# Referierte Publikationen

## 2020

J. Hofbrucker, L. Eiri, A. V. Volotka, and S. Fritzsche
Photoelectron Angular Distributions of Nonresonant Two-Photon Atomic Ionization Near Nonlinear Cooper Minima
Atoms 8, 54 (2020)

Abstract: Photoelectron angular distributions of the two-photon ionization of neutral atoms are theoretically investigated. Numerical calculations of two-photon ionization cross sections and asymmetry parameters are carried out within the independent-particle approximation and relativistic second-order perturbation theory. The dependence of the asymmetry parameters on the polarization and energy of the incident light as well as on the angular momentum properties of the ionized electron are investigated. While dynamic variations of the angular distributions at photon energies near intermediate level resonances are expected, we demonstrate that equally strong variations occur near the nonlinear Cooper minimum. The described phenomena is demonstrated on the example of two-photon ionization of magnesium atom.

W. Eschen, G. Tadesse, Y. Peng, M. Steinert, T. Pertsch, J. Limpert, and J. Rothhardt
Single-shot characterization of strongly focused coherent XUV and soft X-ray beams
Optics Letters 45, 1 (2020)

Abstract: In this Letter, we present a novel, to the best of our knowledge, single-shot method for characterizing focused coherent beams. We utilize a dedicated amplitude-only mask, in combination with an iterative phase retrieval algorithm, to reconstruct the amplitude and phase of a focused beam from a single measured far-field diffraction pattern alone. In a proof-of-principle experiment at a wavelength of 13.5 nm, we demonstrate our new method and obtain an RMS phase error of better than $\lambda /70$. This method will find applications in the alignment of complex optical systems, real-time feedback to adaptive optics, and single-shot beam characterization, e.g., at free-electron lasers or high-order harmonic beamlines.

D. Leimbach, J. Karls, Y. Guo, R. Ahmed, J. Ballof, L. Bengtsson, F. Boix Pamies, A. Borschevsky, K. Chrysalidis, E. Eliav, D. Fedorov, V. Fedosseev, O. Forstner, N. Galland, R. Garcia Ruiz, C. Granados, R. Heinke, K. Johnston, A. Koszorus, U. Köster, M. Kristiansson, Y. Liu, B. Marsh, P. Molkanov, L. Pašteka, J. Ramos, E. Renault, M. Reponen, A. Ringvall-Moberg, R. Rossel, D. Studer, A. Vernon, J. Warbinek, J. Welander, K. Wendt, S. Wilkins, D. Hanstorp, and S. Rothe
The electron affinity of astatine
Nature Communications 11, 3824 (2020)

Abstract: One of the most important properties influencing the chemical behavior of an element is the electron affinity (EA). Among the remaining elements with unknown EA is astatine, where one of its isotopes, 211At, is remarkably well suited for targeted radionuclide therapy of cancer. With the At− anion being involved in many aspects of current astatine labeling protocols, the knowledge of the electron affinity of this element is of prime importance. Here we report the measured value of the EA of astatine to be 2.41578(7)\thinspaceeV. This result is compared to state-of-the-art relativistic quantum mechanical calculations that incorporate both the Breit and the quantum electrodynamics (QED) corrections and the electron--electron correlation effects on the highest level that can be currently achieved for many-electron systems. The developed technique of laser-photodetachment spectroscopy of radioisotopes opens the path for future EA measurements of other radioelements such as polonium, and eventually super-heavy elements.

F. Karbstein, and E. A. Mosman
Enhancing quantum vacuum signatures with tailored laser beams
Physical Review D 101, 113002 (2020)

Abstract: We demonstrate that tailored laser beams provide a powerful means to make quantum vacuum signatures in strong electromagnetic fields accessible in experiment. Typical scenarios aiming at the detection of quantum vacuum nonlinearities at the high-intensity frontier envision the collision of focused laser pulses. The effective interaction of the driving fields mediated by vacuum fluctuations gives rise to signal photons encoding the signature of quantum vacuum nonlinearity. Isolating a small number of signal photons from the large background of the driving laser photons poses a major experimental challenge. The main idea of the present work is to modify the far-field properties of a driving laser beam to exhibit a fieldfree hole in its center, thereby allowing for an essentially backgroundfree measurement of the signal scattered in the forward direction. Our explicit construction makes use of a peculiar far-field/focus duality.

H. Bernhardt, A. Schmitt, B. Grabiger, B. Marx-Glowna, R. Loetzsch, H.-C. Wille, D. Bessas, A. Chumakov, R. Rüffer, R. Röhlsberger, T. Stöhlker, I. Uschmann, G. Paulus, and K. Schulze
Ultra-high precision x-ray polarimetry with artificial diamond channel cuts at the beam divergence limit
Physical Review Research 2, 023365 (2020)

Abstract: We report on the use of synthetic single-crystal diamonds for high definition x-ray polarimetry. The diamonds are precision mounted to form artificial channel-cut crystals (ACCs). Each ACC supports four consecutive reflections with a scattering angle 2ΘB of 90°. We achieved a polarization purity of 3.0×10−10 at beamline ID18 of the European Synchrotron Radiation Facility (ESRF). When the x-ray beam's horizontal divergence was reduced through additional collimation from 17 to 8.4μrad, the polarization purity improved to 1.4×10−10. Precision x-ray polarimetry thus has reached the limit, where the purity is determined by the divergence of the beam. In particular, this result is important for polarimetry at fourth generation x-ray sources, which provide diffraction-limited x-ray beams. The sensitivity expected as a consequence of the present work will pave the way for exploring new physics such as the investigation of vacuum birefringence.

V. Hilbert, M. Tschernajew, R. Klas, J. Limpert, and J. Rothhardt
A compact, turnkey, narrow-bandwidth, tunable, and high-photon-flux extreme ultraviolet source

Abstract: We report on a compact high-photon-flux extreme ultraviolet (XUV) source based on high harmonic generation. A high XUV-photon flux (>10¹³ photons/s) is achieved at 21.8 eV and 26.6 eV. The narrow spectral bandwidth (ΔE/E < 10⁻³) of the generated harmonics is in the range of state-of-the-art synchrotron beamlines and enables high resolution spectroscopy experiments. The robust design based on a fiber– laser system enables turnkey-controlled and even remotely controlled operation outside specialized laser laboratories, which opens the way for a variety of applications.

O. Forstner, J. Müller, and K. Stiebing
Opportunities for negative ions studies at the Frankfurt Low-energy Storage Ring (FLSR)
Hyperfine Interactions 241, 53 (2020)

Abstract: The room-temperature electrostatic heavy ion storage ring FLSR was originally designed to study the collision dynamics of atoms and molecules. Recently it has been equipped with a RF plasma ion source combined with a charge exchange cell to be able to perform studies with negative ions in the ring. In preliminary experiments beams of He−, O− and OH− could successfully be stored. The measured lifetime of the metastable He−--ion is in good agreement with previous results, showing that the lifetime measurement in this case is not limited by the storage time due to collisional detachment. In the case of O− and OH− storage times in the order of seconds have been achieved. In a next step laser beams will be introduced in the ring allowing photodetachment studies of vibrationally cold molecules.

