Theses

2019

M. Bilal
High precision many-electron calculations for multiply-charged ions
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: Recent advances in measurements/observations have made it possible to test small and minute fundamental physical eff ects for transition rates and line strengths in many-electron atomic systems with unprecedented accuracies. This thesis provides high-precision calculations of line strengths and lifetimes for diff erent atomic systems where we accurately account for various higher-order eff ects. In all these systems, systematically enlarged multiconfi guration Dirac-Hartree-Fock (MCDHF) wave functions are employed for calculation of the atomic states involved in the transitions to account for the relativistic correlation corrections.
Firstly, the QED sensitive magnetic dipole (M1) line strengths between the fi ne-structure levels of the ground confi gurations in B-, F-, Al- and Cl-like ions are calculated for the four elements argon, iron, molybdenum and tungsten. For these transitions, in addition to relativistic correlation corrections, the QED corrections are evaluated to all orders in αZ utilizing an eff ective potential approach. As a result, our calculations have reached an accuracy of 10−4 for the M1 line strengths. These accurate theoretical predictions provide the prerequisite for a test of QED by lifetime measurements at diff erent frequencies and timescales. This will help to find a reason for the present discrepancies between theory and experiment for B-like Ar and Al-like Fe.
Secondly, the line strength of the 1s 2 2s2p 1 P 1 – 1s 2 2s 2 1 S 0 spin allowed E1 transition in Be-like carbon is calculated. For this highly correlated transition, different correlation models are developed to account for all major electron-electron correlation contributions. The fi nite nuclear mass eff ect is accurately calculated taking into account the energy, wave functions as well as operator contributions. As a result, a reliable theoretical benchmark of E1 line strength with a relative accuracy of 1.5×10−4 is provided to support high precision lifetime measurement at GSI Darmstadt for the 1s 2 2s2p 1 P 1 state in Be-like carbon.
Finally, large-scale calculations are performed for all allowed (E1) and forbidden (M1, E2, M2) transitions among the fi ne structure levels of the 3s 2 3p 5 , 3s3p 6 and 3s 2 3p 4 3d confi gurations for Ni XII. Here, we validate all recently identifi ed tentative experimental lines with one exception. Moreover, we present ab initio lifetimes that are better than previously reported ab initio and semi-empirical values as compared to available experimental data. Thus, we provide reliable predictions in the prospects of future experiments.

R. Beerwerth
Electron Correlation in Relativistic Multiconfiguration Calculations of Isotope Shift Parameters, Hyperfine Coupling Constants and Atomic Processes
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: Electron correlation denotes the corrections to central field approximations applied in Hartree--Fock methods that arise from the electron-electron interaction. As a consequence, wave functions for atomic states are represented as a mixture of different electronic configurations. Corresponding highly correlated multiconfiguration wave functions allow precise computations of atomic parameters such as energy levels, transition rates, isotope shift parameters and hyperfine coupling constants.
In this work, multiconfiguration Dirac–Hartree–Fock computations are utilized to compute precise isotope shift parameters and hyperfine coupling constants for actinium, nobelium and iron. As a prerequisite, extensive computations of the atomic level structure for actinium were performed to assign the computed energies to measured transitions, and as a consequence several unknown levels are predicted. In order to estimate uncertainties of the computed results, systematically enlarged configuration spaces are utilized and the results of several model computations that probe different correlation effects are compared.
Furthermore, electron correlation is crucial to describe higher order processes such as shake transitions that accompany photoionization or Auger processes. These processes are in addition caused by the non-orthogonality of the single electron orbitals obtained in Hartree–Fock computations. The latter can be transformed into electronic correlation by a biorthonormal transformation and we evaluate its application to the efficient computation of Auger transition rates. With this approach, large scale calculations for complex atoms with multiple open shells can be extended to include shake transitions. These transition rates are utilized in Auger cascade models that describe the ionization or excitation of core electrons from atoms or ions into highly excited states and the subsequent decay of these inner-shell holes by the emission of a cascade of Auger electrons.

D. Hoff
Elektronendynamik in fokussierten Einzelzyklenpulsen
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: This work investigates light-driven electron re-scattering from atomic gases and metal nanotips in focused few-cycle laser pulses. In particular, the work concentrates on the investigation of the evolution of the electric ﬁeld of few-cycle pulses during focussing. Electrons emitted from a Tungsten nanotip are used to probe the electric ﬁeld. With this insight the diﬀerences between the noble gas Xenon and nanotips made of Tungsten and Gold can be understood.
To measure such fast processes, ultra-short laser pulses consisting of merely a few optical cycles (<2) are employed. When dealing with pulses as short as this, the relative position between the optical carrier wave and envelope becomes important. This value is called the carrier-envelope phase and is responsible for how the re-scattering takes place. Having control over this phase means being able to control the re-scattering process. As determining this value at the site of interaction is extremely diﬃcult, measurements have been almost exclusively determining the “relative” carrier-envelope phase dependence, i.e. the eﬀects of the change in carrier-envelope phase without an absolute reference. As examination of the phenomena investigated herein requires a knowledge of the “absolute” carrier-envelope phase, a method for determining this value is proposed and implemented. To this end, the phase dependencies of the photo-electron spectra of Xenon are compared to those of atomic Hydrogen, which can in turn be calibrated with ab initio calculations. This insight makes it possible to use the relatively easy determination of the carrier-envelope phase dependence of Xe-spectra as a ruler in other measurements. For instance, further photo electron spectra of Argon and Krypton are shown.
Because the carrier-envelope phase shifts through the focus it is necessary to know these changes in order to understand local interactions. The metal nanotip, being an extremely localized electron emitter, serves splendidly as a tool to quantify the focussing of the electric ﬁeld of few-cycle pulses. For the ﬁrst time the carrier-envelope phase of a wide range of the focus, both on and oﬀ axis, was scanned without complications from volume averaging. Signiﬁcant deviations from the often assumed arcustangent-shaped evolution described previously by Gouy on the optical axis for the monochromatic case were observed. The behaviour is well reproduced with an analytic model calculated by Porras and can be drawn back to the spectral geometry of the laser beam, which can be easily accessed experimentally and used for a coarse estimation of the focusing properties. The insight into the relationship between input beam properties and focussing behaviour allows for better interpretation and design of light-matter interactions in the future.
Here, this technique is utilised to compare the absolute carrier-envelope phase dependence of electron re-scattering at metal nanotips, i.e. Tungsten and Gold, and in Noble gasses. We ﬁnd that the observed shift can be attributed to the shape of the ionization potential of the diﬀerent species and that in case of the nanotips the optical near-ﬁeld due to the geometry of the tip causes an additional phase shift.

P. Luckner
Entwicklung, Aufbau und Charakterisierung eines optischen, hochgenauen Target-Positioniersystems
Bachelor thesis
Ernst-Abbe-Hochschule Jena, Fachbereich Feinwerktechnik (2019)

