Peer-Review Publications


R. Ferrer, A. Barzakh, B. Bastin, R. Beerwerth, M. Block, P. Creemers, H. Grawe, R. Groote, P. Delahaye, X. Fléchard, S. Franchoo, S. Fritzsche, L. P. Gaffney, L. Ghys, W. Gins, C. Granados, R. Heinke, L. Hijazi, M. Huyse, T. Kron, Y. Kudryavtsev, M. Laatiaoui, N. Lecesne, M. Loiselet, F. Lutton, I. D. Moore, Y. Martínez, E. Mogilevskiy, P. Naubereit, J. Piot, S. Raeder, S. Rothe, H. Savajols, S. Sels, V. Sonnenschein, J.-C. Thomas, E. Traykov, C. Beveren, P. Bergh, P. Duppen, K. Wendt, and A. Zadvornaya
Towards high-resolution laser ionization spectroscopy of the heaviest elements in supersonic gas jet expansion
Nat. Commun., 8 :14520 (February 2017)
Resonant laser ionization and spectroscopy are widely used techniques at radioactive ion beam facilities to produce pure beams of exotic nuclei and measure the shape, size, spin and electromagnetic multipole moments of these nuclei. However, in such measurements it is difficult to combine a high efficiency with a high spectral resolution. Here we demonstrate the on-line application of atomic laser ionization spectroscopy in a supersonic gas jet, a technique suited for high-precision studies of the ground- and isomeric-state properties of nuclei located at the extremes of stability. The technique is characterized in a measurement on actinium isotopes around the N=126 neutron shell closure. A significant improvement in the spectral resolution by more than one order of magnitude is achieved in these experiments without loss in efficiency.
J. Thomas, M. M. Günther, and A. Pukhov
Beam load structures in a basic relativistic interaction model
Phys. Plasmas, 24 :013101 (January 2017)
Some recent experiments have shown that the beam load in bubble and blow-out experiments is located in a volume as small as a few μm^3. Now, we show what kinds of inner structures are possible in such a high dense electron ensemble. Our analysis starts from a first principles model for relativistically corrected mutual electron interaction in a phenomenological bubble model. Discussing 2D and 3D beam load configurations, we show that, depending on the bunch emittance, the beam load might be in a highly ordered and dense configuration, a less ordered but still dense state, or a configuration where each electron performs an individual random motion.
H. Gies, and G. Torgrimsson
Critical Schwinger pair production. II. Universality in the deeply critical regime
Phys. Rev. D, 95 :016001 (January 2017)
We study electron-positron pair production by spatially inhomogeneous electric fields. Depending on the localization of the field, a critical point (critical surface) exists in the space of field configurations where the pair production probability vanishes. Near criticality, pair production exhibits universal properties similar to those of continuous phase transitions. We extend results previously obtained in the semiclassical (weak-field) critical regime to the deeply critical regime for arbitrary peak field strength. In this regime, we find an enhanced universality, featuring a unique critical exponent β=3 for all sufficiently localized fields. For a large class of field profiles, we also compute the nonuniversal amplitudes.