K. Schoenberg, V. Bagnoud, A. Blazevic, V. E. Fortov, D. O. Gericke, A. Golubev, D. H. H. Hoffmann, D. Kraus, I. V. Lomonosov, V. Mintsev, S. Neff, P. Neumayer, A. R. Piriz, R. Redmer, O. Rosmej, M. Roth, T. Schenkel, B. Sharkov, N. A. Tahir, D. Varentsov, and Y. Zhao
High-energy-density-science capabilities at the Facility for Antiproton and Ion Research
Physics of Plasmas 27, 043103 (2020)

Abstract: The Facility for Antiproton and Ion Research (FAIR) will employ the World's highest intensity relativistic beams of heavy nuclei to uniquely create and investigate macroscopic (millimeter-sized) quantities of highly energetic and dense states of matter. Four principal themes of research have been identified: properties of materials driven to extreme conditions of pressure and temperature, shocked matter and material equation of state, basic properties of strongly coupled plasma and warm dense matter, and nuclear photonics with a focus on the excitation of nuclear processes in plasmas, laser-driven particle acceleration, and neutron production. The research program, principally driven by an international collaboration of scientists, called the HED@FAIR collaboration, will evolve over the next decade as the FAIR project completes and experimental capabilities develop. The first programmatic research element, called “FAIR Phase 0, officially began in 2018 to test components, detectors, and experimental techniques. Phase-0 research employs the existing and enhanced infrastructure of the GSI Helmholtzzentrum für Schwerionenforschung (GSI) heavy-ion synchrotron coupled with the PHELIX high-energy, high-intensity laser. The “FAIR Day one” experimental program, presently scheduled to begin in 2025, commences the use of FAIR's heavy-ion synchrotron, coupled to new experimental and diagnostic infrastructure, to realize the envisaged high-energy-density-science research program.

E. Appi, C. C. Papadopoulou, J. L. Mapa, N. Wesavkar, C. Jusko, P. Mosel, S. Ališauskas, T. Lang, C. M. Heyl, B. Manschwetus, M. Brachmanski, M. Braune, H. Lindenblatt, F. Trost, S. Meister, P. Schoch, R. Treusch, R. Moshammer, I. Hartl, U. Morgner, and M. Kovacev
A synchronized VUV light source based on high-order harmonic generation at FLASH
Scientific Reports 10, 6867 (2020)

Abstract: Ultrafast measurements in the extreme ultraviolet (XUV) spectral region targeting femtosecond timescales rely until today on two complementary XUV laser sources: free electron lasers (FELs) and high-harmonic generation (HHG) based sources. The combination of these two source types was until recently not realized. The complementary properties of both sources including broad bandwidth, high pulse energy, narrowband tunability and femtosecond timing, open new opportunities for two-color pump-probe studies. Here we show first results from the commissioning of a high-harmonic beamline that is fully synchronized with the free-electron laser FLASH, installed at beamline FL26 with permanent end-station including a reaction microscope (REMI). An optical parametric amplifier synchronized with the FEL burst mode drives the HHG process. First commissioning tests including electron momentum measurements using REMI, demonstrate long-term stability of the HHG source over more than 14 hours. This realization of the combination of these light sources will open new opportunities for time-resolved studies targeting different science cases including core-level ionization dynamics or the electron dynamics during the transformation of a molecule within a chemical reaction probed on femtosecond timescales in the ultraviolet to soft X-ray spectral region.

J. Deprince, M. Bautista, S. Fritzsche, J. García, T. Kallman, C. Mendoza, P. Palmeri, and P. Quinet
K-line X-ray fluorescence from highly charged iron ions under dense astrophysical plasma conditions
X-Ray Spectrometry 49, 29 (2020)

Abstract: In the present work, we report an investigation of plasma environment effects on the atomic parameters associated with the K-vacancy states in highly charged iron ions within the astrophysical context of accretion disks around black holes. More particularly, the sensitivity of K-line X-ray fluorescence parameters (wavelengths, radiative transition probabilities, and Auger rates) in Fe XVII–Fe XXV ions has been estimated for plasma conditions characterized by an electron temperature ranging from 10⁵ to 10⁷ K and an electron density ranging from 10¹⁸ to 10²² cm⁻³. In order to do this, relativistic multiconfiguration Dirac-Fock atomic structure calculations have been carried out by considering a time averaged Debye-Hückel potential for both the electron–nucleus and electron–electron interactions.

S. Schippers, T. Buhr, A. Borovik Jr., K. Holste, A. Perry-Sassmannshausen, K. Mertens, S. Reinwardt, M. Martins, S. Klumpp, K. Schubert, S. Bari, R. Beerwerth, S. Fritzsche, S. Ricz, J. Hellhund, and A. Müller
The photon-ion merged-beams experiment PIPE at PETRA III—The first five years
X-Ray Spectrometry 49, 11 (2020)

Abstract: The Photon-Ion Spectrometer at PETRA III—in short, PIPE—is a permanently installed user facility at the 'Variable Polarization XUV Beamline' P04 of the synchrotron light source PETRA III operated by DESY in Hamburg, Germany. The careful design of the PIPE ion-optics in combination with the record-high photon flux at P04 has lead to a breakthrough in experimental studies of photon interactions with ionized small quantum systems. This short review provides an overview over the published scientific results from photon-ion merged-beams experiments at PIPE that were obtained since the start of P04 operations in 2013. The topics covered comprise photoionization of ions of astrophysical relevance, quantitative studies of multi-electron processes upon inner-shell photoexcitation and photoionization of negative and positive atomic ions, precision spectroscopy of photoionization resonances, photoionization and photofragmentation of molecular ions, and of endohedral fullerene ions.

M. Afshari, J. Hornung, A. Kleinschmidt, P. Neumayer, D. Bertini, and V. Bagnoud
Proton acceleration via the TNSA mechanism using a smoothed laser focus

Abstract: In this work, we present the results of an experiment aiming at proton acceleration using a focus with a homogeneous intensity distribution, called smoothed focus. To achieve this goal, we implemented a phase plate before the pre-amplifier of the Petawatt High-Energy Laser for Heavy Ion EXperiments laser facility. The phase plate was used for the first time at a high-power short-pulse laser. Demonstrating a low divergent ion beam was the main goal of this work. Numerical simulations using the particle-in-cell code Extendable PIC Open Collaboration estimated a 2–5 times reduction in the angular divergence of the proton beam using a phase plate due to a smoother sheath at the rear side of the target. However, the reduction in the angular divergence was not sensible according to the experimental data. A positive point is that the spectrum of protons that are generated with the smoothed beam is shifted toward lower energies, provided that the laser absorption is kept in check, compared to the Gaussian proton spectrum. Moreover, the number of protons that are generated with the smoothed beam is higher than the ones generated with the Gaussian beam.

