonic generation with Laguerre-Gaussian beams
Phys. Rev. A, 100 :013422 (July 2019)

Abstract:

We investigate phase matching for high-order harmonic generation with linearly polarized Laguerre-Gaussian (LG) beams with nonzero orbital angular momentum (OAM). We compare the conditions for efficient phase matching for LG beams with those of Gaussian beams. In particular, we show how the OAM of the incident beams affects the phase-matching conditions for the short and long trajectories that arise from the saddle-point approximation of the dipole moment. Thereby we illustrate that the coherence length for the short trajectories decreases for LG beams near the focus compared to Gaussian beams, whereas efficient phase matching can be achieved before and behind the focus. Furthermore, we demonstrate that the coherence length for the long trajectory behind the focus plane can be controlled by the OAM. This paper provides a route for the experiment in order to have good coherence control to enhance the conversion efficiency for high-order harmonic generation with beams carrying OAM.

L. Mercadier, A. Benediktovitch, C. Weninger, M. Blessenohl, S. Bernitt, H. Bekker, S. Dobrodey, A. Sanchez-Gonzalez, B. Erk, C. Bomme, R. Boll, Z. Yin, V. Majety, R. Steinbrügge, M. Khalal, F. Penent, J. Palaudoux, P. Lablanquie, A. Rudenko, D. Rolles, J. C. López-Urrutia, and N. Rohringer
Evidence of Extreme Ultraviolet Superfluorescence in Xenon
Phys. Rev. Lett., 123 :023201 (July 2019)

Abstract:

We present a comprehensive experimental and theoretical study on superfluorescence in the extreme ultraviolet wavelength regime. Focusing a free-electron laser pulse in a cell filled with Xe gas, the medium is quasi-instantaneously population inverted by 4d-shell ionization on the giant resonance followed by Auger decay. On the timescale of ∼10 ps to ∼100 ps (depending on parameters) a macroscopic polarization builds up in the medium, resulting in superfluorescent emission of several Xe lines in the forward direction. As the number of emitters in the system is increased by either raising the pressure or the pump-pulse energy, the emission yield grows exponentially over four orders of magnitude and reaches saturation. With increasing yield, we observe line broadening, a manifestation of superfluorescence in the spectral domain. Our novel theoretical approach, based on a full quantum treatment of the atomic system and the irradiated field, shows quantitative agreement with the experiment and supports our interpretation.

B. Lei, T. Teter, J. W. Wang, V. Yu. Kharin, C. B. Schroeder, M. Zepf, and S. G. Rykovanov
Flexible x-ray source with tunable polarization and orbital angular momentum from Hermite-Gaussian laser modes driven plasma channel wakefield
Phys. Rev. Accel. Beams, 22 :071302 (July 2019)

Abstract:

A plasma channel undulator/wiggler may be created through the plasma wakefield excited by the beating of several Hermite-Gaussian laser modes propagating in a parabolic plasma channel. Control over both the betatron and undulator forces is conveniently achieved by tuning the amplitude ratios, colors, and order numbers of the modes. A special structure of the undulator/wiggler field without the focusing force near the propagation axis is generated inside the plasma wakefield by matching the strengths of the fundamental and first-order Hermite-Gaussian modes. The electron beam only experiences forced undulator oscillations in such a field, which significantly improves the quality of the emitted radiation. Since the value of the undulator strength parameter could be in a wide range, less or larger than unity, it is capable of generating narrow bandwidth x-ray, as well as the synchrotronlike high-energy x/γ-ray, radiation by harmonics. Additionally, controlling the relative phases between the laser modes allows for polarization control of the plasma undulator. High-order harmonics produced from a circularly polarized plasma undulator clearly show the vortex nature and carry well-defined orbital angular momentum.