Abstract: Die Bachelorarbeit wurde am Institut für Optik und Quantenelektronik Jena erstellt. Die Arbeitsgruppe der relativistischen Laserphysik untersucht die Wechselwirkung hochintensiver Laserstrahlung mit Materie. Eines der aktuellen Projekte ist der Aufbau, die Entwicklung und die Anwendung des POLARIS-Lasersystems. POLARIS steht für Petawatt Optical Laser Amplifier for Radiation Intensive ExperimentS und ist das derzeit leistungsstärkste, vollständig dioden-gepumpte Hochleistungslasersystem der Welt mit Pulsspitzenleistungen von bis zu 170 TW. Hintergrund des POLARIS-Projektes ist zum einen die Entwicklung von dioden- gepumpten Lasersystemen und zum anderen die Untersuchung von lasergetriebenen Beschleunigungsmechanismen. Ziel der Bachelorarbeit ist die Entwicklung, der Aufbau und die Charakterisierung eines hochgenauen optischen Target-Positioniersystems für das Hochleistungslasersystem POLARIS. Aufgrund der sehr kleinen Fokusgröße, ist eine hochgenaue Positionierung der Targets notwendig. Das Target soll somit möglichst präzise innerhalb der Rayleigh-Länge des POLARIS Lasers positioniert werden. Hierfür wird das Target mit einem Laser-basierten optischen Aufbau vermessen. Momentan erfolgt das Vermessen der Targets noch manuell durch einen Mitarbeiter, der vor jedem Experiment ca. zwei Stunden für diesen Vorgang benötigt. Um diesen Prozess nicht nur deutlich schneller, sondern auch genauer zu gestalten, soll dieser weitestgehend automatisiert werden. Zunächst erfolgt in Kapitel 2 eine kurze Einführung in die Grundlagen des POLARIS Lasers und es werden verschiedene Methoden der Positionsbestimmung und Bildverarbeitung diskutiert. In Kapitel 3 wird der optische Versuchsaufbau charakterisiert. Hierbei liegt der Schwerpunkt auf der Target-Positionierung und dem Weglängenmesssystem. In Kapitel 4 wird das herausgearbeitete Konzept zum Autofokussystem näher erläutert und aufgetretene Probleme analysiert. Anschließend erfolgt die Umsetzung der Ansätze, wo das Autofokussystem auf seine Genauigkeit und Reproduzierbarkeit überprüft wird. In Kapitel 5 werden schließlich die Ergebnisse diskutiert und ein kurzer Ausblick gegeben. Die Idee ist, dass das Target - nach Eingabe weniger Parameter - vermessen und anschließend nach jedem Schuss positioniert werden soll. Hierzu wird über den selbst entwickelten Auto-Fokus eine Referenzstelle für den Laserfokus auf dem Target scharf gestellt und die zu beschießenden Stellen mit einem konfokal-chromatischen Sensor entlang der optischen Achse vermessen.

Z. Samsonova
Relativistic interaction of ultra-short laser pulses with nanostructured solids
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: Relativistic interaction of ultra-intense laser pulses with nanostructured solids is widely considered to be one of the most promising directions for research in high energy density physics. This thesis investigates the influence of the target morphology on the plasma parameters and produced hard X-ray emission. The study is rather broad and covering a range of emerging applications such as a development of efficient X-ray sources and generation of the extreme states of matter for laboratory astrophysics.
We have performed a sequence of experimental campaigns starting from a benchmark experiment at moderate laser intensities and continuing with measurements at relativistic intensities (Iλ^2 ≥1.3 × 10^18 Wcm−2μm2). A set of fundamental questions regarding the laser energy absorption and morphology dependent plasma dynamics were addressed. Measurements of the bremsstrahlung emission and K-shell emission helped to draw some very important conclusions. First of all, nanowire targets are impractical for the generation of the cold line emission since they demonstrate essentially the same photon flux as the flat targets. However, according to the detected emission from the highly charged ion states (He- and H-like), nanowire morphology enables an effective generation of hot dense plasmas. Spectroscopic analysis of the produced X-ray emission, as the main diagnostic tool, revealed keV temperatures and solid density (≥10^23 cm−3) plasmas. In fact, such plasmas can be generated also with a planar target, however only in a thin top layer since the laser cannot deposit energy deeper. The use of NW arrays, on the other hand, increases the laser energy absorption and the interaction volume, resulting in an effective plasma heating, which does not take place for the flat targets. We have also experimentally observed higher flux and higher energies of the ions accelerated away from the front surface of the target matching with the other observations.
The experimental results were supported by numerical simulations. For the chosencases, we have synthesized X-ray line spectra using the plasma parameters provided by the Particle-in-Cell (PIC) and Hydrodynamic (HD) simulations. A good correlation between the measured and synthetic spectra has been achieved. The plasma dynamics for the case of flat and nanostructured solids is strikingly different. For hot high-density plasmas, the collisional rates (e.g., ionization, excitation) are high and, therefore, radiative cooling of the plasmas may overrun hydrodynamic cooling, as it happens for nanowire targets. This naturally causes a great increase in the X-ray yield.
The response of the flat and nanowire targets was investigated in the interaction with short- and long-wavelength laser pulses (0.4 μm and 3.9 μm), corresponding to completely different regimes of interaction. While ultra-short laser pulses in UV, visible and near-infrared are commonly used in laser-induced plasma studies, femtosecond mid-infrared pulses have not been yet extensively applied. In this thesis, we highlight the potential of such long-wavelength drivers to generate hot and dense plasmas. We demonstrate that this becomes feasible only with nanowire targets.

P. Wustelt
Atome und Moleküle fundamentaler Bedeutung in intensiven Laserfeldern: He, He+ und HeH+
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: This work focuses the control of fundamental single- and two-electron systems using intense, ultra-short laser fields and includes new measurements, novel data evaluation techniques, and interpretation using various theoretical techniques. The measurements were carried out using an ion-beam apparatus that produces a beam of atomic or molecular ions, which is exposed to the controlling laser pulses causing fragmentation and/or ionization. The three-dimensional momenta of these fragments are then detected in coincidence, which allows for reconstruction of the interaction dynamics.

In this thesis, to understand the fundamental timing of the laser-induced electron tunneling, the attoclock method was applied to the helium ion, a single-electron system with twice the charge of hydrogen. This serves to test and refine models of tunneling ionization and the larger intensity required for ionization enables the investigation of the tunneling process close to the ideal case - in the quasi-static tunnel regime. Evaluation of the measured electron-emission angle as a function of the radial momentum for He+ is significantly smaller than for, the typically used, atoms with lower ionization potential. Moreover, using He+ results in a much lower Keldysh parameter, which significantly reduces the importance of nonadiabatic effects that can complicate interpretation. The results are in good agreement with TDSE solutions as well as semiclassical simulations that do not include tunneling times.

Further, double ionization of the helium atom by nearly circularly polarized few-cycle laser pulses was investigated. The dependence of the sequential double ionization on the subcycle shape of the ionizing few-cycle laser field was demonstrated by comparing measured ion momentum distributions with classical Monte Carlo simulations. Simulations based on a purely sequential ionization model show a remarkable good agreement with the experimental observations and reproduce the characteristic 6-peak structure of the measured ion momentum distribution after double ionization with few-cycle laser pulses.

In addition to laser-induced ionization of fundamental atomic systems with strong laser fields, in this work the first experimental investigation of the simplest asymmetric molecule, the helium hydride ion, in strong laser fields was performed. Helium hydride is only stable as an ion and, therefore, an ion beam apparatus is required for its investigation. This study focused on how the asymmetric structure, and the resulting permanent dipole moment of the HeH+, influence laser-induced fragmentation. Both experiment and theory for dissociation, single ionization and double ionization of HeH+ and the isotopologue HeD+ reveal, that for the asymmetric molecule, direct vibrational excitation, with almost no electronic excitation as the initial process, dictates the fragmentation process. The dynamics of this extremely asymmetric molecule contrasts the symmetric molecules and gives new and fundamental insights into the behavior of molecular systems in general.

E. Menz
A Scintillation Particle Detector for Recombination Experiments at CRYRING@ESR
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: The following work describes the implementation of a single-particle detector based on a YAP:Ce scintillation crystal at the CRYRING heavy-ion storage ring at GSI. YAP:Ce is a durable and non-hygroscopic crystal that is bakeable to a certain degree and is thus suitable for installation directly in the ultra-high vacuum of the storage ring. The photons produced by the scintillator are detected by a photomultiplier tube. The detector is located downstream from a dipole magnet and is used to detect reaction products that undergo a change of their charge-to-mass ratio in the preceding straight section of the ring which houses the electron cooler. This positioning facilitates a number of applications for the setup that include the observation of beam losses both from interaction with residual gas atoms and molecules and with electrons in the cooler section. It can also be used for future recombination studies in the cooler section, providing detailed insight into the atomic structure of highly charged ions. The detector has been assembled and installed at CRYRING and was used during two beamtimes in August and November of 2018 to test its functionality and gather ﬁrst experimental data. During these tests a number of issues concerning the detector itself and the signal read-out were identiﬁed and solved and the setup demonstrated its suitability for detecting single ions even at low energies of ∼300 keV. Moreover for the November beamtime a data acquisition system was implemented and tested.