R. Hollinger, D. Hoff, P. Wustelt, S. Skruszewicz, Y. Zhang, H. Kang, D. Würzler, T. Jungnickel, M. Dumergue, A. Nayak, R. Flender, L. Haizer, M. Kurucz, B. Kiss, S. Kühn, E. Cormier, C. Spielmann, G. G. Paulus, P. Tzallas, and M. Kübel
Carrier-envelope-phase measurement of few-cycle mid-infrared laser pulses using high harmonic generation in ZnO
Optics Express 28, 7314 (2020)

Abstract: High-harmonic generation (HHG) in crystals offers a simple, affordable and easily accessible route to carrier-envelope phase (CEP) measurements, which scales favorably towards longer wavelengths. We present measurements of HHG in ZnO using few-cycle pulses at 3.1 µm. Thanks to the broad bandwidth of the driving laser pulses, spectral overlap between adjacent harmonic orders is achieved. The resulting spectral interference pattern provides access to the relative harmonic phase, and hence, the CEP.

R. Klas, W. Eschen, A. Kirsche, J. Rothhardt, and J. Limpert
Generation of coherent broadband high photon flux continua in the XUV with a sub-two-cycle fiber laser
Optics Express 28, 6188 (2020)

Abstract: High harmonic sources can provide ultrashort pulses of coherent radiation in the XUV and X-ray spectral region. In this paper we utilize a sub-two-cycle femtosecond fiber laser to efficiently generate a broadband continuum of high-order harmonics between 70 eV and 120 eV. The average power delivered by this source ranges from > 0.2 µW/eV at 80 eV to >0.03 µW/eV at 120 eV. At 92 eV (13.5 nm wavelength), we measured a coherent record-high average power of 0.1 µW/eV, which corresponds to 7 · 109 ph/s/eV, with a long-term stability of 0.8% rms deviation over a 20 min time period. The presented approach is average power scalable and promises up to 1011 ph/s/eV in the near future. With additional carrier-envelop phase control even isolated attosecond pulses can be expected from such sources. The combination of high flux, high photon energy and ultrashort (sub-) fs duration will enable photon-hungry time-resolved and multidimensional studies.

P. Balla, A. B. Wahid, I. Sytcevich, C. Guo, A.-L. Viotti, l. Silletti, A. Cartella, S. Alisauskas, H. Tavakol, U. Grosse-Wortmann, A. Schönberg, M. Seidel, A. Trabattoni, B. Manschwetus, T. Lang, F. Calegari, A. Couairon, A. LHuillier, C. Arnold, I. Hartl, and C. M. Heyl
Post-compression of picosecond pulses into the few-cycle regime
Optics Letters 45, 2572 (2020)

Abstract: In this work, we demonstrate post-compression of 1.2 picosecond laser pulses to 13 fs via gas-based multipass spectral broadening. Our results yield a singlestage compression factor of about 40 at 200 W in-burst average power and a total compression factor >90 at reduced power. The employed scheme represents a route towards compact few-cycle sources driven by industrial-grade Yb:YAG lasers at high average power.

L. Klar
Detectable Optical Signatures of QED Vacuum Nonlinearities Using High-Intensity Laser Fields
Particles 3, 223 (2020)

Abstract: Up to date, quantum electrodynamics (QED) is the most precisely tested quantum field theory. Nevertheless, particularly in the high-intensity regime it predicts various phenomena that so far have not directly been accessible in all-optical experiments, such as photon-photon scattering phenomena induced by quantum vacuum fluctuations. Here, we focus on all-optical signatures of quantum vacuum effects accessible in the high-intensity regime of electromagnetic fields. We present an experimental setup giving rise to signal photons distinguishable from the background. This configuration is based on two optical pulsed petawatt lasers: one generates a narrow but high-intensity scattering center to be probed by the other one. We calculate the differential number of signal photons attainable with this field configuration analytically and compare it with the background of the driving laser beams.

D. Würzler, S. Skruszewicz, A. M. Sayler, D. Zille, M. Möller, P. Wustelt, Y. Zhang, J. Tiggesbäumker, and G. G. Paulus
Accurate retrieval of ionization times by means of the phase-of-the-phase spectroscopy, and its limits
Physical Review A 101, 033416 (2020)

Abstract: By applying recently introduced, phase-of-the-phase spectroscopy [S. Skruszewicz et al., Phys. Rev. Lett. 115, 043001 (2015)], we analyze the phase-dependent photoelectron signal from Xe ionized in intense, parallel, two-color (1800 nm and 900 nm) laser fields. With such a field configuration, tuning of the relative phase between the ionizing, ω , and the perturbative, 2ω, field results in a modulation of the ionization rate, as well as modifications of the trajectories of electrons propagating in the laser-dressed continuum. Based on a semiclassical model, we confirm that phase dependencies, due to the perturbation of the ionization rate, encode the ionization times of the electrons. Here, using the fork structure, a well-known feature originating from well-defined dynamics allows us to distinguish between electrons ionized within distinct time windows. However, due to the simultaneous perturbation of the electron trajectories, the assignment of the ionization times can be distorted by up to 80 as, i.e., a 10° phase shift, which is independent of the degree of the perturbation.

B. Böning, W. Paufler, and S. Fritzsche
Polarization-dependent high-intensity Kapitza-Dirac effect in strong laser fields
Physical Review A 101, 031401 (2020)

Abstract: We study the deflection of photoelectrons in intense elliptically polarized standing light waves, known as the high-intensity Kapitza-Dirac effect. In order to compute the longitudinal momentum transfer to the photoelectron in above-threshold ionization, we utilize a complete description of the quantum dynamics in the spatially dependent field of the standing light wave. We propose experimental conditions under which low-energy photoelectrons can be generated with remarkably high longitudinal momenta that can be controlled via the polarization of the standing wave. We expect that future experimental realizations will provide additional insights into the momentum transfer in intense laser-atom interactions.

M. B. Schwab, E. Siminos, T. Heinemann, D. Ullmann, F. Karbstein, S. Kuschel, A. Sävert, M. Yeung, D. Hollatz, A. Seidel, J. Cole, S. P. D. Mangles, B. Hidding, M. Zepf, S. Skupin, and M. C. Kaluza
Visualization of relativistic laser pulses in underdense plasma
Physical Review Accelerators and Beams 23, 032801 (2020)

Abstract: We present experimental evidence of relativistic electron-cyclotron resonances (RECRs) in the vicinity of the relativistically intense pump laser of a laser wakefield accelerator (LWFA). The effects of the RECRs are visualized by imaging the driven plasma wave with a few-cycle, optical probe in transverse geometry. The probe experiences strong, spectrally dependent and relativistically modified birefringence in the vicinity of the pump that arises due to the plasma electrons’ relativistic motion in the pump’s electromagnetic fields. The spectral birefringence is strongly dependent on the local magnetic field distribution of the pump laser. Analysis and comparison to both 2D and 3D particle-in-cell simulations confirm the origin of the RECR effect and its appearance in experimental and simulated shadowgrams of the laser-plasma interaction. The RECR effect is relevant for any relativistic, magnetized plasma and in the case of LWFA could provide a nondestructive, in situ diagnostic for tracking the evolution of the pump’s intensity distribution with propagation through tenuous plasma.

