Referierte Publikationen

2019

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
Appl. Phys. B, 32 :56 (November 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.
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S. Hendi, S. Panahiyan, B. Panah, and M. Jamil
Alternative approach to thermodynamic phase transitions
Chinese Phys. C, 43 :113106 (November 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.
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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]
J. Opt. Soc. Am. B, 36 :3024 (November 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.
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G. Becker, M. Schwab, R. Lötzsch, S. Tietze, D. Klöpfel, M. Rehwald, H.-P. Schlenvoigt, A. Sävert, U. Schramm, M. Zepf, and M. Kaluza
Characterization of laser-driven proton acceleration from water microdroplets
Sci. Rep., 9 :17169 (November 2019)
Abstract:
We report on a proton acceleration experiment in which high-intensity laser pulses with a wavelength of 0.4 mm and with varying temporal intensity contrast have been used to irradiate water droplets of 20 mm diameter. Such droplets are a reliable and easy-to-implement type of target for proton acceleration experiments with the potential to be used at very high repetition rates. We have investigated the influence of the laser's angle of incidence by moving the droplet along the laser polarization axis. This position, which is coupled with the angle of incidence, has a crucial impact on the maximum proton energy. Central irradiation leads to an inefficient coupling of the laser energy into hot electrons, resulting in a low maximum proton energy. The introduction of a controlled pre-pulse produces an enhancement of hot electron generation in this geometry and therefore higher proton energies. However, two-dimensional particle-in-cell simulations support our experimental results confirming, that even slightly higher proton energies are achieved under grazing laser incidence when no additional pre-plasma is present. Illuminating a droplet under grazing incidence generates a stream of hot electrons that flows along the droplet's surface due to self-generated electric and magnetic fields and ultimately generates a strong electric field responsible for proton acceleration. The interaction conditions were monitored with the help of an ultra-short optical probe laser, with which the plasma expansion could be observed.
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J. Polz, A. P. L. Robinson, A. Kalinin, G. A. Becker, R. Fraga, M. Hellwing, M. Hornung, S. Keppler, A. Kessler, D. Klöpfel, H. Liebetrau, F. Schorcht, J. Hein, M. Zepf, R. E. Grisenti, and M.C. Kaluza
Efficient Laser-Driven Proton Acceleration from a Cryogenic Solid Hydrogen Target
Sci. Rep., 9 :16534 (November 2019)
Abstract:
We report on the successful implementation and characterization of a cryogenic solid hydrogen target in experiments on high-power laser-driven proton acceleration. When irradiating a solid hydrogen filament of 10 mm diameter with 10-Terawatt laser pulses of 2.5 J energy, protons with kinetic energies in excess of 20?MeV exhibiting non-thermal features in their spectrum were observed. The protons were emitted into a large solid angle reaching a total conversion efficiency of several percent. Two-dimensional particle-in-cell simulations confirm our results indicating that the spectral modulations are caused by collisionless shocks launched from the surface of the the high-density filament into a low-density corona surrounding the target. The use of solid hydrogen targets may significantly improve the prospects of laser-accelerated proton pulses for future applications.
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T. Nagy, S. Hädrich, P. Simon, A. Blumenstein, N. Walther, R. Klas, J. Buldt, H. Stark, S. Breitkopf, P. Jójárt, I. Seres, Z. Várallyay, T. Eidam, and J. Limpert
Generation of three-cycle multi-millijoule laser pulses at 318  W average power
Optica, 6 :1423 (November 2019)
Abstract:
The generation of three-cycle multi-millijoule pulses at 318 W power is reported by compressing pulses of a Yb-fiber chirped pulse amplifier in a 6 m long stretched flexible hollow fiber. This technique brings high-power lasers to the few-cycle regime.
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J. Nathanael, M. Wünsche, S. Fuchs, T. Weber, J. Abel, J. Reinhard, F. Wiesner, U. Hübner, S. Skruszewicz, G. Paulus, and C. Rödel
Laboratory setup for extreme ultraviolet coherence tomography driven by a high-harmonic source
Rev. Sci. Instrum., 90 :113702 (November 2019)
Abstract:
We present a laboratory beamline dedicated to nanoscale subsurface imaging using extreme ultraviolet coherence tomography (XCT). In this setup, broad-bandwidth extreme ultraviolet (XUV) radiation is generated by a laser-driven high-harmonic source. The beamline is able to handle a spectral range of 30-130 eV and a beam divergence of 10 mrad (full width at half maximum). The XUV radiation is focused on the sample under investigation, and the broadband reflectivity is measured using an XUV spectrometer. For the given spectral window, the XCT beamline is particularly suited to investigate silicon-based nanostructured samples. Cross-sectional imaging of layered nanometer-scale samples can be routinely performed using the laboratory-scale XCT beamline. A depth resolution of 16 nm has been achieved using the spectral range of 36-98 eV which represents a 33% increase in resolution due to the broader spectral range compared to previous work.