H. Bernhardt
Hochpräzise Röntgenpolarimetrie mit Diamantkristallen
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2019)

Abstract: The dissertation describes the development and application of several diamond crystal x-ray polarizers. The polarizers are based on the channel-cut principle, in which an X-ray beam is diffracted several times under a Bragg angle of 45° and linearly polarized. The diamond crystals were characterized and the effect of defects (dislocations and stacking faults) on X-ray polarimetry were investigated. Since the diamonds were unsuitable for the fabrication of monolithic channel-cut crystals, special quasi-channel cuts (QCC's) out of invar alloy and mirror mounts were developed. With these QCC's up to four diamonds could be adjusted parallel to each other with a precision of sub-μrad. These diamond QCC’s were used in experiments at the European synchrotron in Grenobel, where an unprecedented polarization purity of 1.3 x 10^(-10) was achieved. As a further result, it was proved that the polarization purity is limited by the divergence of the synchrotron and that a better purity can be measured with reduced divergence. Thus, even better polarization purity can be achieved at x-ray sources with lower divergence, e.g. Synchrotron 4th generation and X-ray lasers. This is an important result for the measurement of vacuum birefringence in future. Al in al the dissertation shows that even diamond crystals with dislocation densities in the range of 10^4 to 10^6 cm^-2 are suitable for high-precision X-ray polarimetry and the production of highly pure linear polarized X-ray beams.

2018

B. Arndt
Time-of-flight Measurements at HILITE
Bachelor thesis
Johann Wolfgang Goethe-Universität Frankfurt, Fachbereich Physik (2018)

Abstract: -

S. Fuchs
Optische Kohärenztomographie mit extrem ultravioletter Strahlung
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2018)

Abstract: In this thesis, the concept and the realization of laboratory-based optical coherence tomography in the extreme ultraviolet (XUV) spectral range is presented. XUV coherence tomography (XCT) is a three-dimensional imaging technique with an axial resolution down to a few nanometer. A theoretical XCT model has been developed for the reconstruction of the sample structure, which includes the interaction between the XUV light and the sample. It is valid for absorbing samples illuminated under arbitrary angles of incidence and thus extends a common model of optical coherence tomography (OCT). As the information about the absorption and dispersion of the sample is contained in the XCT model, an additional reconstruction of material properties of the sample will be enabled.
The demonstration of laboratory-based XCT, which before has only been implemented at synchrotron facilities, was a major gaol of this thesis. Using high harmonic generation (HHG) of a femtosecond infrared laser pulse, a broadband laboratory-based XUV source with sufficient photon flux (approximately 0,2 nW/eV over the full bandwidth) in the so-called silicon transmission window between 30 eV − 100 eV was realized. A revised XCT microscope has been designed, constructed and adapted to the new laser-based XUV source, which routinely facilitates XCT measurements in the laboratory. The microscope is a three meter long vacuum beamline consisting of XUV source, focusing mirror, and sample chamber.
A comparison between laser-based and synchrotron-based measurements shows good agreement. With laser-based XCT, an axial resolution of approximately 30 nm has been achieved. This is comparable to the achieved synchrotron-based axial resolution of approximately 20 nm. Accordingly, the axial resolution of XCT is 2-3 orders of magnitude higher than in conventional OCT.
Unlike conventional OCT, the realized XCT setup does not use a beamsplitter for the generation of a reference wave. Instead, the surface of the sample serves as a reference. Therefore, the interferometric stability is intrinsically achieved and simplifies the experimental setup significantly. However, such a setup has the disadvantage that the reconstruction is ambiguous, since autocorrelation artifacts appear. A non-ambiguous reconstruction of the axial structure was so far not possible. In this thesis, a novel one-dimensional phase-retrieval algorithm is presented, which is able to remove the artifacts from the signal and allows a non-ambiguous reconstruction of the structure. Three-dimensional structured silicon-based samples have been investigated and processed with the new algorithm, which is referred to as PR-XCT. With the removal of artifacts and thus the possibility to use XCT on samples, whose inner structure is unknown before the measurement, a further goal of this thesis was achieved.
In fact, during laser-based PR-XCT measurements, an unexpected nanometer-thin layer was found inside the sample, which was not intentionally planned in the production process. The existence of this layer and thus the XCT measurement could only be confirmed by a transmission electron microscope. To this end, a thin slice was cut out of the sample, which was thus destroyed. The resolution of a scanning electron microscope was not high enough to resolve the layer. Later it turned out, that the vacuum chamber was vented for a short amount of time during the production process and a 1-2 nm layer of SiO2 was formed. Hereby, a striking advantage of XUV microscopy becomes apparent. Lighter elements like oxygen produce a high contrast in the XUV albeit they are almost indistinguishable from surrounding light elements like silicon in an electron microscope.
In this work, XCT is realized using optics with low numerical aperture (NA) since the fabrication of high NA optics in the XUV is technically extremely demanding. Therefore, the lateral resolution of the laboratory-based XCT setup is limited to approximately 23 μm. At least, the lateral resolution has been improved by a factor of 10 compared to the synchrotron-based measurements. However, the axial resolution of XCT is still orders of magnitudes better than the lateral resolution. Even with this technical limitation of the current XCT setup, several applications are within reach, e.g., threedimensional investigation of (multilayer-)coatings of optical mirrors or even XUV-mirrors, axial structured devices like solar cells or axial-structured semiconductor devices like graphene-based electronics. In addition, imaging of laterally homogeneous biological membranes might be possible. XCT with high numerical aperture and thus high lateral resolution could even have further applications, e.g., non-destructive three-dimensional imaging of semiconductor devices, lithographic masks, and biological structures. A combination of XCT with lensless imaging techniques like „Coherent Diffraction Imaging“ or Ptychography might be a promising approach to improve the lateral resolution of XCT. Furthermore, the intrinsic time resolution of the HHG source in the range of femto- or even attoseconds may allow time-resolved imaging of ultrafast processes in solids.

M. Reuter
Characterisation of a Laser Wakefield Accelerator with Ultra-Short Probe Pulses
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2018)

Abstract: Within the frame of this thesis, aspects of the acceleration of electrons with high-intensity laser pulses inside an underdense plasma were investigated. The basic acceleration mechanism, which is referred to as laser wakefield acceleration relies on the generation of a plasma wave by an intense laser pulse. Since the plasma wave co-propagates with the laser pulse, its longitudinally alternating electric field moves with a velocity close to the speed of light and electrons trapped in the accelerating phase of the wave can be accelerated to relativistic energies. While basic principles such as the generation of a plasma wave, the injection of electrons into the accelerating phase of the wave and limits to the acceleration process are known, the exact processes occurring during the nonlinear interaction of laser pulse and plasma wave still need to be explored in more detail. The consequence of those nonlinear processes is a drastic change of the electron parameters – e.g. final electron energy, bandwidth and pointing – through slight changes in the initial conditions. In this context, the position in the plasma at which electrons are injected into the plasma wave plays a key role for the maximum achievable electron energy. Therefore, the injection of electrons at a defined position is a possibility to reduce shot-to-shot fluctuations and might make the electron pulses applicable, e.g. as a stable source of secondary radiation for temporally and spatially highly resolving imaging techniques. The investigation of controlled injection of electrons at an electron density transition demonstrated a correlation of electron pulse parameters such as electron energy gain and accelerated charge to the properties of the transition, and thus, might be a promising method to generate custom designed electron pulses. Nevertheless, shot-to-shot fluctuations in the electron parameters were still observed and are most likely caused by the nonlinear evolution of the laser pulse inside the plasma. To further reduce instabilities, deeper insight into these nonlinear processes is required and hence, a method to observe the plasma wave and the laser pulse. Combining an ultra short probe pulse with a highly resolving imaging system as successfully implemented at the institute of Optics and Quantumelectronics in Jena, more light can be shed on these processes, which take place on femtosecond and micrometer scales. With that system, characteristics of the magnetic fields inextricably connected to the acceleration process could be studied in unprecedented detail. This deeper insight allowed to observe signatures of the magnetic field of the driving laser pulse for the first time, which paves the way for the indirect observation of the main laser pulse during the interaction.