P. A. Walker, P. D. Alesini, A. S. Alexandrova, M. P. Anania, N. E. Andreev, I. Andriyash, A. Aschikhin, R. W. Assmann, T. Audet, A. Bacci, I. F. Barna, A. Beaton, A. Beck, A. Beluze, A. Bernhard, S. Bielawski, F. G. Bisesto, J. Boedewadt, F. Brandi, O. Bringer, R. Brinkmann, E. Bründermann, M. Büscher, M. Bussmann, G. C. Bussolino, A. Chance, J. C. Chanteloup, M. Chen, E. Chiadroni, A. Cianchi, J. Clarke, J. Cole, M. E. Couprie, M. Croia, B. Cros, J. Dale, G. Dattoli, N. Delerue, O. Delferriere, P. Delinikolas, J. Dias, U. Dorda, K. Ertel, A. F. Pousa, M. Ferrario, F. Filippi, J. Fils, R. Fiorito, R. A. Fonseca, M. Galimberti, A. Gallo, D. Garzella, P. Gastinel, D. Giove, A. Giribono, L. A. Gizzi, F. J. Grüner, A. F. Habib, L. C. Haefner, T. Heinemann, B. Hidding, B. J. Holzer, S. M. Hooker, T. Hosokai, A. Irman, D. A. Jaroszynski, S. Jaster-Merz, C. Joshi, M.C. Kaluza, M. Kando, O. S. Karger, S. Karsch, E. Khazanov, D. Khikhlukha, A. Knetsch, D. Kocon, P. Koester, O. Kononenko, G. Korn, I. Kostyukov, L. Labate, C. Lechner, W. P. Leemans, A. Lehrach, F. Y. Li, X. Li, V. Libov, A. Lifschitz, V. Litvinenko, W. Lu, A. R. Maier, V. Malka, G. G. Manahan, S. P. D. Mangles, B. Marchetti, A. Marocchino, l. Ossa, J. L. Martins, F. Massimo, F. Mathieu, G. Maynard, T. J. Mehrling, A. Y. Molodozhentsev, A. Mosnier, A. Mostacci, A. S. Mueller, Z. Najmudin, P. A. P. Nghiem, F. Nguyen, P. Niknejadi, J. Osterhoff, D. Papadopoulos, B. Patrizi, R. Pattathil, V. Petrillo, M. A. Pocsai, K. Poder, R. Pompili, L. Pribyl, D. Pugacheva, S. Romeo, A. R. Rossi, E. Roussel, A. A. Sahai, P. Scherkl, U. Schramm, C. B. Schroeder, J. Schwindling, J. Scifo, L. Serafini, Z. M. Sheng, L. O. Silva, T. Silva, C. Simon, U. Sinha, A. Specka, M. J. V. Streeter, E. N. Svystun, D. Symes, C. Szwaj, G. Tauscher, A. G. R. Thomas, N. Thompson, G. Toci, P. Tomassini, C. Vaccarezza, M. Vannini, J. M. Vieira, F. Villa, C.-G. Wahlström, R. Walczak, M. K. Weikum, C. P. Welsch, C. Wiemann, J. Wolfenden, G. Xia, M. Yabashi, L. Yu, J. Zhu, and A. Zigler
Horizon 2020 EuPRAXIA design study
J. Phys.: Conf. Ser., 874 :012029 (January 2017)
The Horizon 2020 Project EuPRAXIA (“European Plasma Research Accelerator with eXcellence In Applications”) is preparing a conceptual design report of a highly compact and cost-effective European facility with multi-GeV electron beams using plasma as the acceleration medium. The accelerator facility will be based on a laser and/or a beam driven plasma acceleration approach and will be used for photon science, high-energy physics (HEP) detector tests, and other applications such as compact X-ray sources for medical imaging or material processing. EuPRAXIA started in November 2015 and will deliver the design report in October 2019. EuPRAXIA aims to be included on the ESFRI roadmap in 2020.
H. Liebetrau, M. Hornung, S. Keppler, M. Hellwing, A. Kessler, F. Schorcht, J. Hein, and M.C. Kaluza
Intracavity stretcher for chirped-pulse amplification in high-power laser systems
Opt. Lett., 42 :326 (January 2017)
We present pulse stretching with an intracavity Offner-type pulse stretcher applied to a high-energy, short-pulse laser system. The compact intracavity design, offering a tunable stretching factor, allows the pulses to be stretched to several nanoseconds and, at the same time, to be amplified to 100 μJ. The stretched pulses have been further amplified with the high-power laser system Polaris and have been recompressed to durations as short as 102 fs, reaching peak powers of 100 TW. Furthermore, the temporal intensity contrast is investigated and compared to the formerly used stretcher setup.
