Peer-Review Publications

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.

We discuss the electron-optical properties of a toroidal magnetic sector spectrometer and its suitablilty for electron-positron pair spectroscopy in relativistic ion-atom collisions in the future HESR storage ring at FAIR. With the simultaneous mapping of electrons and positrons and geometric invariants in the lepton trajectorties this instrument offers a very high efficiency for studies of vector momentum correlation in free-free pair production.

We report on a comparative study of strong-field ionization of alkaline-earth-metal atoms by intense femtosecond laser pulses from near-infrared to midinfrared wavelengths. By collecting the ionization signals only produced within the central portion of the laser focus, the focus volume effect is largely reduced and the saturation intensities for different alkaline-earth-metal atoms are reliably determined, which permits us to directly test the strong-field-ionization theories. We demonstrate that the Perelomov-Popov-Terent'ev model accurately predicts the experimental ionization yields and saturation intensities in general for arbitrary values of the Keldysh parameter, while the Ammosov-Delone-Krainov simulations agree with the experiments for the tunneling-ionization regime and also for the regime when the Keldysh parameter is around 1. Our work presents benchmark data for strong-field ionization of alkaline-earth metals over a broad range of laser parameters and confirms the validity of Keldysh's picture for such atoms.

A concept of a high resolution asymmetric von Hamos X-ray spectrometer for the CRYRING@ESR electron cooler is described and its characteristics obtained by ray-tracing Monte-Carlo simulations are presented. The spectrometer will be used to study the QED e-ects in H-like medium-Z ions by measuring the energies of X-rays from radiative recombination of highly charged ions with cooling electrons, with a ppm precision of energy determination.

Luminosity is a measure of the colliding frequency between beam and target and it is a crucial parameter for the measurement of absolute values, such as reaction cross sections. In this paper, we make use of experimental data from the ESR storage ring to demonstrate that the luminosity can be precisely determined by modelling the measured Rutherford scattering distribution. The obtained results are in good agreement with an independent measurement based on the x-ray normalization method. Our new method provides an alternative way to precisely measure the luminosity in low-energy stored-beam configurations. This can be of great value in particular in dedicated low-energy storage rings where established methods are difficult or impossible to apply.

The exact knowledge of optical material parameters is crucial for laser systems design. Therefore, the work reported herein was dedicated to the determination of an important parameter that is typically not known or only known with insufficient precision: the Kerr coefficient determined by the third order non-linearity, also called the n2-parameter. The optical Kerr effect is responsible for the accumulated nonlinear phase (the B-integral) in high energy laser amplifiers, which often represents a serious limitation. Therefore, the knowledge of n2 is especially required for new types of laser materials. In this paper we report measurements carried out on the widely used optical material Ytterbium-doped Yttrium Aluminium Garnet (Yb:YAG) ceramics. Furthermore, the new Neodymium-doped Calcium Fluoride (Nd:CaF2) crystal was investigated. Specifically, three different approaches have been employed to determine experimentally the nonlinear refractive index of these materials. Thus classical Z-scan technique (at two different wavelengths), the degenerated four waves mixing and the time-resolved digital holography techniques, were compared. These different approaches have permitted the precise measurements of these parameters as well as their dispersion estimations.

A detector setup for registering ion species between the poles of a dipole magnet at CRYRING@ESR has been developed. It is based on a scintillator delivering light via a quartz light guide onto a semiconductor photomultiplier. The detector is capable of operating in a strong magnetic field. It can be swiftly retracted from the exposition area during the beam injection into the ring and repositioned back for the measurement cycle to avoid unnecessary exposition and, thus, to increase the scintillator life time.

In this work, we demonstrate a discrete dispersion scan scheme using a low number of flat windows to vary the dispersion of laser pulses in discrete steps. Monte Carlo simulations indicate that the pulse duration can be retrieved accurately with less than 10 dispersion steps, which we verify experimentally by measuring few-cycle pulses and material dispersion curves at 3 and 10 µm wavelength. This minimal measuring scheme using only five optical components without the need for linear positioners and interferometric alignment can be readily implemented in many wavelength ranges and situations.