A. Peshkov
Interaction of atoms with twisted light
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2018)

Abstract: Twisted photons are particles which carry a nonzero projection of the orbital angular momentum onto their propagation direction. During the last years, the interaction between twisted photons and atoms became an active area of fundamental and applied research. In the present work, we show how the “twistedness” of Bessel and Laguerre-Gauss photons may affect a number of fundamental light-matter interaction processes in comparison with the results for standard plane-wave radiation. In particular, we perform an analysis of the photoionization of hydrogen molecular ions by twisted photons. It is shown that the oscillations in the angular and energy distributions of photoelectrons are affected by the intensity profile of twisted photons. We also investigate the excitation of atoms by these twisted photons. We demonstrate here that the orbital angular momentum of light leads to the alignment or specific magnetic sublevel population of excited atoms. Apart from these studies, we explore the elastic Rayleigh scattering of twisted photons by hydrogenlike ions. Our results indicate that the “twistedness” of incident photons may significantly influence the polarization properties of scattered light.

F. Kröger
Charge State Tailoring for Relativistic Heavy Ion Beams
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2018)

Abstract: In this work charge state distributions of heavy ions have been calculated for the production of effective stripper foils for heavy ion acceleration facilities. In this context, the FAIR facility at GSI and the proposed Gamma Factory at CERN are presented, where the use of partially stripped, relativistic ions will be of special interest for upcoming experiments. To determine the charge state distribution as a function of penetration depth, various programmes have been applied, depending on the respective energy regime. For stripping scenarios in the lower energy regime, the GLOBAL code was applied, that allows to take into account up to twenty-eight projectile electrons for energies up to 2000 MeV/u. Since the GSI/FAIR facility can accelerate even low-charged uranium ions up to 2700 MeV/u, and the Gamma Factory at CERN considers a stripping scenario at 5900 MeV/u, another programme was needed. This is why for the stripping scenarios in the high energy regime, first the well-known CHARGE code was used. However, even though it can operate in the very high energy regime, it only takes into account bare, hydrogen- and heliumlike projectile charge states. To overcome this limitation, the recently developed BREIT code was verified and used for stripping scenarios in the high energy regime. As this code has no built-in treatment of the various charge-changing processes, it needs a multitude of information about the electron capture and loss cross sections as input parameters. Thus, for the calculation of charge state distributions with the BREIT code, cross sections were computed by well-tested theories and codes. The BREIT code together with the codes for the cross section computation were then applied for two studies: first for an exemplification study for the upcoming GSI/FAIR facility to show the practicability of the BREIT code together with the cross section programmes, and then for a study to find optimal stripper foils for the Gamma Factory study group at the CERN facility, in order to efficiently produce Pb⁸⁰⁺ and Pb⁸¹⁺ ions from a Pb⁵⁴⁺ beam before entering the LHC. Furthermore, experimental data of a beam time at ESR at GSI in 2016 was analysed, where a Xe⁵⁴⁺ ion beam of several MeV/u was colliding with a hydrogen gas target. The data allowed the derivation of experimental NRC cross sections, and it was shown that the predictions of the EIKONAL code are in good agreement with these cross sections in an energy range most relevant for upcoming experiments at CRYRING@GSI.

S. Kuschel
Erzeugung dichter Elektronenpulse mit Laser-Plasma-Beschleunigern für QED-Experimente in hohen Feldern
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2018)

Abstract: Quantum electrodynamics (QED) is widely considered to be one of the most accurately tested theories. Nevertheless fundamental processes such as pair production from the vacuum or the motion of the electron in extreme fields have not been measured in the laboratory to date. Their measurement requires a high intensity laser together with a high intensity electron or γ-beam, which can be produced by a high density electron bunch.

A recent development within the last two decades are plasma based accelerators. The high fields that can be sustained by a plasma are used to deliver extremely short and dense electron bunches while shrinking size and costs of the device. Importantly, they are automatically co-located with and synchronized to a high intensity laser pulse, providing an ideal basis for investigating QED in high fields.The availability of generating dense electron bunches brings new QED experiments within reach. However, the quality and stability of laser wake field accelerated (LWFA) electron beams still has to be improved to make these experiments possible. Beyond the tests of QED, the stability and quality of the electron beam is also crucial for highly demanding applications such as LWFA-driven free-electron lasers.

The first part of this thesis is devoted to the LWFA process and its improvements with a particular emphasis on improving the stability of laser plasma accelerators. It is shown that the gas dynamics on a 10 μm scale plays an important role in LWFA, which has not been fully appreciated yet. Density modulations on a 10 μm scale were measured in a gas jet using a few-cycle probe pulse. It is shown that self-injection can be triggered by these modulations. Particle-in-cell (PIC) simulations and analytical modeling confirm the experimental results. A gas cell providing a homogeneous plasma density has been developed in order to reduce self-injection. Using this gas cell, it was possible to suppress self-injection. The experiments show that self-injection was suppressed in the gas cell. Using ionization injection and the gas cell, the beam shape as well as the pointing stability were strongly improved. This finding paves the way towards self-injection free acceleration in a plasma based accelerator. It also establishes a new requirement on the homogenouity of the plasma density – not only for LWFA, but also in a broader context, for example in particle driven plasma wake field acceleration (PWFA).

In the second part of this, the possibility of focusing the ultra-short electron bunch by passive plasma lensing is studied. LWFA-beams typically have a very small source size and a divergence of the order or a few mrad, resulting in a rapid drop in electron beam density during free-space propagation. Many of the envisioned experiments, however, require intense focused electron bunches. Therefore, the concept of passive plasma lensing has been applied to ultra-short LWFA-bunches for the first time. The passive plasma lens effect was demonstrated experimentally by using a second gas target with predefined density. PIC simulations and analytical modeling describe the measured effect. Notably, the observed focusing strength of the passive plasma lens is larger compared to a conventional magnetic quadrupole lens. The analytical model predicts that the focusing strength can be further enhanced by increasing the bunch charge.

2017

A. K. Arunchalam
Investigation of laser-plasma interactions at near-critical densities
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: During the high-intensity laser-plasma experiments conducted at the high-power laser system JETI40 at IOQ, the two qualitatively different laser side-scattering processes have been observed. The side-scattering observed during the first experiment was found to be non-symmetric in nature with respect to the laser’s propagation direction and it was estimated to occur from under-dense to quarter critical plasma densities. The scattering angle was found to gradually decrease, as the laser pulse propagates towards regions of higher densities (i.e. the gas jet centre). For increasing nozzle backing pressures, the scattering was also found to gradually change from upward to downward directions. In this thesis, this side-scattering process is shown to a consequence of the laser propagation in non-uniform plasma, where the scattering angle was found to be oriented along the direction of the plasma gradient. In the second experiment, a symmetric side-scattering process with respect to the laser’s propagation direction was observed from the intense central laser-plasma interaction region. This scattering process was found to originate from a longitudinally narrow laser-plasma interaction region and vary over +-50° with respect to the laser’s transverse direction. It was found to primarily occur in the nearcritical plasma density regime (0.09 n_c to 0.25_nc, where n_c is the plasma critical density). In contrast to the previous experiment, Raman scattering has been shown to be the cause of this symmetric scattering process, where the scattering occurs as the result of the wave vector non-alignment between the main laser pulse and the resulting plasma wave.

P. Pfäfflein
Entwicklung und Aufbau eines Teilchendetektors für erste Experimente am Ionenspeicherring CRYRING
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: This thesis describes the development of a particle counter based on a Cerium activated yttrium aluminium perovskite (YAP:Ce) scintillator. The detector is designed for charge exchange experiments at the ion storage ring CRYRING at the GSI Helmholtz Zentrum für Schwerionenforschung in Darmstadt. It will be used for charge exchange experiments. The suggested detector design was tailored for the requirements set by the desired ultra-high vacuum conditions of up to 1E-12 mbar at CRYRING in combination with a high radiation hardness against ion irradiation. The design was kept as simple as possible, offering an easy exchange of the scintillator (not limited to YAP:Ce) if necessary.
For an estimation of the detector lifetime the radiation hardness was systematically investigated for hydrogen, oxygen and iodine irradiation in the energy regime of 1–10 MeV. The measurement took place at the JULIA tandem accelerator operated by the Institute of Solid State Physics at the University of Jena. As the measurement of detector degradation the light yield was used. Values determined for the critical ﬂuence, defined as ﬂuence at half the initial light yield, varied from 1E15/cm2 in the case of hydrogen down to 1.7E12/cm2 for iodine irradiation.
Prior to the hardness investigation, the used photomultiplier tube (PMT) was tested for temporal drifts of the output signal and whether the signal depends on the position of illumination on the sensitive surface. To the limit of the experimental uncertainties, no such dependencies could be observed. It was concluded that the investigated PMT was well suited for the use in the experiment as well as in the particle counter.