A. Hoffmann, C. Egelkamp, D. Winters, T. Kühl, and C. Spielmann
Online Monitoring of Laser-Generated XUV Radiation Spectra by Surface Reflectivity Measurements with Particle Detectors
Appl. Sci., 7 :70 (January 2017)
In this contribution, we present a wavelength-sensitive method for the detection of extreme ultraviolet (XUV) photon energies between 30 eV and 120 eV. The method is based on 45° reflectivity from either a cesium iodide-coated or an uncoated metal surface, which directs the XUV beam onto an electron or ion detector and its signal is used to monitor the XUV beam. The benefits of our approach are a spectrally sensitive diagnosis of the XUV radiation at the interaction place of time-resolved XUV experiments and the detection of infrared leak light though metal filters in high-harmonic generation (HHG) experiments. Both features were tested using spectrally shaped XUV pulses from HHG in a capillary, and we have achieved excellent agreement with XUV spectrometer measurements and reflectivity calculations. Our obtained results are of interest for time-resolved XUV experiments presenting an additional diagnostic directly in the interaction region and for small footprint XUV beamline diagnostics.
M. Kahle, J. Körner, J. Hein, and M.C. Kaluza
Performance of a quantum defect minimized disk laser based on cryogenically cooled Yb:CaF2
Opt. Laser Technol., 92 :19 (January 2017)
Abstract A low quantum defect is the fundamental key to a high efficiency of any laser. To study the anticipated performance boost for a 980 nm-diode pumped cryogenically cooled Yb:CaF2 disk laser we compared its operation at output wavelengths of 991 nm, 996 nm, and 1032 nm. Despite the higher quantum defect a maximum efficiency of 74% (output versus incident power) with an output power of 15.8 W was achieved at the 1032 nm output wavelength. This observation led to a detailed analysis of remaining loss mechanisms we are reporting on in this paper.


K. Minamisono, D. M. Rossi, R. Beerwerth, S. Fritzsche, D. Garand, A. Klose, Y. Liu, B. Maass, P. F. Mantica, A. J. Miller, P. Müller, W. Nazarewicz, W. Nörtershäuser, E. Olsen, M. R. Pearson, P.-G. Reinhard, E. E. Saperstein, C. Sumithrarachchi, and S. V. Tolokonnikov
Charge Radii of Neutron Deficient ⁵²,⁵³Fe Produced by Projectile Fragmentation
Phys. Rev. Lett., 117 :252501 (December 2016)
Bunched-beam collinear laser spectroscopy is performed on neutron deficient Fe52,53 prepared through in-flight separation followed by a gas stopping. This novel scheme is a major step to reach nuclides far from the stability line in laser spectroscopy. Differential mean-square charge radii δ⟨r2⟩ of Fe52,53 are determined relative to stable Fe56 as δ⟨r2⟩56,52=−0.034(13)  fm2 and δ⟨r2⟩56,53=−0.218(13)  fm2, respectively, from the isotope shift of atomic hyperfine structures. The multiconfiguration Dirac-Fock method is used to calculate atomic factors to deduce δ⟨r2⟩. The values of δ⟨r2⟩ exhibit a minimum at the N=28 neutron shell closure. The nuclear density functional theory with Fayans and Skyrme energy density functionals is used to interpret the data. The trend of δ⟨r2⟩ along the Fe isotopic chain results from an interplay between single-particle shell structure, pairing, and polarization effects and provides important data for understanding the intricate trend in the δ⟨r2⟩ of closed-shell Ca isotopes.