Y. X. Zhang, S. Rykovanov, M. Shi, C. L. Zhong, X. T. He, B. Qiao, and M. Zepf
Giant Isolated Attosecond Pulses from Two-Color Laser-Plasma Interactions
Physical Review Letters 124, 114802 (2020)

Abstract: A new regime in the interaction of a two-color (ω,2ω) laser with a nanometer-scale foil is identified, resulting in the emission of extremely intense, isolated attosecond pulses—even in the case of multicycle lasers. For foils irradiated by lasers exceeding the blow-out field strength (i.e., capable of fully separating electrons from the ion background), the addition of a second harmonic field results in the stabilization of the foil up to the blow-out intensity. This is then followed by a sharp transition to transparency that essentially occurs in a single optical cycle. During the transition cycle, a dense, nanometer-scale electron bunch is accelerated to relativistic velocities and emits a single, strong attosecond pulse with a peak intensity approaching that of the laser field.

Y. Zhang, D. Zille, D. Hoff, P. Wustelt, D. Würzler, M. Möller, A. M. Sayler, and G. Paulus
Observing the Importance of the Phase-Volume Effect for Few-Cycle Light-Matter Interactions
Physical Review Letters 124, 133202 (2020)

Abstract: The spatially dependent phase distribution of focused few-cycle pulses, i.e., the focal phase, is much more complex than the well-known Gouy phase of monochromatic beams. As the focal phase is imprinted on the carrier-envelope phase (CEP), for accurate modeling and interpretation of CEP-dependent few-cycle laser-matter interactions, both the coupled spatially dependent phase and intensity distributions must be taken into account. In this Letter, we demonstrate the significance of the focal phase effect via comparison of measurements and simulations of CEP-dependent photoelectron spectra. Moreover, we demonstrate the impact of this effect on few-cycle light-matter interactions as a function of their nonlinear intensity dependence to answer the general question: if, when, and how much should one be concerned about the focal phase?

A. McIlvenny, H. Ahmed, C. Scullion, D. Doria, L. Romagnani, P. Martin, K. Naughton, A. Sgattoni, D. R. Symes, A. Macchi, P. McKenna, M. Zepf, S. Kar, and M. Borghes
Characteristics of ion beams generated inthe interaction of ultra-short laser pulseswith ultra-thin foils
Plasma Physics and Controlled Fusion 62, 054001 (2020)

Abstract: Experiments investigating ion acceleration from laser-irradiated ultra-thin foils on the GEMINI laser facility at the Rutherford appleton laboratory indicate a transition to 'light sail' radiation pressure acceleration when using circularly polarised, high contrast laser pulses. This paper complements previously published results with additional data and modelling which provide information on the multispecies dynamics taking place during the acceleration, and provides an indication on expected scaling of these processes at higher laser intensities.

C. Hahn, E. Menz, P. Pfäfflein, G. Weber, and T. Stöhlker
A scintillator‐based particle detector for CRYRING@ESR
X-Ray Spectrometry 49, 338 (2020)

Abstract: With the unprecedented range of ion species and energies offered by the newly commissioned CRYRING facility, the availability of single ion detectors is of significant importance as part of standard instrumentation as well as for novel experiments. A detector system was constructed on the basis of the YAP:Ce crystal scintillator, which is at once radiation‐hard, fast, and affordable. Results of a characterization experiment confirmed the feasibility of the setup for incident ion rates on the order of MHz and found a critical fluence of some 10¹³ cm⁻² upon which the crystal is rendered locally blind to further ion irradiation. The device was first used in CRYRING commissioning runs in August and November 2018. Future efforts will complete the integration of the detector into the GSI control and data acquisition system MBS.

V. Dinu, and G. Torgrimsson
Trident process in laser pulses
Physical Review D 101, 056017 (2020)

Abstract: We study the trident process in laser pulses. We provide exact numerical results for all contributions, including the difficult exchange term. We show that all terms are in general important for a short pulse. For a long pulse, we identify a term that gives the dominant contribution even if the intensity is only moderately high, a0≳1, which is an experimentally important regime where the standard locally constant field (LCF) approximation cannot be used. We show that the spectrum has a richer structure at a0∼1, compared to the LCF regime a0≫1. We study the convergence to LCF as a0 increases and how this convergence depends on the momentum of the initial electron. We also identify the terms that dominate at high energy.

J. Hofbrucker, A. V. Volotka, and S. Fritzsche
Breakdown of the electric dipole approximation at Cooper minima in direct two-photon ionisation
Scientific Reports 10, 3617 (2020)

Abstract: We predict breakdown of the electric dipole approximation at nonlinear Cooper minimum in direct two-photon K–shell atomic ionisation by circularly polarised light. According to predictions based on the electric dipole approximation, we expect that tuning the incident photon energy to the Cooper minimum in two-photon ionisation results in pure depletion of one spin projection of the initially bound 1s electrons, and hence, leaves the ionised atom in a fully oriented state. We show that by inclusion of electric quadrupole interaction, dramatic drop of orientation purity is obtained. The low degree of the remaining ion orientation provides a direct access to contributions of the electron-photon interaction beyond the electric dipole approximation in the two-photon ionisation of atoms and molecules. The orientation of the photoions can be experimentally detected either directly by a Stern-Gerlach analyzer, or by means of subsequent Kα fluorescence emission, which has the information about the ion orientation imprinted in the polarisation of the emitted photons.

N. Stallkamp, S. Ringleb, B. Arndt, M. Kiffer, S. Kumar, T. Morgenroth, G. G. Paulus, W. Quint, Th. Stöhlker, and M. Vogel
HILITE—A tool to investigate interactions of matter and light
X-Ray Spectrometry 49, 188 (2020)

Abstract: Detailed investigations of laser–ion interactions require well‐defined ion targets and detection techniques for high‐sensitivity measurements of reaction educts and products. To this end, we have designed and built the High‐Intensity Laser‐Ion Trap Experiment Penning trap setup, which features various ion‐target preparation techniques including selection, cooling, compression, and positioning as well as destructive and non‐destructive measurement techniques to determine the number of stored ions for all charge states individually and simultaneously. We have recently performed first commissioning experiments of ion deceleration and dynamic ion capture with highly charged ion bunches from an electron beam ion source. We have characterized our single‐pass non‐destructive ion counter in detail and were able to determine the ion velocity as well as the number of ions from the signals acquired.