M. Kienel
Power Scaling of Ultrashort Pulses by Spatial and Temporal Coherent Combining
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: Ytterbium-doped solid-state lasers are versatile tools for the generation of intense ultrashort pulses, which are the key for many industrial and scientific applications. The performance requirements on the driving laser have become very demanding. High pulse-peak powers and high average powers are desired at the same time, e.g. to initiate a physical process of interest while providing fast data-acquisition times. Although sophisticated state-of-the-art laser concepts have already demonstrated remarkable performance figures, their working principles hamper the simultaneous delivery of both high peak power and high average power. Coherent combination of pulses provided by an amplifier array constitutes a novel concept for scaling both the average power and the peak power. Although this technique is applicable to any laser concept, it is especially well suited for fibers due to their high single-pass gain and their reproducible, excellent beam quality. As the number of amplifier channels may become too large for the ambitious energy levels being targeted, divided-pulse amplification (DPA) – the coherent combination of a pulse burst into a single pulse – can be applied as another energy-scaling approach, which is the focus of this thesis. In this regard, the energy-scalability of DPA implementations as an extension to well established chirped-pulse amplification (CPA) is analyzed. In a first experiment, high-energy operation is demonstrated using an actively-controlled DPA implementation and challenges that occurred are discussed. Next, in a proof-of-principle experiment, the potential of merging spatial and temporal coherent combining concepts in a power- and energy-scalable architecture has been demonstrated. Furthermore, phase stabilization of actively-controlled temporal and spatio-temporal combination implementations is investigated. Based on the findings, the layout of a state-of-the-art high-power fiber-CPA system is improved and extended by eight parallel main-amplifier channels, in which bursts of up to four pulse replicas are amplified that are eventually stacked into a single pulse. With this technique < 300 fs pulses of 12 mJ pulse energy at 700 W average power have been achieved, which is an order of magnitude improvement in both energy and average power compared to the state-of-the-art at the beginning of this work.

M. Möller
Probing Strong-field Photoionization of Atoms and Diatomic Molecules with Short-wave Infrared Radiation
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: The availability of pico- and femtosecond laser pulses, which can be focused to peak intensities in the range between 10^12 and 10^16 W/cm2, allows the investigation of the interaction between atoms or diatomic molecules with strong laser fields. It has revealed fascinating phenomena such as above-threshold ionization (ATI), high energy above-threshold ionization (HATI), non-sequential ionization (NSDI), high-harmonic generation (HHG) and, most recently, frustrated tunnel ionization (FTI). Today, these characteristic strong-field phenomena are the backbone of the burgeoning field of attosecond science. Derived applications presently mature to standard techniques in the field of ultrafast atomic and molecular dynamics. Examples are HHG as table-top source of coherent extreme ultraviolet radiation with attosecond duration or the application of HATI for the characterization of few-cycle laser pulses. Although experimental and theoretical considerations have shown that using longer laser wavelength is interesting for applications as well as for fundamental aspects, primary due to technological limitations, the vast majority of measurements has been performed at laser wavelengths below 1.0 μm.
In this thesis, an optic parametric amplification laser source of intense femtosecond laser pulses with short-wave infrared (SWIR) and infrared (IR) wavelength is put to operation, characterized and compressed to intense few-cycle pulses. Further, it is applied to investigate strong-field photoionization (SFI) of atoms and diatomic molecules using two different experimental techniques for momentum spectroscopy of laser-induced fragmentation processes.
For SFI of atoms, the velocity map imaging technique is used to measure three-dimensional momentum distributions from strong-field photoionization of Xenon by strong SWIR fields with different pulse duration. Besides observation of the pulse duration dependence of characteristic features, like the low-energy structures, which are particularly pronounced in the SWIR, an eye-catching off-axis low-energy feature, called the “fork”, which appears close to right angle to the polarization axis of the laser, is investigated in detail. The corresponding modeling with an improved version of the semi-classical model, demonstrates that on- and off-axis low-energy features can be traced to rescattering between the laser-driven photoelectron and the remaining ion. They can, thus, be understood on the same footing as HATI, where the electron scatters into high energy states.
SFI of diatomic molecules is investigated using an apparatus for Ion Target Recoil Ion Momentum Spectroscopy (ITRIMS). Besides measuring intensity dependent vector momentum distributions of the protons from SFI of the hydrogen molecular ion, it is shown that momentum conservation can be used to extract the correlated electron momentum from the measured data, although the electron is not detected. The capability of having experimental access to the momenta of all fragments, i.e. two protons and one electron, enables the analysis of correlated electron-nuclear momentum distributions. Together, with a one-dimensional two-level model, this sheds light on correlated electron-nuclear ionization dynamics during SFI of diatomic molecules by SWIR fields.

J. Ullmann
Laserspektroskopie an hochgeladenen Bismutionen zum Test der Quantenelektrodynamik
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: Ths dissertation concerns a test of the theory of quantum electrodynamics in strong fields by laser spectroscopy of the ground state hyperfine splitting of highly charged bismuth ions. The experiment was performed and analyzed at the storage ring ESR at Helmholtzzentrum für Schwerionenforschung in Darmstadt. A systematic study of space charge effects was carried out and the laser wavelength measurement was verified by absorption spectroscopy of iodine. The determination of the ion velocity by an in-situ measurement of the electron cooler voltage reduced the main systematic uncertainty of the previous experiment by over an order of magnitude. This indicated the necessity to establish a permanent high voltage measurement at the electron cooler, which was promoted in this work. The measured wavelengths were combined in a specific difference which deviates significantly from the theoretical predictions. None of the investigated systematics has the magnitude to explain this deviation. Apart from doubts regarding theory, the literature value of the nuclear magnetic moment of Bismuth-209 is indicated as a possible explanation. Follow-up experiments to solve this puzzle are described in the outook.

B. Landgraf
Stimulated Raman backscattering in transient laser generated plasmas with ultra-short seed pulses
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: Stimulated Raman backscattering (SRBS) is a promising concept to create ultra-intense laser pulses. State-of-the-art SRBS experiments find best conditions close to the Langmuir wave-breaking limit, which is one reason, why it cannot be applied at high energy class systems, as focal lengths of hundreds of meters would be necessary. A solution is offered, if the scheme can be transferred to high pump intensities in the strong wave-breaking regime. One of the dominant competitors of SRBS at high intensities is Langmuir wave-breaking, which increases significantly at 10^{14} W/cm^2. Recent studies propose the existence of a time frame in which wave-breaking starts, but is too slow to dephase the electron distribution resulting in efficient amplification with ultra-short seed pulses. In this work SRBS, in transient plasma distributions is demonstrated leading to broadband amplification of up to 80 nm. To understand the temporal dynamics, particle in cell (PIC) simulations are performed. The highest conversion efficiency of up to 1.2 % is found at 5 x 10^{15} W/cm^2, which corresponds to the strong wave-breaking regime. At even higher intensities efficiency drops again, resulting in a lower average efficiency, but conserving its transform limited pulse duration.
After wave-breaking at high intensities a decrease of the pulse energy is observed. To minimize this effect $\mu$m-sized nozzle orifices are manufactured for perfect matching between overlap length and plasma dimension achieving the best conversion efficiency of 2.3 % in this work. To explore static linear density gradients, trapezoid shaped nozzle orifices are sintered by a 3D printer. They provide exceptional stability and an SRBS spectrum of up to 30 nm bandwidth, which should only be accessible in the non-linear case. PIC simulations agree very well with negative density gradients (pump frame), which can partly compensate the pump chirp. Spectral features in the PIC simulation related to the absence of wave-breaking are not observed, possibly pointing to higher dimensional effects. There is no agreement of 1D PIC simulations and positive gradients in two consecutive experiments, which reinforces the thesis, that higher dimensional PIC simulations are necessary. One candidate for two dimensional mechanisms limiting SRBS efficiency is identified as angular chirp whose influence is extrapolated. This potentially allows the conversion efficiency to be increased by a factor of two. By inserting two glass wedges inside the seed setup, it is possible to change the pulse duration by dispersion. A strong correlation between efficiency function and pulse duration is found, where the former oscillates with the plasma period. This important feature has consequences for future experiments trying to explore the coherent wave breaking regime as it is not only necessary to achieve a sufficient growth time, but now also higher plasma densities are necessary for sub-20 fs seed pulses.