M. Yeung, S. Rykovanov, J. Bierbach, L. Li, E. Eckner, S. Kuschel, A. Woldegeorgis, C. Rödel, A. Sävert, G.G. Paulus, M. Coughlan, B. Dromey, and M. Zepf
Experimental observation of attosecond control over relativistic electron bunches with two-colour fields
Nat. Photonics, 32 :11 (December 2016)
Energy coupling during relativistically intense laser–matter interactions is encoded in the attosecond motion of strongly driven electrons at the pre-formed plasma–vacuum boundary. Studying and controlling this motion can reveal details about the microscopic processes that govern a vast array of light–matter interaction phenomena, including those at the forefront of extreme laser–plasma science such as laser-driven ion acceleration, bright attosecond pulse generation and efficient energy coupling for the generation and study of warm dense matter. Here we experimentally demonstrate that by precisely adjusting the relative phase of an additional laser beam operating at the second harmonic of the driving laser it is possible to control the trajectories of relativistic electron bunches formed during the interaction with a solid target at the attosecond scale. We observe significant enhancements in the resulting high-harmonic yield, suggesting potential applications for sources of ultra-bright, extreme ultraviolet attosecond radiation to be used in atomic and molecular pump–probe experiments.
S. Breitkopf, S. Wunderlich, T. Eidam, E. Shestaev, S. Holzberger, T. Gottschall, H. Carstens, A. Tünnermann, I. Pupeza, and J. Limpert
Extraction of enhanced, ultrashort laser pulses from a passive 10-MHz stack-and-dump cavity
Appl. Phys. B, 122 :297 (December 2016)
Periodic dumping of ultrashort laser pulses from a passive multi-MHz repetition-rate enhancement cavity is a promising route towards multi-kHz repetition-rate pulses with Joule-level energies at an unparalleled average power. Here, we demonstrate this so-called stack-and-dump scheme with a 30-m-long cavity. Using an acousto-optic modulator, we extract pulses of 0.16 mJ at 30-kHz repetition rate, corresponding to 65 stacked input pulses, representing an improvement in three orders of magnitude over previously extracted pulse energies. The ten times longer cavity affords three essential benefits over former approaches. First, the time between subsequent pulses is increased to 100 ns, relaxing the requirements on the switch. Second, it allows for the stacking of strongly stretched pulses (here from 800 fs to 1.5 ns), thus mitigating nonlinear effects in the cavity optics. Third, the choice of a long cavity offers increased design flexibility with regard to thermal robustness, which will be crucial for future power scaling. The herein presented results constitute a necessary step towards stack-and-dump systems providing access to unprecedented laser parameter regimes.
M. Drągowski, M. Wlodarczyk, G. Weber, J. Ciborowski, J. Enders, Y. Fritzsche, and A. Poliszczuk
Monte Carlo study of the effective Sherman function for electron polarimetry
Nucl. Instr. Meth. Phys. Res. B, 389 :48 (December 2016)
The PEBSI Monte Carlo simulation was upgraded towards usefulness for electron Mott polarimetry. The description of Mott scattering was improved and polarisation transfer in Moller scattering was included in the code. An improved agreement was achieved between the simulation and available experimental data for a 100 keV polarised electron beam scattering off gold foils of various thicknesses. The dependence of the effective Sherman function on scattering angle and target thickness, as well as the method of finding optimal conditions for Mott polarimetry measurements were analysed.
L. Filippin, R. Beerwerth, J. Ekman, S. Fritzsche, M. Godefroid, and P. Jönsson
Multiconfiguration calculations of electronic isotope shift factors in Al I
Phys. Rev. A, 94 :062508 (December 2016)
The present work reports results from systematic multiconfiguration Dirac–Hartree–Fock calculations of electronic isotope shift factors for a set of transitions between low-lying levels of neutral aluminium. These electronic quantities together with observed isotope shifts between different pairs of isotopes provide the changes in mean-square charge radii of the atomic nuclei. Two computational approaches are adopted for the estimation of the mass- and field-shift factors. Within these approaches, different models for electron correlation are explored in a systematic way to determine a reliable computational strategy and to estimate theoretical error bars of the isotope shift factors.