O. Forstner, D. Bemmerer, T. Cowan, R. Dressler, A. Junghans, D. Schumann, T. Stöhlker, T. Szücs, A. Wagner, and K. Zuber
Opportunities for measurements of astrophysical-relevant alpha-capture reaction rates at CRYRING@ESR
X-Ray Spectrometry 49, 129 (2020)

Abstract: The heavy-ion storage ring CRYRING@ESR has recently been installed and commissioned at GSI as one of the first installations of the upcoming Facility for Antiproton and Ion Research (FAIR). It is designed to store highly charged ions in the energy range between 300?keV/u and about 10?MeV/u. It will incorporate a gas-jet target providing high-density jets of, among other gases, hydrogen and helium. This will allow to study alpha-capture reaction rates of astrophysical interest in the energy range of the Gamow window for core-collapse supernovae. Special interest comes from the long-lived radio-isotope 44Ti (t1/2?=?58.9?years), which is supposed to be produced in the alpha-rich freeze-out during such an event. The nucleosynthesis of this isotope is of great interest, as the amount of material produced can be estimated by direct observation in remnants of recent supernovae. The disagreements between the observations and the estimations from astrophysical models show the need of more experimental data for the production and consumption reactions in the energy range of a core-collapse supernova. In this article, we will describe the proposed method of injecting beams of 44Ti into CRYRING@ESR and performing the actual reaction rate measurements.

G. Weber, A. Gumberidze, M. Herdrich, R. Märtin, U. Spillmann, A. Surzhykov, D. Thorn, S. Trotsenko, N. Petridis, C. Fontes, and Th. Stöhlker
Towards a determination of absolute cross sections for projectile excitation of hydrogen‐like uranium in collisions with neutral atoms
X-Ray Spectrometry 49, 239 (2020)

Abstract: Recently, the contribution of the generalized Breit interaction to electron impact ionization was identified for the first time in a high‐Z system, namely, hydrogen‐like uranium. This study employed a measurement of the relative population of the j = 1/2 and j = 3/2 states of the L shell by projectile excitation in collision of U91+ with hydrogen and nitrogen targets. However, for a rigorous test of ion–atom collision theory, also the absolute excitation cross sections are of great importance. In the present work, we report on our efforts to extend the previous study to a determination of the absolute projectile excitation cross sections by normalization to the well‐known radiative electron capture process.

R. Sánchez, A. Braeuning-Demian, J. Glorius, S. Hagmann, P.-M. Hillenbrand, A. Kalinin, T. Köhler, Y. A. Litvinov, N. Petridis, S. Sanjari, U. Spillmann, and T. Stöhlker
Towards experiments with highly charged ions at HESR
X-Ray Spectrometry 49, 33 (2020)

Abstract: The atomic physics collaboration SPARC is a part of the APPA pillar at the future Facility for Antiproton and Ion Research. It aims at atomic‐physics research across virtually the full range of atomic matter. An emphasis of this contribution are the atomic physics experiments addressing the collision dynamics in strong electro‐magnetic fields as well as the fundamental interactions between electrons and heavy nuclei at the HESR. Here we give a short overview about the central instruments for SPARC experiments at this storage ring.

E. Shestaev, D. Hoff, A. M. Sayler, A. Klenke, S. Hädrich, F. Just, T. Eidam, P. Jójárt, Z. Várallyay, K. Osvay, G. G. Paulus, A. Tünnermann, and J. Limpert
High-power ytterbium-doped fiber laser delivering few-cycle, carrier-envelope phase-stable 100 µJ pulses at 100 kHz
Optics Letters 45, 97 (2020)

Abstract: We present a carrier-envelope phase (CEP)-stable Yb-doped fiber laser system delivering 100 µJ few-cycle pulses at a repetition rate of 100 kHz. The CEP stability of the system when seeded by a carrier-envelope offset-locked oscillator is 360 mrad, as measured pulse-to-pulse with a stereographic above-threshold ionization (stereo-ATI) phase meter. Slow CEP fluctuations have been suppressed by implementing a feedback loop from the phase meter to the pulse picking acousto-optic modulator. To the best of our knowledge, this is the highest CEP stability achieved to date with a fiber-based, high-power few-cycle laser.

F. Karbstein
Probing Vacuum Polarization Effects with High-Intensity Lasers
Particles 3, 39 (2020)

Abstract: These notes provide a pedagogical introduction to the theoretical study of vacuum polarization effects in strong electromagnetic fields as provided by state-of-the-art high-intensity lasers. Quantum vacuum fluctuations give rise to effective couplings between electromagnetic fields, thereby supplementing Maxwell’s linear theory of classical electrodynamics with nonlinearities. Resorting to a simplified laser pulse model, allowing for explicit analytical insights, we demonstrate how to efficiently analyze all-optical signatures of these effective interactions in high-intensity laser experiments. Moreover, we highlight several key features relevant for the accurate planning and quantitative theoretical analysis of quantum vacuum nonlinearities in the collision of high-intensity laser pulses.

S. Hagmann, P. Hillenbrand, Y. A. Litvinov, U. Spillmann, and T. Stöhlker
A magnetic spectrometer for electron‐positron pair spectroscopy in storage rings
X-Ray Spectrometry 49, 115 (2020)

Abstract: We report an analysis of electron‐optical properties of a toroidal magnetic sector spectrometer and examine parameters for its implementation in a relativistic heavy‐ion storage ring, for example the High Energy Storage ring (HESR) at the future Facility for Antiproton and Ion Research (FAIR) facility. For studies of free–free pair production in heavy‐ion atom collisions, this spectrometer exhibits very high efficiencies for coincident e+–e− pair spectroscopy over a wide range of momenta of emitted lepton pairs. The high coincidence efficiency of the spectrometer is the key for stringent tests of theoretical predictions for the phase space correlation of lepton vector momenta in free–free pair production.

F. M. Kröger, G. Weber, V. P. Shevelko, S. Hirlaender, M. W. Krasny, and Th. Stöhlker
Charge state tailoring of relativistic heavy ion beams for the Gamma Factory project at CERN
X-Ray Spectrometry 49, 25 (2020)

Abstract: We present charge‐state evolution studies for Pb⁵⁴⁺ ion beams passing through stripper foils at relativistic energies of 5.9 GeV/u. The purpose of this investigation is to determine the optimum target material and non‐equilibrium thickness for the efficient production of few‐electron lead ions, that is, Pb⁸⁰⁺ and Pb⁸¹⁺, at the present European Organization for Nuclear Research, CERN, accelerator facility at energies as high as 5.9 GeV/u. Based on these predictions, an Al stripper foil has been selected for a proof‐of‐principle measurement in the frame of the Gamma Factory study group. The experimental data confirms a substantial yield of non‐bare Pb ions. In addition, a charge‐state evolution study for the production of Li‐like lead ions Pb⁷⁹⁺ is presented, which will be subject of a follow‐up experiment in the near future.