Z. Wu
Angular Correlation and Polarization of X-rays Emitted from Highly Charged Ions
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: In collisions of highly charged ions with electrons or light, ions are usually excited to one of their excited states and, then, may stabilize radiatively under the emission of fluorescence photons. Detailed studies on the emitted photons can help understand the structure and collision dynamics of the ions. When compared with total decay rates, angle-resolved properties such as angular correlation and polarization of emitted photons were found more sensitive to various interactions and effects and, actually, have helped provide new insights into electron-electron and electron-photon interactions in the presence of strong Coulomb fields. For this reason, such kind of studies has attracted considerable interest in both theory and experiment.

Until now, however, almost all studies of x-ray angular correlation and polarization were performed for photons emitted from well-isolated energy levels. Little attention was paid so far to photon emissions from two or more overlapping resonances of ions. In this thesis, we develop a novel theoretical formalism to study radiative decay from the overlapping resonances. Special attention is paid to the question of how the splitting of these resonances affect the angular and polarization properties of emitted photons. Calculations are performed based on the density matrix theory and multi-configuration Dirac-Fock method. The obtained results from several case studies show that the photon angular distribution and polarization are strongly affected by the splitting and sequence of the overlapping resonances. Therefore, we suggest that accurate angle-resolved measurements of photon emissions may serve as a tool to identify level splitting and sequence of overlapping resonances in excited highly charged ions, even if they cannot be spectroscopically resolved. When applied to the isotopes with non-zero nuclear spin, moreover, such a tool can also be used to determine hyperfine splitting and associated nuclear parameters.

B. Marx-Glowna
Hochauflösende Röntgenpolarimetrie
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: Polarimetry has a long history with versatile applications in chemistry and pharmacy in the visible spectral range. In the field of X-ray radiation, the interest in high-resolution polarimeters has only increased in recent years. This work is based on a project aiming to observe the vacuum birefringence in an ultra-intense laser field.
The present dissertation describes the development of a precision polarimeter based on multiple reflections at a Bragg angle of 45° in silicon channel-cut crystals. A degree of polarization purity of 10^-10 could be achieved. This improves the best x-ray polarimeters to date by more than two orders of magnitude. In this thesis, experimental and theoretical factors are investigated, which currently limit the degree of polarization purity of precision polarimeters, such as multiple-beam cases, surface treatment of the crystals and source parameters. A new methodology of thin crystals is presented with which the degree of polarization purity can be improved in the future. The high purity of the precision polarimeter allows numerous new applications in nuclear resonant scattering and quantum optics as well as the characterization of X-ray sources of the 3rd and 4th generation.

S. Stock
Auger cascades in resonantly excited neon
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: The Auger cascades following the resonant 1s -> 3p and 1s -> 4p excitation of neutral neon are studied theoretically. In order to accurately predict Auger electron spectra, shake probabilities, ion yields, and the population of final states, the complete cascade of decays from neutral to doubly-ionized neon is simulated bymeans of extensive MCDF calculations. Experimentally known values for the energy levels of neutral, singly and doubly ionized neon are utilized in order to further improve the simulated spectra. The obtained results are compared to experimental findings. For the most part, quite good agreement between theory and experiment is found. However, for the lifetime widths of certain energy levels of Ne+, larger differences between the calculated values and the experiment are found. It is presumed that these discrepancies originate from the approximations that are utilized in the calculations of the Auger amplitudes.

N. Seegert
Signatures of the quantum vacuum in inhomogeneous electromagentic fields
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2017)

Abstract: According to the theory of quantum electrodynamics, zero-point fluctuations of the vacuum manifest themselves through the ubiquitous creation and annihilation of virtual electron-positron pairs. These give rise to classically forbidden nonlinear interactions between strong electromagnetic fields in vacuum, first described by Heisenberg and Euler in 1936. As these interactions only become sizable for large strengths of the involved fields, an experimental verification of purely optical signatures of the quantum vacuum nonlinearity is yet to be achieved.
This thesis deals with various signatures of the quantum vacuum nonlinearity in the presence of inhomogeneous electromagnetic fields, putting emphasis on analytical methods. The proper treatment of inhomogeneities is motivated by the rapid development of high-intensity lasers capable of generating enormous field strengths in their focal spot, making them promising tools for upcoming discovery experiments. In the first part of this work we introduce “quantum reflection” as a new signature of the quantum vacuum nonlinearity, requiring manifestly inhomogeneous pump fields. To this end we start with an analytical expression of the “two-photon” polarization tensor (photon two-point function) in constant pump fields, and develop a formalism to generalize it to inhomogeneous pump fields. In the experimentally relevant weakfield limit our formalism permits a detailed study of various types of inhomogeneities and configurations. Additionally, we also gain insight into the nonperturbative strongfield limit. The investigation of quantum reflection is concluded by giving estimates for the attainable number of quantum reflected photons in experiments consisting of state-of-the-art high-intensity lasers. We then turn to the investigation of photon splitting and merging in inhomogeneous pump fields. For the first time, we compute the “three-photon” polarization tensor for slowly-varying but otherwise arbitrary pump field inhomogeneities in the low-energy limit. With its help we discuss in detail the polarization properties and selection rules governing these two processes. For photon merging we perform an elaborate study of possible experimental set-ups employing parameters of present-day state-of-the-art high-intensity lasers. The combination of polarization shifts, frequency conversion and the emission of the signal into background-free areas establishes photon merging as an ideal candidate to experimentally verify the nonlinear nature of the quantum vacuum in upcoming experiments.

2016

T. Gassner
High Precision X-Ray Spectroscopy of Highly Charged Heavy Ions
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2016)

Abstract: In the present thesis, the advantages of two new and complementary detector concepts for x-ray spectroscopy of highly charged ions over conventional semiconductor detectors have been worked out. These two detectors are the twin crystal spectrometer FOCAL and the metallic magnetic microcalorimeter maXs. Although the maXs microcalorimeter is still under development, first very promising x-ray spectra could be recorded at the ESR storage ring at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt. With the crystal spectrometer FOCAL, which was fully equipped for the first time, a dedicated beam time at the ESR, aiming for the precise determination of the 1s Lamb shift of hydrogen-like gold (Au^78+), could be conducted. The obtained result for the Lyman-a1 transition energy is afflicted with a small statistical uncertainty, however, the encountered systematic effects are still posing a challenge to overcome. In the outlook, it will be discussed in detail how the accuracy of a future measurement could be improved, and in which way both detector concepts could support each other optimally.

I. Tamer
Investigation of Pump-Induced Phase Aberrations for Solid-State Laser Amplifiers
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2016)

Abstract: Recent scientific endeavors in the realm of relativistic laser plasma physics benefit from the increase of the on-target intensity (~10^21 W/cm2) generated by a sufficiently powerful laser. With the POLARIS laser system in Jena, Germany, the processes of ion or electron acceleration, laser-based x-ray generation, high intensity laser physics, and laser-based proton radiography can be more adequately understood and improved towards higher particle energies. In order to further fuel the investigation of these interactions, the question remains as to how a state-of-the-art petawatt class laser system can be upgraded. Several paths can be taken to increase the intensity: through improved beam profiles via wavefront aberration corrections, higher energies, and shorter pulse durations. Although diode-pumped solid-state laser systems employing Yb3+-doped active materials, such as POLARIS, can achieve femtosecond pulses with energies in the Joule regime, the focal spot intensity is nevertheless limited by the quality of the laser beam, resulting from strong phase distortions within the beam profile.