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I. Engin, Z. Chitgar, O. Deppert, L. Lucchio, R. Engels, P. Fedorets, S. Frydrych, P. Gibbon, A. Kleinschmidt, A. Lehrach, R. Maier, D. Prasuhn, M. Roth, F. Schlüter, C. Schneider, T. Stöhlker, K. Strathmann, and M. Büscher
Laser-induced acceleration of Helium ions from unpolarized gas jets
Plasma Phys. Contr. F., 61 :115012 (November 2019)
Abstract:
In order to develop a laser-driven spin-polarized He-3-ion beam source available for nuclear-physics experiments as well as for the investigation of polarized nuclear fusion, several challenges have to be overcome. Apart from the provision of a properly polarized He-3 gas-jet target, one of the biggest milestones is the demonstration of the general feasibility of laser-induced ion acceleration out of gas-jet targets. Of particular importance is the knowledge about the main ion-emission angles as well as the achievable ion-energy spectra (dependent on the optimal set of laser and target parameters). We report on the results of such a feasibility study performed at PHELIX, GSI Darmstadt. Both He-3- and He-4-gas jets (n(gas) similar to 10(19) cm(-3)) were illuminated with high-intensity laser pulses, I-L similar to O(10(19) W cm(-2)). The main ion-emission angles could be identified (+/- 90 degrees with respect to the laser-propagation direction) and the ion-energy spectra for all ion species could be extracted: for the optimal laser and target parameters, the high-energy cut-offs for He-2+,He-1+ ions were 4.65 MeV (with a normalized energy uncertainty of Delta epsilon epsilon(-1) = 0.033) and 3.27 MeV (Delta epsilon epsilon(-1) = 0.055), respectively.
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A. Woldegeorgis, S. Herzer, M. Almassarani, S. Marathapalli, and A. Gopal
Modeling terahertz emission from the target rear side during intense laser-solid interactions
Phys. Rev. E, 100 :053204 (November 2019)
Abstract:
Relativistic laser-solid target interaction is a powerful source of terahertz radiation where broadband terahertz radiation is emitted from the front and rear surfaces of the target. Even though several experimental works have reported the generation of subpicosecond duration gigawatt peak power terahertz pulses from the target rear surface, the underlying physical process behind their origin is still an open question. Here we discuss a numerical model that can accurately reproduce several aspects of the experimental results. The model is based on the charged particle dynamics at the target rear surface and the evolution of the charge separation field. We identify the major contributors that are responsible for broadband terahertz emission from the rear surface of the target.
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S. Franchino-Viñas, and H. Gies
Propagator from nonperturbative worldline dynamics
Phys. Rev. D, 100 :105020 (November 2019)
Abstract:
We use the worldline representation for correlation functions together with numerical path integral methods to extract nonperturbative information about the propagator to all orders in the coupling in the quenched limit (small-Nf expansion). Specifically, we consider a simple two-scalar field theory with cubic interaction (S²QED) in four dimensions as a toy model for QED-like theories. Using a worldline regularization technique, we are able to analyze the divergence structure of all-order diagrams and to perform the renormalization of the model nonperturbatively. Our method gives us access to a wide range of couplings and coordinate distances. We compute the pole mass of the S²QED electron and observe sizable nonperturbative effects in the strong-coupling regime arising from the full photon dressing. We also find indications for the existence of a critical coupling where the photon dressing compensates the bare mass such that the electron mass vanishes. The short distance behavior remains unaffected by the photon dressing in accordance with the power-counting structure of the model.