M. Herdrich, A. Fleischmann, D. Hengstler, S. Allgeier, C. Enss, S. Trotsenko, T. Morgenroth, R. Schuch, G. Weber, and T. Stöhlker
High-precision X-ray spectroscopy of Fe ions in an EBIT using a micro-calorimeter detector: First results
X-Ray Spectrometry 49, 184 (2020)

Abstract: A micro-calorimeter X-ray detector of the maXs-30 type was used to record the X-ray radiation from Fe ions, being produced in the S-EBIT-I electron beam ion trap at the site of GSI. The resulting spectra demonstrate the superior energy resolving power of micro-calorimeter detectors compared with conventional semiconductor detectors. The experiment serves as another proof of principle for the application of calorimeters as dedicated high-resolution X-ray spectrometers at an ion facility. Together with the development of an improved analysis algorithm for online readout, these results present a step towards the use of maXs-type detectors as standard instrumentation at GSI/FAIR.

T. Gassner, A. Gumberidze, M. Trassinelli, R. Heß, U. Spillmann, D. Banaś, K.-H. Blumenhagen, F. Bosch, C. Brandau, W. Chen, C. Dimopoulou, E. Förster, R. Grisenti, S. Hagmann, P.-M. Hillenbrand, P. Indelicato, P. Jagodzinski, T. Kämpfer, M. Lestinsky, D. Liesen, Y. Litvinov, R. Lötzsch, B. Manil, R. Märtin, F. Nolden, N. Petridis, M. Sanjari, K. Schulze, M. Schwemlein, A. Simionovici, M. Steck, Th. Stöhlker, C. Szabo, S. Trotsenko, I. Uschmann, G. Weber, O. Wehrhan, N. Winckler, D. Winters, N. Winters, E. Ziegler, and H. Beyer
High-resolution wavelength-dispersive spectroscopy of K-shell transitions in hydrogen-like gold
X-Ray Spectrometry 49, 204 (2020)

Abstract: We present a measurement of K‐shell transitions in H‐like gold (Au78+) using specially developed transmission type crystal spectrometers combined with Ge(i) microstrip detectors. The experiment has been carried out at the Experimental Storage Ring at GSI in Darmstadt. This is a first high‐resolution wavelength‐dispersive measurement of a K‐shell transition in a high‐Z H‐like ion, thus representing an important milestone in this field. Ideas on possible future improvements are discussed as well.

J. Rothhardt, M. Bilal, R. Beerwerth, A. Volotka, V. Hilbert, T. Stöhlker, S. Fritzsche, and J. Limpert
Lifetime measurements of ultrashort-lived excited states in Be-like ions
X-Ray Spectrometry 49, 165 (2020)

Abstract: We propose to measure the lifetime of short-lived excited states in highly charged ions by pump-probe experiments. Utilizing two synchronized and delayed Femtosecond pulses allows accessing these lifetimes with Femtosecond precision. Such measurements could provide sensitive tests of state-of-the art atomic structure calculations beyond the capabilities of established methods.

W. Paufler, B. Böning, and S. Fritzsche
Quantum Collisions and Confinement of Atomic and Molecular Species, and Photons
(2020)

Abstract: We study strong-field ionization of a hydrogenic target by few-cycle Bessel pulses. In order to investigate the interplay between the carrier envelope phase (CEP) and the orbital angular momentum of a few-cycle pulse (OAM), we apply a semiclassical two-step model. In particular, we here compute and discuss photoelectron momentum distributions (PEMD) for localized atomic targets. We show how these momentum distributions are affected by the CEP and TAM of the incident pulse. In particular, we find that the OAM affects the PEMD in a similar way as the CEP, depending on the initial position of our target.

## 2019

L. Schmidl, G. Schmidl, A. Gawlik, J. Dellith, U. Hübner, V. Tympel, F. Schmidl, J. Plentz, C. Geis, and H. Haselmann
Combining super-resolution microscopy with neuronal network recording using magnesium fluoride thin films as cover layer for multi-electrode array technology
Scientific Reports 9, 16110 (2019)

Abstract: We present an approach for fabrication of reproducible, chemically and mechanically robust functionalized layers based on MgF₂ thin films on thin glass substrates. These show great advantages for use in super-resolution microscopy as well as for multi-electrode-array fabrication and are especially suited for combination of these techniques. The transparency of the coated substrates with the low refractive index material is adjustable by the layer thickness and can be increased above 92%. Due to the hydrophobic and lipophilic properties of the thin crystalline MgF₂ layers, the temporal stable adhesion needed for fixation of thin tissue, e.g. cryogenic brain slices is given. This has been tested using localization-based super-resolution microscopy with currently highest spatial resolution in light microscopy. We demonstrated that direct stochastic optical reconstruction microscopy revealed in reliable imaging of structures of central synapses by use of double immunostaining of post- (homer1 and GluA2) and presynaptic (bassoon) marker structure in a 10 µm brain slice without additional fixing of the slices. Due to the proven additional electrical insulating effect of MgF2 layers, surfaces of multi-electrode-arrays were coated with this material and tested by voltage-current-measurements. MgF₂ coated multi-electrode-arrays can be used as a functionalized microscope cover slip for combination with live-cell super-resolution microscopy.

V. Bagnoud, J. Hornung, M. Afshari, U. Eisenbarth, C. Brabetz, Z. Major, and B. Zielbauer
Implementation of a phase plate for the generation of homogeneous focal-spot intensity distributions at the high-energy short-pulse laser facility PHELIX
High Power Laser Science and Engineering 7, E62 (2019)

Abstract: We propose and demonstrate the use of random phase plates (RPPs) for high-energy sub-picosecond lasers. Contrarily to previous work related to nanosecond lasers, an RPP poses technical challenges with ultrashort-pulse lasers. Here, we implement the RPP near the beginning of the amplifier and image-relay it throughout the laser amplifier. With this, we obtain a uniform intensity distribution in the focus over an area 1600 times the diffraction limit. This method shows no significant drawbacks for the laser and it has been implemented at the PHELIX laser facility where it is now available for users.

T. Saule, S. Heinrich, J. Schötz, N. Lilienfein, M. Högner, O. DeVries, M. Plötner, J. Weitenberg, D. Esser, J. Schulte, P. Russbueldt, J. Limpert, M. Kling, U. Kleineberg, and I. Pupeza
High-flux ultrafast extreme-ultraviolet photoemission spectroscopy at 18.4 MHz pulse repetition rate
Nature Communications 10, 458 (2019)

Abstract: Laser-dressed photoelectron spectroscopy, employing extreme-ultraviolet attosecond pulses obtained by femtosecond-laser-driven high-order harmonic generation, grants access to atomic-scale electron dynamics. Limited by space charge effects determining the admissible number of photoelectrons ejected during each laser pulse, multidimensional (i.e. spatially or angle-resolved) attosecond photoelectron spectroscopy of solids and nanostructures requires high-photon-energy, broadband high harmonic sources operating at high repetition rates. Here, we present a high-conversion-efficiency, 18.4-MHz-repetition-rate cavity-enhanced high harmonic source emitting 5 x 10(5) photons per pulse in the 25-to-60-eV range, releasing 1 x 10(10) photoelectrons per second from a 10-mu m-diameter spot on tungsten, at space charge distortions of only a few tens of meV. Broadband, time-of-flight photoelectron detection with nearly 100% temporal duty cycle evidences a count rate improvement between two and three orders of magnitude over state-of-the-art attosecond photoelectron spectroscopy experiments under identical space charge conditions. The measurement time reduction and the photon energy scalability render this technology viable for next-generation, high-repetition-rate, multidimensional attosecond metrology.