Scattering of light on relativistic heavy ion beams is widely used for characterizing and tuning the properties of both the light and the ion beam. Its elastic component-Rayleigh scattering-is investigated in this work for photon energies close to certain electronic transitions because of its potential usage in the Gamma Factory initiative at CERN. The angle-differential cross-section, as well as the degree of polarization of the scattered light are investigated for the cases of 1s - 2p1/2 and 1s - 2p3/2 resonance transitions in H-like lead ions. In order to gauge the validity and uncertainty of frequently used approximations, we compare different methods. In particular, rigorous quantum electrodynamics calculations are compared with the resonant electric-dipole approximation evaluated within the relativistic and nonrelativistic formalisms. For better understanding of the origin of the approximation, the commonly used theoretical approach is explained here in detail. We find that in most cases, the nonrelativistic resonant electric-dipole approximation fails to describe the properties of the scattered light. At the same time, its relativistic variant agrees with the rigorous treatment within a level of 10% to 20%. These findings are essential for the design of an experimental setup exploiting the scattering process, as well as for the determination of the scattered light properties.

We present a study of laser-driven ion acceleration with micrometre and sub-micrometre thick targets, which focuses on the enhancement of the maximum proton energy and the total number of accelerated particles at the PHELIX facility. Using laser pulses with a nanosecond temporal contrast of up to and an intensity of the order of, proton energies up to 93 MeV are achieved. Additionally, the conversion efficiency at incidence angle was increased when changing the laser polarization to p, enabling similar proton energies and particle numbers as in the case of normal incidence and s-polarization, but reducing the debris on the last focusing optic.

A compact, femtosecond-pumped noncollinear optical parametric amplifier (NOPA) is presented with a passive spectral shaping technique, employed to produce a flat-top-like ultrabroadband output spectrum. The NOPA is pumped by a dedicated 2 mJ, 120 fs Yb3+- based CPA system, which generates both the second harmonic pump pulse and white light supercontinuum as the signal pulse. A chirped mirror pair pre-compensates the material GVD within the optical path of the signal pulse to produce a near-FTL pulse duration at the OPA crystal output. By optimizing both the pump/signal cross angle and the pump/signal delay, the 40 cm × 40 cm footprint, single-pass, fs-pumped, direct NOPA (non-NOPCPA) system generates a record 20 μJ, 11 fs pulses at 820 nm central wavelength with a bandwidth of 230 nm FWHM, to be used as an ultrashort optical probe pulse for relativistic laser-plasma interactions at the petawatt-class POLARIS laser system.

We present on THz generation in the two-color gas plasma scheme driven by a high-power, ultrafast fiber laser system. The applied scheme is a promising approach for scaling the THz average power but it has been limited so far by the driving lasers to repetition rates up to 1 kHz. Here, we demonstrate recent results of THz generation operating at a two orders of magnitude higher repetition rate. This results in a unprecedented THz average power of 50 mW. The development of compact, table-top THz sources with high repetition rate and high field strength is crucial for studying nonlinear responses of materials, particle acceleration or faster data acquisition in imaging and spectroscopy.

We present a flexible all-polarization-maintaining (PM) mode-locked ytterbium (Yb):fiber laser based on a nonlinear amplifying loop mirror (NALM). In addition to providing detailed design considerations, we discuss the different operation regimes accessible by this versatile laser architecture and experimentally analyze five representative mode-locking states. These five states were obtained in a 78-MHz configuration at different intracavity group delay dispersion (GDD) values ranging from anomalous (-0.035 ps2) to normal (+0.015 ps2). We put a particular focus on the characterization of the intensity noise as well as the free-running linewidth of the carrier-envelope-offset (CEO) frequency as a function of the different operation regimes. We observe that operation points far from the spontaneous emission peak of Yb (~1030 nm) and close to zero intracavity dispersion can be found, where the influence of pump noise is strongly suppressed. For such an operation point, we show that a CEO linewidth of less than 10-kHz at 1 s integration can be obtained without any active stabilization.