A. V. Volotka, A. Surzhykov, S. Trotsenko, G. Plunien, T. Stöhlker, and S. Fritzsche
Nuclear Excitation by Two-Photon Electron Transition
Phys. Rev. Lett., 117 :243001 (December 2016)
A new mechanism of nuclear excitation via two-photon electron transitions (NETP) is proposed and studied theoretically. As a generic example, detailed calculations are performed for the E1E1 1s2sS01→1s2S01 two-photon decay of a He-like Ac87+225 ion with a resonant excitation of the 3/2+ nuclear state with an energy of 40.09(5) keV. The probability for such a two-photon decay via the nuclear excitation is found to be PNETP=3.5×10−9 and, thus, is comparable with other mechanisms, such as nuclear excitation by electron transition and by electron capture. The possibility for the experimental observation of the proposed mechanism is thoroughly discussed.
T. Saule, S. Holzberger, O. de Vries, M. Plötner, J. Limpert, A. Tünnermann, and I. Pupeza
Phase-stable, multi-textmuJ femtosecond pulses from a repetition-rate tunable Ti:Sa-oscillator-seeded Yb-fiber amplifier
Appl. Phys. B, 123 :17 (December 2016)
We present a high-power, MHz-repetition-rate, phase-stable femtosecond laser system based on a phase-stabilized Ti:Sa oscillator and a multi-stage Yb-fiber chirped-pulse power amplifier. A 10-nm band around 1030 nm is split from the 7-fs oscillator output and serves as the seed for subsequent amplification by 54 dB to 80 W of average power. The textmuJ-level output is spectrally broadened in a solid-core fiber and compressed to textasciitilde30 fs with chirped mirrors. A pulse picker prior to power amplification allows for decreasing the repetition rate from 74 MHz by a factor of up to 4 without affecting the pulse parameters. To compensate for phase jitter added by the amplifier to the feed-forward phase-stabilized seeding pulses, a self-referencing feed-back loop is implemented at the system output. An integrated out-of-loop phase noise of less than 100 mrad was measured in the band from 0.4 Hz to 400 kHz, which to the best of our knowledge corresponds to the highest phase stability ever demonstrated for high-power, multi-MHz-repetition-rate ultrafast lasers. This system will enable experiments in attosecond physics at unprecedented repetition rates, it offers ideal prerequisites for the generation and field-resolved electro-optical sampling of high-power, broadband infrared pulses, and it is suitable for phase-stable white light generation.
J. Hofbrucker, A. V. Volotka, and S. Fritzsche
Relativistic calculations of the nonresonant two-photon ionization of neutral atoms
Phys. Rev. A, 94 :063412 (December 2016)
The nonresonant, two-photon, one-electron ionization of neutral atoms is studied theoretically in the framework of relativistic second-order perturbation theory and independent particle approximation. In particular, the importance of relativistic and screening effects in the total two-photon ionization cross section is investigated. Detailed computations have been carried out for the K-shell ionization of neutral Ne, Ge, Xe, and U atoms. The relativistic effects significantly decrease the total cross section; for the case of U, for example, they reduce the total cross section by a factor of two. Moreover, we have found that the account for the screening effects of the remaining electrons leads to occurrence of an unexpected minimum in the total cross section at the total photon energies equal to the ionization threshold; for the case of Ne, for example, the cross section drops there by a factor of three.
M. Hornung, H. Liebetrau, S. Keppler, A. Kessler, M. Hellwing, F. Schorcht, G. A. Becker, M. Reuter, J. Polz, J. Körner, J. Hein, and M.C. Kaluza
54  J pulses with 18  nm bandwidth from a diode-pumped chirped-pulse amplification laser system
Opt. Lett., 41 :5413 (November 2016)
We report on results from the fully diode-pumped chirped-pulse amplification laser system Polaris. Pulses were amplified to a maximum energy of 54.2 J before compression. These pulses have a full width at half-maximum spectral bandwidth of 18 nm centered at 1033 nm and are generated at a repetition rate of 0.02 Hz. To the best of our knowledge, these are the most energetic broadband laser pulses generated by a diode-pumped laser system so far. Due to the limited size of our vacuum compressor, only attenuated pulses could be compressed to a duration of 98 fs containing an energy of 16.7 J, which leads to a peak power of 170 TW. These pulses could be focused to a peak intensity of 1.3×1021  W/cm2. Having an ultra-high temporal contrast of 1012 with respect to amplified spontaneous emission these laser pulses are well suited for high-intensity laser–matter experiments.