J. Hofbrucker, A. V. Volotka, and S. Fritzsche
Fluorescence polarization as a precise tool for understanding nonsequential many-photon ionization
Phys. Rev. A, 100 :011401 (July 2019)

Abstract:

Nonsequential two-photon ionization of inner-shell np subshell of neutral atoms by circularly polarized light is investigated. Detection of subsequent fluorescence as a signature of the process is proposed and the dependence of fluorescence degree of polarization on incident photon beam energy is studied. It is generally expected that the degree of polarization remains approximately constant, except when the beam energy is tuned to an intermediate n′ resonance. However, strong unexpected change in the polarization degree is discovered for nonsequential two-photon ionization at specific incident beam energy due to a zero contribution of the otherwise dominant ionization channel. Polarization degree of the fluorescence depends less on the beam parameters, and its measurements at this specific beam energy, whose position is very sensitive to the details of the employed theory, are highly desirable for evaluation of theoretical calculations of nonlinear ionization at hitherto unreachable accuracy.

W. Paufler, B. Böning, and S. Fritzsche
High harmonic generation with Laguerre-Gaussian beams
J. Opt., 21 :094001 (July 2019)

Abstract:

We summarize the development of high harmonic generation (HHG) with linearly polarized Laguerre–Gaussian (LG) beams and their superpositions to explain the non-perturbative aspects of HHG. Furthermore, we show that circularly polarized extreme ultraviolet vortices with well-defined orbital angular momentum (OAM) can be generated by HHG with bicircular LG beams. We introduce photon diagrams in order to explain how to calculate the OAM and the polarization of the generated harmonics by means of simultaneous conservation of spin angular momentum and OAM. Moreover, we show how the intensity ratio of the driving fields in HHG with bicircular LG beams further enhances the generation of circularly polarized twisted attosecond pulse trains.

D. Wu, W. Yu, S. Fritzsche, and X. T. He
High-order implicit particle-in-cell method for plasma simulations at solid densities
Phys. Rev. E, 100 :013207 (July 2019)

Abstract:

A high-order implicit multidimensional particle-in-cell (PIC) method is developed for simulating plasmas at solid densities. The space-time arrangement is based on Yee and a leapfrog algorithm for electromagnetic fields and particle advancement. The field solver algorithm completely eliminates numerical instabilities found in explicit PIC methods with relaxed time step and grid resolution. Moreover, this algorithm eliminates the numerical cooling found in the standard implicit PIC methods by using a pseudo-electric-field method. The particle pusher algorithm combines the standard Boris particle pusher with the Newton-Krylov iteration method. This algorithm increases the precision accuracy by several orders of magnitude when compared with the standard Boris particle pusher and also significantly decreases the iteration time when compared with the pure Newton-Krylov method. The code is tested with several benchmarks, including Weibel instability, and relativistic laser plasma interactions at both low and solid densities.

J. Helfrich, J. Vorberger, S. Frydrych, G. Schaumann, A. Ravasio, M. Gauthier, L. Fletcher, B. Nagler, B. Barbrel, B. Bachmann, E. Gamboa, S. Göde, E. Granados, H. Lee, P. Neumayer, W. Schumaker, T. Döppner, R. Falcone, S. Glenzer, M. Roth, and D. Kraus
Investigation of the temperature in dense carbon near the solid-liquid phase transition between 100 GPa and 200 GPa with spectrally resolved X-ray scattering
HEDP, 32 :56 (July 2019)

Abstract:

We present experiments investigating dense carbon at pressures between 100 GPa and 200 GPa and temperatures between 5,000 K and 15,000 K. High-pressure samples with different temperatures were created by laser-driven shock compression of graphite and varying the initial density from 1.53 g/cm³ to 2.21 g/cm³ and the drive laser intensity from 7.1 TW/cm² to 14.2 TW/cm². In order to deduce temperatures, spectrally resolved X-ray scattering was applied to determine ion-ion structure factors at a scattering vector of k = 4.12x10¹⁰ m⁻¹ which shows high sensitivity to temperature for the investigated sample conditions. After comparison to corresponding DFT-MD simulations, we were able to assign each structure factor a temperature. This information is indicative of the expected temperature range for the melting line of carbon at high pressures and can be compared to theoretical predictions.

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