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M. Kiffer, S. Ringleb, N. Stallkamp, B. Arndt, I. Blinov, S. Kumar, S. Stahl, T. Stöhlker, and M. Vogel
Single-pass non-destructive electronic detection of charged particles
Rev. Sci. Instrum., 90 :113301 (November 2019)
Abstract:
We have devised an experimental method and apparatus for the simultaneous nondestructive determination of the absolute ion number, ion kinetic energy, and length of bunches of charged particles. We have built and operated a corresponding electronic detector that is based on induced charges and their subsequent low-noise amplification at cryogenic temperatures. We have performed measurements with bunches of low-energy highly charged ions from an electron-beam ion source that show the capability of the methods and their implementation. We discuss requirements for, and applications of, such detectors with a particular view on the obtainable information and their sensitivity.
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A. Gopal, A. Woldegeorgis, S. Herzer, and M. Almassarani
Spatiotemporal visualization of the terahertz emission during high-power laser-matter interaction
Phys. Rev. E, 100 :053203 (November 2019)
Abstract:
Single-cycle pulses with multimillion volts per centimeter field strengths and spectra in the terahertz (THz) band have attracted great interest due to their ability to coherently manipulate molecular orientations and electron spins resonantly and nonresonantly. The tremendous progress made in the development of compact and powerful terahertz sources have identified intense laser-thin foil interaction as a potential candidate for high-power broadband terahertz radiation. They are micrometers in size and deliver radially polarized terahertz pulses with millijoule energy and gigawatt peak power. Although several works have been carried out to investigate the terahertz generation process, their origin and angular distribution are still debated. We present here an indisputable study on their spatiotemporal characteristics and elaborate the underlying physical processes via recording the three-dimensional beam profile along with transient dynamics. These results are substructured with the quantitative visualization of the charge particle spectra.
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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
J. Phys.: Conf. Ser., 1350 :012059 (November 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.
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Z. Sun, F. Tuitje, and C. Spielmann
Toward high contrast and high-resolution microscopic ghost imaging
Opt. Express, 27 :33652 (November 2019)
Abstract:
In this study, the influence of speckle size on contrast-to-noise ratio (CNR) and resolution is examined based on the object dimensions in the macroscopic and microscopic regimes. This research shows that for microscopic samples the conventional scaling laws are no longer effective and the CNR does not counter-propagate in the same manner as the resolution. To our knowledge, a deviation in CNR scaling on speckle size is observed for the first time in the field of microscopic ghost imaging. This result was verified using two different sample shapes. In addition, numerical analysis revealed that the noise of the photodiode is a limiting factor for the CNR. Based on these findings, the conditions for identifying the parameter set that maximizes the CNR and provides high resolution images was defined, which achieving high-quality microscopic ghost images.
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M. Zhekova, G. Maleshkov, L. Stoyanov, I. Stefanov, G. Paulus, and A. Dreischuh
Formation of multi-spot focal arrays by square-shaped optical vortex lattices
Opt. Commun., 449 :110 (October 2019)
Abstract:
In this work, we present numerical simulations and experimental evidence for the creation of controllable multi-spot focal arrays composed of bright beams with flat phase profiles. The input phase structures sent to spatial light modulators were square-shaped optical vortex (OV) lattices containing hundreds of vortices. In order to stabilize each of these lattices in space, all used OVs were singly charged and their topological charges varied alternatively across the structures. It is proven that the OV lattice node spacing can be used as a control parameter for reshaping the generated multi-spot focal arrays. Each peak of these arrays is shown to be able to additionally host a singular beam.
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D. A. Glazov, F. Köhler-Langes, A. V. Volotka, K. Blaum, F. Heiße, G. Plunien, W. Quint, S. Rau, V. M. Shabaev, S. Sturm, and G. Werth
g Factor of Lithiumlike Silicon: New Challenge to Bound-State QED
Phys. Rev. Lett., 123 :173001 (October 2019)
Abstract:
The recently established agreement between experiment and theory for the g factors of lithiumlike silicon and calcium ions manifests the most stringent test of the many-electron bound-state quantum electrodynamics (QED) effects in the presence of a magnetic field. In this Letter, we present a significant simultaneous improvement of both theoretical gth=2.000 889 894 4 (34) and experimental gexp=2.000 889 888 45 (14) values of the g factor of lithiumlike silicon 28Si11+. The theoretical precision now is limited by the many-electron two-loop contributions of the bound-state QED. The experimental value is accurate enough to test these contributions on a few percent level.
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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.
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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).
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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.
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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.
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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.
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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)
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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.
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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.
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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.
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