J. W. Wang, M. Zepf, and S. Rykovanov
Intense attosecond pulses carrying orbital angularmomentum using laser plasma interactions
Nature Communications 10, 5554 (2019)

Abstract: Light beams with helical phase-fronts are known to carry orbital angular momentum (OAM) and provide an additional degree of freedom to beams of coherent light. While OAM beams can be readily derived from Gaussian laser beams with phase plates or gratings, this is far more challenging in the extreme ultra-violet (XUV), especially for the case of high XUV intensity. Here, we theoretically and numerically demonstrate that intense surface harmonics carrying OAM are naturally produced by the intrinsic dynamics of a relativistically intense circularly-polarized Gaussian beam (i.e. non-vortex) interacting with a target at normal incidence. Relativistic surface oscillations convert the laser pulses to intense XUV harmonic radiation via the well-known relativistic oscillating mirror mechanism. We show that the azimuthal and radial dependence of the harmonic generation process converts the spin angular momentum of the laser beam to orbital angular momentum resulting in an intense attosecond pulse (or pulse train) with OAM.

S. Hagmann, P. Hillenbrand, Y. Litvinov, U. Spillmann, and Th. Stöhlker
The magnetic toroidal sector as a broad-band electron–positron pair spectrometer I. lepton trajectories
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 946, 162641 (2019)

Abstract: We report an analysis of electron-optical properties of a toroidal magnetic sector spectrometer and examine parameters for its implementation in a relativistic heavy-ion storage ring like HESR. For studies of free-free pair production in heavy-ion atom collisions this spectrometer exhibits very high efficiencies for coincident e(+)- e(-) pair spectroscopy over a wide range of momenta of emitted lepton pairs. The high coincidence efficiency of the spectrometer is the key for stringent tests of theoretical predictions for the coincident positron- and electron emission characteristics and for the phase space correlation of lepton vector momenta in free-free pair production.

S. Panahiyan, and S. Fritzsche
Simulation of the multiphase configuration and phase transitions with quantum walks utilizing a step-dependent coin
Physical Review A 100, 062115 (2019)

Abstract: Quantum walks are versatile simulators of topological phases and phase transitions as observed in condensed-matter physics. Here, we utilize a step-dependent coin in quantum walks and investigate what topological phases we can simulate with it, their topological invariants, bound states, and possibility of phase transitions. These quantum walks simulate nontrivial phases characterized by topological invariants (winding number) ±1, which are similar to the ones observed in topological insulators and polyacetylene. We confirm that the number of phases and their corresponding bound states increase step dependently. In contrast, the size of topological phase and distance between two bound states are decreasing functions of steps resulting into formation of multiple phases as quantum walks proceed (multiphase configuration). We show that, in the bound states, the winding number and group velocity are ill defined and the second moment of the probability density distribution in position space undergoes an abrupt change. Therefore, there are phase transitions taking place over the bound states and between two topological phases with different winding numbers.

L. Stoyanov, G. Maleshkov, M. Zhekova, I. Stefanov, G. Paulus, and A. Dreischuh
Controllable beam reshaping by mixing square-shaped and hexagonal optical vortex lattices
Scientific Reports 9, 2128 (2019)

Abstract: In the present work we show experimentally and by numerical calculations a substantial far-field beam reshaping by mixing square-shaped and hexagonal optical vortex (OV) lattices composed of vortices with alternatively changing topological charges. We show that the small-scale structure of the observed pattern results from the OV lattice with the larger array node spacing, whereas the large-scale structure stems from the OV lattice with the smaller array node spacing. In addition, we demonstrate that it is possible to host an OV, a one-dimensional, or a quasi-two-dimensional singular beam in each of the bright beams of the generated focal patterns. The detailed experimental data at different square-to-hexagonal vortex array node spacings shows that this quantity could be used as a control parameter for generating the desired focused structure. The experimental data are in excellent agreement with the numerical simulations.

R. Beerwerth, T. Buhr, A. Perry-Sassmannshausen, S. O. Stock, S. Bari, K. Holste, A. L. D. Kilcoyne, S. Reinwardt, S. Ricz, D. W. Savin, K. Schubert, M. Martins, A. Müller, S. Fritzsche, and S. Schippers
Near L-edge Single and Multiple Photoionization of Triply Charged Iron Ions
The Astrophysical Journal 887, 189 (2019)

Abstract: Relative cross sections for m-fold photoionization (m = 1,…, 5) of Fe3+ by single-photon absorption were measured employing the photon-ion merged-beams setup PIPE at the PETRA III synchrotron light source operated at DESY in Hamburg, Germany. The photon energies used spanned the range of 680–950 eV, covering both the photoexcitation resonances from the 2p and 2s shells, as well as the direct ionization from both shells. Multiconfiguration Dirac–Hartree–Fock (MCDHF) calculations were performed to simulate the total photoexcitation spectra. Good agreement was found with the experimental results. These computations helped to assign several strong resonance features to specific transitions. We also carried out Hartree–Fock calculations with relativistic extensions taking into account both photoexcitation and photoionization. Furthermore, we performed extensive MCDHF calculations of the Auger cascades that result when an electron is removed from the 2p and 2s shells of Fe3+. Our theoretically predicted charge-state fractions are in good agreement with the experimental results, representing a substantial improvement over previous theoretical calculations. The main reason for the disagreement with the previous calculations is their lack of inclusion of slow Auger decays of several configurations that can only proceed when accompanied by de-excitation of two electrons. In such cases, this additional shake-down transition of a (sub)valence electron is required to gain the necessary energy for the release of the Auger electron.

J. B. Ohland, U. Eisenbarth, M. Roth, and V. Bagnoud
A study on the effects and visibility of low-order aberrations on laser beams with orbital angular momentum
Applied Physics B 32, 56 (2019)

Abstract: Laguerre–Gaussian-like laser beams have been proposed for driving experiments with high-intensity lasers. They carry orbital angular momentum and exhibit a ring-shaped intensity distribution in the far field which make them particularly attractive for various applications. We show experimentally and numerically that this donut-like shape is extremely sensitive to off-axis wavefront deformations. To support our claim, we generate a Laguerre–Gaussian-like laser beam and apply a selection of common low-order wavefront aberrations. We investigate the visibility of those wavefront deformations in the far field. Under use of established tolerance criteria, we determine the thresholds for the applied aberration and compare the findings with simulations for verification.