H. Heylen, C. Babcock, R. Beerwerth, J. Billowes, M. L. Bissell, K. Blaum, J. Bonnard, P. Campbell, B. Cheal, T. Day Goodacre, D. Fedorov, S. Fritzsche, R. F. Garcia Ruiz, W. Geithner, Ch. Geppert, W. Gins, L. K. Grob, M. Kowalska, K. Kreim, S. M. Lenzi, I. D. Moore, B. Maass, S. Malbrunot-Ettenauer, B. Marsh, R. Neugart, G. Neyens, W. Nörtershäuser, T. Otsuka, J. Papuga, R. Rossel, S. Rothe, R. Sanchez, Y. Tsunoda, C. Wraith, L. Xie, X. F. Yang, and D. T. Yordanov
Changes in nuclear structure along the Mn isotopic chain studied via charge radii
Phys. Rev. C, 94 :054321 (November 2016)
The hyperfine spectra of Mn51,53−64 were measured in two experimental runs using collinear laser spectroscopy at ISOLDE, CERN. Laser spectroscopy was performed on the atomic 3d54s2S5/26→3d54s4pP3/26 and ionic 3d54s5S2→3d54p5P3 transitions, yielding two sets of isotope shifts. The mass and field shift factors for both transitions have been calculated in the multiconfiguration Dirac-Fock framework and were combined with a King plot analysis in order to obtain a consistent set of mean-square charge radii which, together with earlier work on neutron-deficient Mn, allow the study of nuclear structure changes from N=25 across N=28 up to N=39. A clear development of deformation is observed towards N=40, confirming the conclusions of the nuclear moments studies. From a Monte Carlo shell-model study of the shape in the Mn isotopic chain, it is suggested that the observed development of deformation is not only due to an increase in static prolate deformation but also due to shape fluctuations and triaxiality. The changes in mean-square charge radii are well reproduced using the Duflo-Zuker formula except in the case of large deformation.
J. Körner, J. Hein, and M. Kaluza
Compact Aberration-Free Relay-Imaging Multi-Pass Layouts for High-Energy Laser Amplifiers
Appl. Sci., 6 :353 (November 2016)
We present the results from a theoretical investigation of laser beam propagation in relay imaging multi-pass layouts, which recently found application in high-energy laser amplifiers. Using a method based on the well-known ABCD-matrix formalism and proven by ray tracing, it was possible to derive a categorization of such systems. Furthermore, basic rules for the setup of such systems and the compensation for low order aberrations are derived. Due to the introduced generalization and parametrization, the presented results can immediately be applied to any system of the investigated kinds for a wide range of parameters, such as number of round-trips, focal lengths and optics sizes. It is shown that appropriate setups allow a close-to-perfect compensation of defocus caused by a thermal lens and astigmatism caused by non-normal incidence on the imaging optics, as well. Both are important to avoid intensity spikes leading to damages of optics in multi-pass laser amplifiers.
G. K. Tadesse, R. Klas, S. Demmler, S. Hädrich, I. Wahyutama, M. Steinert, C. Spielmann, M. Zürch, T. Pertsch, A. Tünnermann, J. Limpert, and J. Rothhardt
High speed and high resolution table-top nanoscale imaging
Opt. Lett., 41 :5170 (November 2016)
We present a table-top coherent diffractive imaging (CDI) experiment based on high-order harmonics generated at 18 nm by a high average power femtosecond fiber laser system. The high photon flux, narrow spectral bandwidth, and high degree of spatial coherence allow for ultrahigh subwavelength resolution imaging at a high numerical aperture. Our experiments demonstrate a half-pitch resolution of 15 nm, close to the actual Abbe limit of 12 nm, which is the highest resolution achieved from any table-top extreme ultraviolet (XUV) or x-ray microscope. In addition, sub-30 nm resolution was achieved with only 3 s of integration time, bringing live diffractive imaging and three-dimensional tomography on the nanoscale one step closer to reality. The current resolution is solely limited by the wavelength and the detector size. Thus, table-top nanoscopes with only a few-nanometer resolutions are in reach and will find applications in many areas of science and technology.