These phase aberrations are mainly a product of the optical pumping process of the active material, necessary in order to generate population inversion and optical gain. A percentage of the energy from the pump laser is translated into heat through non-radiative transitions, which results in a temperature increase and subsequent refractive index change, based on the dn/dT, photoelastic effect, and expansion of the material. A non-uniform change in the refractive index due to the intensity profile of the pump laser causes the incident wavefront of the seed laser to experience an additional, spatially and temporally varying, phase-shift. This effect, due to the temperature rise in the active material, is referred to as a thermal lens". An additional type of aberration is also formed when a difference exists in the charge distribution of the dopant ions at different energy levels. Since this is based on the amount of population inversion encouraged by the pump, this spatiotemporal phase-shift effect is called a population lens". Both of these aberrations affect the phase profile of the incident seed laser with comparable amplitudes, yet occur on difference time scales. Therefore, revealing the full behavior of the pump-induced phase aberrations in diode-pumped active materials requires a spatially and temporally resolved study. The investigation presented in this thesis accomplishes this through high-resolution interference measurements, gain measurements, and a thermal simulation with COMSOL, verified with a thermal imaging camera.

A testbed amplifier was constructed afterwards, which can be used to further observe these aberrations within normal amplifier operation and test relay-imaging vs multi-pass amplification, multiple active materials, and additional accessories utilized within amplifier stages, through multiple diagnostics. The combination of the pump source, active material, and amplifier design topics in this thesis grants a well-rounded insight into the field of diode-pumped solid-state laser systems.

J. Polz
Laser proton acceleration from water micro-droplets and solid hydrogen targets
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2016)

Abstract: *

M. Kiffer
Selektive Breitbandanregung von Ionen in einer Penningfalle
Bachelor thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2016)

Abstract: This thesis examines the Stored waveform inverse Fourier transform SWIFT-procedure. With this procedure one can excite and remove several ions types selectively from a Penning trap. An excitation with SWIFT is performed by an external electric signal. The first part summaries the foundations of the movement and the excitation in an ideal Penning trap.
Afterwards the excitation with SWIFT in a real Penningtrap is analyzed. Here a big discrepancy between the ideal and real trap arise. Therefore a direct selective removal is inefficient. To compensate for this inefficiency the SWIFT-procedure is adapted. The main idea is to use a switch to remove weakly excited ions from the trap. After the excitation the trap voltage is switched to a low value, which reduces the binding energy of the trap.
The last part contains the application of the SWIFT-procedure at the ARTEMIS ion trap. During this application ions where selectively removed from the trap. The obtained findings for the SWIFT-procedure will be applied for an application at the HILITE experiment.

K.-H. Blumenhagen
Experimental studies on polarization correlations in hard x-ray Rayleigh scattering
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2016)

Abstract: This thesis investigates experimentally the elastic scattering of hard x-rays. Combining the novel technologies of a third-generation synchrotron radiation source and a Si(Li) strip detector which acts as a highly efficient x-ray Compton polarimeter allows to measure the linear polarization of the elastically scattered photons for a highly linearly polarized incident beam. Here, such a polarization transfer is considered for the first time in the hard x-ray regime. With a photon energy of 175 keV and gold as scatterer, a highly relativistic regime is chosen where Rayleigh scattering is the only significant elastic scattering contribution. In addition to the polarization of the elastically scattered photons, also the angular distribution is measured. The data are compared to fully relativistic second-order QED calculations. Both observables are well described by these predictions whereas the form factor approximation fails. The simultaneous measurement of angular distribution and polarization allows to identify spurious agreement of the form factor theory in only one observable. At scattering angles around 90°, the assumption that the incident beam is completely linearly polarized is not sufficient to explain the data. The measured linear polarization of the Compton-scattered photons is used to obtain an independent estimate for the incident beam polarization of about 98% which leads to an agreement between experiment and theory at all measured data points. The significant change introduced by this depolarization of 2% indicates a strong sensitivity on the polarization of the incident beam. In the present experiment, this sensitivity limits the precision, but on the other hand, it allows a precise reconstruction of the incident beam polarization when the theory is established. Here, such a reconstruction is performed and the result agrees with the 98% from the Compton polarization, but with a slightly lower uncertainty and with less statistics.

A. Sävert
Few-cycle microscopy of a laser wakefield accelerator
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2016)

Abstract: This thesis describes the development and first application of a novel diagnostic for a laser driven wakefield accelerator. It is termed Few‐Cycle Microscopy (FCM) and consists of a high-resolution imaging system and probe pulses with a duration of a few optical cycles synchronized to a high intensity laser pulse. Using FCM has opened a pristine view into the laser‐plasma interaction and has allowed to record high‐resolution images of the plasma wave in real time. Important stages during the wave’s evolution such as its formation, its breaking and finally the acceleration of electrons in the associated wake fields were observed in the experiment as well as in simulations, allowing for the first time a quantitative comparison between analytical and numerical models and experimental results. Using this diagnostic, the expansion of the wave’s first period, the so‐called ‘bubble’, was identified to be crucial for the injection of electrons into the wave. Furthermore, the shadowgrams taken with FCM in combination with interferograms and backscatter spectra have revealed a new acceleration regime when using hydrogen as the target gas. It was found that in this scheme electron pulses are generated with a higher charge, lower divergence and better pointing stability than with helium gas. The underlying pre‐heating process could be attributed to stimulated Raman scattering, which has been thought up till now to be negligible for short (t < 30 fs) laser pulses. However, as it is shown in this thesis, the interplay of the temporal intensity contrast of the laser pulse 1 ps before the peak of the pulse together with a sufficiently high plasma electron density can provide suitable conditions for this instability to grow, resulting in improved electron pulse parameters.

F. Kurian
Cryogenic Current Comparators for Precise Ion Beam Current Measurements
Doctoral thesis
Johann Wolfgang Goethe-Universität Frankfurt, Fachbereich Physik (2016)

Abstract: The planned Facility for Antiproton and Ion Research (FAIR) at GSI has to cope with a wide range of beam intensities in its high-energy beam transport systems and in the storage rings. To meet the requirements of a non-intercepting intensity measurement down to nA range, it is planned to install a number of Cryogenic Current Comparator (CCC) units at different locations in the FAIR beamlines. In this work, the first CCC system for intensity measurement of heavy ion beams, which was developed at GSI, was re-commissioned and upgraded to be used as a 'GSI - CCC prototype' for extensive optimization and development of an improved CCC for FAIR. After installation of a new SQUID sensor and related electronics, as well as implementation of improved data acquisition components, successful beam current measurements were performed at a SIS18 extraction line. The measured intensity values were compared with those of a Secondary Electron Monitor (SEM). Furthermore, the spill-structure of a slowly extracted beam was measured and analyzed, investigating its improvement due to bunching during the slow-extraction process. Due to the extreme sensitivity of the superconducting sensor, the determined intensity values as well as the adjustment of the system for optimal performance are strongly influenced by the numerous noise sources of the accelerators environment. For this reason, detailed studies of different effects caused by noise have been carried out, which are presented together with proposals to reduce them. Similarly, studies were performed to increase the dynamic range and overcome slew rate limitations, the results of which are illustrated and discussed as well.
By combining the various optimizations and characterizations of the GSI CCC prototype with the experiences made during beam operation, criteria for a more efficient CCC System could be worked out, which are presented in this work. The details of this new design are worked out with respect to the corresponding boundary conditions at FAIR. Larger beam tube diameters, higher radiation resistivity and UHV requirements are of particular importance for the cryostat. At the same time these parameters affect the CCC superconducting magnetic shielding, which again has significant influence on the current resolution of the system. In order to investigate the influence of the geometry of the superconducting magnetic shield on different magnetic field components and to optimize the attenuation, FEM simulations have been performed. Based on the results of these calculations, modifications of the shield geometry for optimum damping behavior are proposed and discussed in the thesis.