S. Hendi, S. Panahiyan, B. Panah, and M. Jamil
Alternative approach to thermodynamic phase transitions
Chinese Physics C 43, 113106 (2019)

Abstract: One of the major open problems in theoretical physics is the lack of a consistent quantum gravity theory. Recent developments in our knowledge on thermodynamic phase transitions of black holes and their van der Waals-like behavior may provide an interesting quantum interpretation of classical gravity. Studying different methods of investigating phase transitions can extend our understanding of the nature of quantum gravity. In this paper, we present an alternative theoretical approach for finding thermodynamic phase transitions in the extended phase space. Unlike the standard methods based on the usual equation of state involving temperature, our approach uses a new quasi-equation constructed from the slope of temperature versus entropy. This approach addresses some of the shortcomings of the other methods and provides a simple and powerful way of studying the critical behavior of a thermodynamical system. Among the applications of this approach, we emphasize the analytical demonstration of possible phase transition points and the identification of the non-physical range of horizon radii for black holes.

R. Hollinger, D. Gupta, M. Zapf, M. Karst, R. Röder, I. Uschmann, U. Reislöhner, D. Kartashov, C. Ronning, and C. Spielmann
Polarization dependent multiphoton absorption in ZnO thin films
Journal of Physics D: Applied Physics 53, 055102 (2019)

Abstract: We present a simple non-destructive approach for studying the polarization dependence of nonlinear absorption processes in semiconductors. The method is based on measuring the yield of the near UV photoluminescence as a function of polarization and intensity of femtosecond laser pulses. In particular, we investigated the polarization dependence of three photon laser absorption in intrinsic and Al-doped ZnO thin films. Both specimen show stronger emission for linearly polarized excitation compared to circular polarization. The ratios for the three-photon absorption coefficients are about 1.8 and independent of the doping. It is shown that Al-doped films have lower threshold for stimulated emission in comparison to the intrinsic films.

M. Weikum, T. Akhter, D. Alesini, A. Alexandrova, M. Anania, N. Andreev, I. Andriyash, A. Aschikhin, R. Assmann, T. Audet, A. Bacci, I. Barna, A. Beaton, A. Beck, A. Beluze, A. Bernhard, S. Bielawski, F. Bisesto, F. Brandi, R. Brinkmann, E. Bruendermann, M. Büscher, M. Bussmann, G. Bussolino, A. Chance, M. Chen, E. Chiadroni, A. Cianchi, J. Clarke, J. Cole, M. Couprie, M. Croia, B. Cros, P. Crump, G. Dattoli, A. Del Dotto, N. Delerue, S. De Nicola, J. Dias, U. Dorda, R. Fedele, A. Ferran Pousa, M. Ferrario, F. Filippi, G. Fiore, R. Fonseca, M. Galimberti, A. Gallo, A. Ghaith, D. Giove, A. Giribono, L. Gizzi, F. Grüner, A. Habib, C. Haefner, T. Heinemann, B. Hidding, B. Holzer, S. Hooker, T. Hosokai, M. Huebner, A. Irman, F. Jafarinia, D. Jaroszynski, C. Joshi, M. Kaluza, M. Kando, O. Karger, S. Karsch, E. Khazanov, D. Khikhlukha, A. Knetsch, D. Kocon, P. Koester, O. Kononenko, G. Korn, I. Kostyukov, K. Kruchinin, L. Labate, C. Blanc, C. Lechner, W. Leemans, A. Lehrach, X. Li, V. Libov, A. Lifschitz, V. Litvinenko, W. Lu, O. Lundh, A. Maier, V. Malka, G. Manahan, S. Mangles, B. Marchetti, A. Martinez de la Ossa, J. Martins, P. Mason, F. Massimo, F. Mathieu, G. Maynard, Z. Mazzotta, A. Molodozhentsev, A. Mostacci, A.- . Mueller, C. Murphy, Z. Najmudin, P. Nghiem, F. Nguyen, P. Niknejadi, J. Osterhoff, D. Oumbarek Espinos, D. Papadopoulos, B. Patrizi, V. Petrillo, M. Pocsai, K. Poder, R. Pompili, L. Pribyl, D. Pugacheva, P. Rajeev, S. Romeo, M. Rossetti Conti, A. Rossi, R. Rossmanith, E. Roussel, A. Sahai, G. Sarri, L. Schaper, P. Scherkl, U. Schramm, C. Schroeder, J. Scifo, L. Serafini, Z. Sheng, C. Siders, L. Silva, T. Silva, C. Simon, U. Sinha, A. Specka, M. Streeter, E. Svystun, D. Symes, C. Szwaj, G. Tauscher, D. Terzani, N. Thompson, G. Toci, P. Tomassini, R. Torres, D. Ullmann, C. Vaccarezza, M. Vannini, J. Vieira, F. Villa, C.- . Wahlstrom, R. Walczak, P. Walker, K. Wang, C. Welsch, S. Wiggins, J. Wolfenden, G. Xia, M. Yabashi, J. Zhu, and A. Zigler
Status of the Horizon 2020 EuPRAXIA conceptual design study
Journal of Physics: Conference Series 1350, 012059 (2019)

Abstract: The Horizon 2020 project EuPRAXIA (European Plasma Research Accelerator with eXcellence In Applications) is producing a conceptual design report for a highly compact and cost-effective European facility with multi-GeV electron beams accelerated using plasmas. EuPRAXIA will be set up as a distributed Open Innovation platform with two construction sites, one with a focus on beam-driven plasma acceleration (PWFA) and another site with a focus on laser-driven plasma acceleration (LWFA). User areas at both sites will provide access to free-electron laser pilot experiments, positron generation and acceleration, compact radiation sources, and test beams for high-energy physics detector development. Support centres in four different countries will complement the pan-European implementation of this infrastructure.

P. Polynkin, Z. Samsonova, A. Englesbe, A. Lucero, J. Elle, and A. Schmitt-Sody
Channeling the dielectric breakdown of air by a sequence of laser-generated plasma filaments [Invited]
Journal of the Optical Society of America B 36, 3024 (2019)

Abstract: We have investigated channeling the DC dielectric breakdown of a 20 cm air gap by a sequence of four concatenated plasma filaments, independently produced by four focused, 5-ps-long laser pulses. The polarity of the applied DC voltage, as well as the temporal delay between the four pulses, was varied from a few to 400 ns, in an attempt to find the optimum direction and speed of the stepping filament sequence. We have found that the filament sequence reliably channeled the breakdown and measurably reduced the breakdown threshold voltage, relative to that in the unguided breakdown. However, no meaningful dependence on either the polarity of the applied DC voltage or the stepping speed of the filament sequence was observed. Our results support the established scenario of channeling the DC air breakdown by laser filaments, which is primarily based on the creation of a reduced-density air channel bridging the discharge gap. The channeling mechanism associated with seeding the discharge leader by the filament plasma plays a negligible role.