H. Li, X. M. Tong, N. Schirmel, G. Urbasch, K. J. Betsch, S. Zherebtsov, F. Süssmann, A. Kessel, S. A. Trushin, G.G. Paulus, K.-M. Weitzel, and M. F. Kling
Intensity dependence of the dissociative ionization of DCl in few-cycle laser fields
J. Phys. B, 49 :015601 (November 2016)
We have studied the dissociative ionization of DCl in 4 fs laser fields at 720 nm central wavelength using intensities in the range (1.3–3.1) × 10^14 W cm−2 . By employing the phase-tagged velocity-map imaging technique, information about the angular distribution of deuterium ions as a function of their kinetic energy and the carrier-envelope phase is obtained. On the basis of the experimental data and semi-classical simulations, three regions are distinguished for the resulting D+ ions with different kinetic energies. The one with the lowest kinetic energy, around 5–7 eV, is from dissociation involving the X-state of DCl+ , populated through direct ionization with the laser field. The second region, around 7–11 eV, originates from rescattering induced dissociative ionization. Above 2 × 10^14 W cm−2 D+ ions with kinetic energies exceeding 15 eV are obtained, which we ascribe to double ionization induced by rescattered electrons.
M. Fernandes, R. Geithner, J. Golm, R. Neubert, M. Schwickert, T. Stöhlker, J. Tan, and C. P. Welsch
Non-perturbative measurement of low-intensity charged particle beams
Supercond. Sci. Technol., 30 :015001 (November 2016)
Non-perturbative measurements of low-intensity charged particle beams are particularly challenging to beam diagnostics due to the low amplitude of the induced electromagnetic fields. In the low-energy antiproton decelerator (AD) and the future extra low energy antiproton rings at CERN, an absolute measurement of the beam intensity is essential to monitor the operation efficiency. Superconducting quantum interference device (SQUID) based cryogenic current comparators (CCC) have been used for measuring slow charged beams in the nA range, showing a very good current resolution. But these were unable to measure fast bunched beams, due to the slew-rate limitation of SQUID devices and presented a strong susceptibility to external perturbations. Here, we present a CCC system developed for the AD machine, which was optimised in terms of its current resolution, system stability, ability to cope with short bunched beams, and immunity to mechanical vibrations. This paper presents the monitor design and the first results from measurements with a low energy antiproton beam obtained in the AD in 2015. These are the first CCC beam current measurements ever performed in a synchrotron machine with both coasting and short bunched beams. It is shown that the system is able to stably measure the AD beam throughout the entire cycle, with a current resolution of 30 nA .
K.-H. Blumenhagen, S. Fritzsche, T. Gassner, A. Gumberidze, R. Märtin, N. Schell, D. Seipt, U. Spillmann, A. Surzhykov, S. Trotsenko, G. Weber, V. A. Yerokhin, and T. Stöhlker
Polarization transfer in Rayleigh scattering of hard x-rays
New J. Phys., 18 :103034 (November 2016)
We report on the first elastic hard x-ray scattering experiment where the linear polarization characteristics of both the incident and the scattered radiation were observed. Rayleigh scattering was investigated in a relativistic regime by using a high- Z target material, namely gold, and a photon energy of 175 keV. Although the incident synchrotron radiation was nearly 100% linearly polarized, at a scattering angle of θ=90° we observed a strong depolarization for the scattered photons with a degree of linear polarization of +27% ± 12% only. This finding agrees with second-order quantum electrodynamics calculations of Rayleigh scattering, when taking into account a small polarization impurity of the incident photon beam which was determined to be close to 98%. The latter value was obtained independently from the elastic scattering by analyzing photons that were Compton-scattered in the target. Moreover, our results indicate that when relying on state-of-the-art theory, Rayleigh scattering could provide a very accurate method to diagnose polarization impurities in a broad region of hard x-ray energies.