Theoretical g-factor calculations for the first excited 2P3/2 state of boronlike ions in the range Z=10–20 are presented and compared with the previously published values. The first-order interelectronic-interaction contribution is evaluated within the rigorous quantum electrodynamics (QED) approach in the effective screening potential. The second-order contribution is considered within the Breit approximation. The QED and nuclear recoil corrections are also taken into account.

We present a novel, to the best of our knowledge, Hartmann wave front sensor for extreme ultraviolet (EUV) spectral range with a numerical aperture (NA) of 0.15. The sensor has been calibrated using an EUV radiation source based on gas high harmonic generation. The calibration, together with simulation results, shows an accuracy beyond λ/39 root mean square (rms) at λ = 32 nm. The sensor is suitable for wave front measurement in the 10 nm to 45 nm spectral regime. This compact wave front sensor is high-vacuum compatible and designed for in situ operations, allowing wide applications for up-to-date EUV sources or high-NA EUV optics.

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.

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.

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.

We present a Penning-trap-based setup for the study of light-matter interactions in the high-power and/or high-intensity laser regime, such as multi-photon ionization and field ionization. The setup applies ioncloud formation techniques to highly charged ions to the end of specific target preparation, as well as nondestructive detection techniques to identify and quantify the interaction educts and products.

In this work, we present a pilot experiment in the experimental storage ring (ESR) at GSI devoted to impact parameter sensitive studies of inner shell atomic processes for bare and He-like xenon ions (Xe54+, Xe52+) colliding with neutral xenon gas atoms. The projectile and target x-rays have been measured at different observation angles for all impact parameters as well as for the impact parameter range of ∼35 - 70 fm.

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.

A new approach to accurately assess multiphoton ionization is suggested. Vanishing of the dominant ionization channel in nonresonant (direct) multiphoton ionization is predicted for a specific incident photon energy. The exact energy position of such nonlinear Cooper minimum can be accurately measured and requires calculations of the complete electronic spectrum. Measurements of various observables at these photon energies are desirable for further evaluation of theoretical calculations at hitherto unreachable accuracy.

In this paper, we investigate the thermodynamics of dyonic black holes in the presence of Born-Infeld electromagnetic field. We show that electric-magnetic duality reported for dyonic solutions with Maxwell field is omitted in case of Born-Infeld generalization. We also confirm that generalization to nonlinear field provides the possibility of canceling the effects of cosmological constant. This is done for nonlinearity parameter with 10−33 eV2 order of magnitude which is high nonlinearity regime. In addition, we show that for small electric/magnetic charge and high nonlinearity regime, black holes would develop critical behavior and several phases. In contrast, for highly charged case and Maxwell limits (small nonlinearity), black holes have one thermal stable phase. We also find that the pressure of the cold black holes is bounded by some constraints on its volume while hot black holes' pressure has physical behavior for any volume. In addition, we report on possibility of existences of triple point and reentrant of phase transition in thermodynamics of these black holes. Finally, we show that if electric and magnetic charges are identical, the behavior of our solutions would be Maxwell like (independent of nonlinear parameter and field). In other words, nonlinearity of electromagnetic field becomes evident only when these black holes are charged magnetically and electrically different.

A systematic study of nonsequential double ionization (NSDI) of alkaline-earth metal atoms with mid-infrared femtosecond pulses is reported. We find that the measured NSDI yield shows a strong target dependence and it is more suppressed for alkaline-earth metal with higher ionization potential. The observation is attributed to the differences in the recollision induced excitation and ionization cross sections of alkaline-earth metals. This work indicates that NSDI of alkaline-earth metals can be generally understood within recollision picture and sheds light on ultrafast control of electron correlation and dynamics of ionic excited states during NSDI of atoms with complex structures.

This contribution is based on the plenary presentation at the 14th International Conference on Heavy Ion Accelerator Technology (HIAT-2018) in Lanzhou, China. Heavy-ion storage rings offer unparalleled opportunities for precision experiments in the realm of nuclear structure, atomic physics and astrophysics. A brief somewhat biased review of the presently ongoing research programs is given as well as the future projects are outlined. The limited space does not allow for detailed description of individual experiments, which shall - to some extent - be compensated by extended bibliography.