R. Klas, S. Demmler, M. Tschernajew, S. Hädrich, Y. Shamir, A. Tünnermann, J. Rothhardt, and J. Limpert
Table-top milliwatt-class extreme ultraviolet high harmonic light source
Optica, 3 :1167 (November 2016)
Extreme ultraviolet (XUV) lasers are essential for the investigation of fundamental physics. Especially high repetition rate, high photon flux sources are of major interest for reducing acquisition times and improving signal-to-noise ratios in a plethora of applications. Here, an XUV source based on cascaded frequency conversion is presented, which, due to the drastic better single atom response for short wavelength drivers, delivers an average output power of (832±204)  μW at 21.7 eV. This is the highest average power produced by any high harmonic generation source in this spectral range, surpassing previous demonstrations by almost an order of magnitude. Furthermore, a narrowband harmonic at 26.6 eV with a relative energy bandwidth of only ΔE/E=1.8·10−3 has been generated that is of high interest for high-precision spectroscopy experiments.
D. Seipt, R. A. Müller, A. Surzhykov, and S. Fritzsche
Two-color above-threshold ionization of atoms and ions in XUV Bessel beams and intense laser light
Phys. Rev. A, 94 :053420 (November 2016)
The two-color above-threshold ionization (ATI) of atoms and ions is investigated for a vortex Bessel beam in the presence of a strong near-infrared (NIR) light field. While the photoionization is caused by the photons from the weak but extreme ultraviolet (XUV) vortex Bessel beam, the energy and angular distribution of the photoelectrons and their sideband structure are affected by the plane-wave NIR field. We here explore the energy spectra and angular emission of the photoelectrons in such two-color fields as a function of the size and location of the target atoms with regard to the beam axis. In addition, analog to the circular dichroism in typical two-color ATI experiments with circularly polarized light, we define and discuss seven different dichroism signals for such vortex Bessel beams that arise from the various combinations of the orbital and spin angular momenta of the two light fields. For localized targets, it is found that these dichroism signals strongly depend on the size and position of the atoms relative to the beam. For macroscopically extended targets, in contrast, three of these dichroism signals tend to zero, while the other four just coincide with the standard circular dichroism, similar as for Bessel beams with a small opening angle. Detailed computations of the dichroism are performed and discussed for the 4s valence-shell photoionization of Ca+ ions.
I. P. Ivanov, D. Seipt, A. Surzhykov, and S. Fritzsche
Elastic scattering of vortex electrons provides direct access to the Coulomb phase
Phys. Rev. D, 94 :076001 (October 2016)
Vortex electron beams are freely propagating electron waves carrying adjustable orbital angular momentum with respect to the propagation direction. Such beams were experimentally realized just a few years ago and are now used to probe various electromagnetic processes. So far, these experiments used the single vortex electron beams, either propagating in external fields or impacting a target. Here, we investigate the elastic scattering of two such aligned vortex electron beams and demonstrate that this process allows one to experimentally measure features which are impossible to detect in the usual plane-wave scattering. The scattering amplitude of this process is well approximated by two plane-wave scattering amplitudes with different momentum transfers, which interfere and give direct experimental access to the Coulomb phase. This phase (shift) affects the scattering of all charged particles and has thus received significant theoretical attention but was never probed experimentally. We show that a properly defined azimuthal asymmetry, which has no counterpart in plane-wave scattering, allows one to directly measure the Coulomb phase as function of the scattering angle.