M. O. Herdrich
Photonen- und Elektronen-Emission von relativistischen Schwerionen beim Durchgang durch Materie
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2016)

Abstract: The planned FAIR-complex on the site of the GSI Helmholtz-Center for Heavy-Ion Research establishes a broad bandwidth of new experimental opportunities especially in the area of heavy-ion physics. New efforts to not only use its high-energy storagering HESR for proton-antiproton collisions, but also to open it up for experiments with relativistic heavy ions, are of great importance for the regime of relativistic collisions. They extend the options for atomic-physical studies into so far unreached areas of energy. This allows collision experiments of intensive, well-defined ion beams with virtually the full range of both energy and charge states with a variable gas-target. Electrons and photons released in those interactions lead the way to detailed observations and analysis of atomic structures and processes within the collision system. The planning of future experiments requires preferably pragmatic and precise methods of describing the cross-sections of the most important interaction-processes that lead to the emission of electrons and photons in ion-atom-colissions. In the frame of this work a basic overview of relevant interaction processes of collisions in the new energy range made available beyond 500 MeV/u is summarized. Furthermore the theoretical description of their emission characteristics is collected from already existing work, and used to calculate the energy and angle differential cross-sections and polarisation behaviours for a few processes in a wide range of parameters. The data sets are condensed into a database and compared to the results of other work, to test their quality. In the second part of this work the aquired data is used to plan a possible experiment at the HESR. For one, this demonstrates the practical usability of the database for future experiments. But also, the proposed experiment could be conducted in the initial phase of the storage-ring’s operation. The functionality of the facility could be checked and the effect of negative-polarized x-rays emitted by the radiative electron capture process, which - because of insufficient experimental capabilities - was not detectable yet, could be measured for the first time. Beyond the sole optimization of the experiment’s parameters using the database, several simulations were executed. The efficiency of a possible detector was studied, as well as the detectability of the effect itself under the precalculated experimental conditions. Secondly an analysis of the fraction of the radiation background was performed, that looked at the electrons which are also emitted and their interaction products with the experiment setup. The newly gained insight shows that a measurement of the negative polarization effect at the new storage-ring seems possible, but new problems and challenges arise from the fact that the emitted particles carry much higher energies. For example, binary encounter electrons can reach kinetic energies in the MeV-regime, which may lead to the emission of high energy secondary Bremsstrahlung. This has to be considered when designing the new target-chamber and detectors, and it is crucial for the planning of experiments to come.

A. Klenke
Performance scaling of laser amplifiers via coherent combination of ultrashort pulses
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2016)

Abstract: Laser systems emitting ultrashort pulses have become an indispensable tool in science. However, the performance of a single amplifier is limited by a variety of physical effects. Hence, the coherent combination of ultrashort pulses has been investigated as a way to provide a new power-scaling opportunity. This concept can provide a simultaneous increase of the average power, pulse energy and peak power while preserving the beam quality and temporal pulse profile of a single-amplifier system. Theoretical considerations were carried out to investigate the impact of differences between the pulses on the combination process. It could be shown that their impact is small enough to realize laser systems based on coherent combination experimentally with a good combination efficiency. Additionally, the total combination efficiency converges to a fixed values for an increasing number of channels. The coherent combination concept was demonstrated experimentally with a fiber-CPA system comprising four parallel state-of-the-art amplifiers. In these experiments, the highest peak power emitted from a fiber laser system so far (22GW) could be achieved. Finally, for future systems with a large channel count, the compact integration of these channels will play a major role in reducing the footprint and component count and, therefore, the cost. Experimentally, this was demonstrated by employing a multicore fiber together with a compact beam-splitter design.

2015

K. S. Schulze
Methoden und Möglichkeiten der hochpräzisen Röntgenpolarimetrie
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2015)

Abstract: In the visible range, polarimetry is a versatile tool in physics, chemistry and life sciences. Also in the x-ray range, the measurement of polarization changes can be found in a large number of scientific fields. The topic of this work is the analysis of such polarization changes with an extremely high precision. Therefore, two methods of creating very pure linear polarization states are investigated theoretically and experimentally, namely polarimetry with channel-cut crystals and polarimetry based on the Borrmann effect. With these methods, polarization purities reaching ten orders of magnitude can be realized, which enable the precise study of birefringence, dichroism and optical activity. This is demonstrated by different experiments. For instance, a rotation of the polarization plane of less than one arc second was detected during the transmission of an x-ray beam through a sugar solution. Various properties of the polarizers are explained using the dynamical theory of x-ray diffraction. These calculations show that especially at high photon energies the polarization purity is limited by so called umweganregung. Besides the measurement of small polarization changes, the high polarization purity leads also to application in nuclear resonant scattering experiments. Photons that change their polarization during scattering can pass the polarimeter whereas the non-resonantly scattered photons are suppressed by many orders of magnitude. Thus, this method allows a pure measurement of nuclear spectra and lead to the discovery of several quantum optical phenomena in the x-ray range.

R. A. Müller
Radiative recombination in the presence of an intense laser field
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2015)

Abstract: In this thesis we present a theoretical study on the radiative recombination of electrons into the ground state of hydrogen like ions in the presence of an intense external laser field. We employ for the description of this process Heisenberg’s S-matrix theory, where the final bound state of the electron is constructed using first order time dependent perturbation theory. Two different initial electron states are considered. First asymptotically plane-wave-like electrons with a separable Coulomb-Volkov continuum wave function and secondly twisted electrons with a well defined orbital angular momentum constructed from Volkov states. Using this approach we perform detailed calculations for the angle-differential and total cross section of laser assisted radiative recombination considering low-Z ions and laser intensities in the range from IL = 10^11 W/cm^2 to IL = 10^13 W/cm^2. Special emphasis is put on the effects arising due to the laser dressing of the residual bound state. It is seen that the bound state dressing remarkably affects the total cross section and manifests moreover as asymmetries in the angular and energy distribution of the emitted photons. For incident Coulomb-Volkov electrons we study moreover the polarization of the emitted recombination radiation. Here we find that the direction of polarization is rotated depending on the energy of the emitted recombination photons.

T. Jahrsetz
Two-photon processes in highly charged ions
Doctoral thesis
Ruprecht-Karls-Universität, Fakultät für Physik und Astronomie (2015)

Abstract: Two-photon processes are atomic processes in which an atom interacts simultaneously with two photons. Such processes describe a wide range of phenomena, such as two-photon decay and elastic or inelastic scattering of photons. In recent years two-photon processes involving highly charged heavy ions have become an active area of research. Such studies do not only consider the total transition or scattering rates but also their angular and polarization dependence. To support such examinations in this thesis I present a theoretical framework to describe these properties in all two-photon processes with bound initial and final states and involving heavy H-like or He-like ions. I demonstrate how this framework can be used in some detailed studies of different two-photon processes. Specifically a detailed analysis of two-photon decay of H-like and He-like ions in strong external electromagnetic fields shows the importance of considering the effect of such fields for the physics of such systems. Furthermore I studied the elastic Rayleigh as well as inelastic Raman scattering by heavy H-like ions. I found a number of previously unobserved phenomena in the angular and polarization dependence of the scattering cross-sections that do not only allow to study interesting details of the electronic structure of the ion but might also be useful for the measurement of weak physical effects in such systems.

2014

S. Höfer
Zeitaufgelöste Röntgenbeugung an einkristallinem Indiumantimonid
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2014)

Abstract: In this work the structural changes in the semiconductor indiumantimonide (InSb) after the excitation with an ultrashort laser pulse (60fs) are investigated, by using ultrashort x-ray pulses (100 fs). The source of this ultrashort x-ray pulses is a laser-plasma-x-ray-source. In this source an ultrashort and intense laser pulse is focused to a 20 µm thick metal foil (intensity up to 8*10^16 W/cm^2, wavelength 800 nm), by the produced plasma characteristic x-rays and bremsstrahlung are emitted. To characterize the emitted radiation a novel timepix-detector is used, with this it was possible to detect bremstrahlung up to 700 keV.

The typical extinction depth of x-rays is several millimeter and therefore much deeper than the absorption depth of the excitation laser with 100 nm. By using a strong asymmetric Bragg reflection it was possible to adapt the extinction depth from the x-rays to the absorption depth of the optical laser pulse used for excitation. Through this small extinction depth was it possible to measure 2 ps after excitation a strain of 4% in a 4 nm thin layer on the surface. The excitation of the semiconductor is described with different theoretical models, the predicted temporal and spatial evolution of the strain is compared with measured results.

H. Ding
Study of Radiative Electron Capture in Relativistic Ion-Atom Collisions
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2014)

Abstract: Within the frame work of this work, the radiative electron capture (REC) was studied with emphasis on the polarization properties.

First, a fast REC calculator was developed, which facilitates the calculation for REC angular differential cross section and degree of linear polarization for initially bare projectiles with kinetic energy between 5 MeV/u and 400 MeV/u. The interpolations of radiative recombination properties performed by this fast calculator are, on the percent level, in agreement with the exact fully relativistic calculations. With the extension of the underlying RR database to 5 GeV/u, this Calculator can be used for the planning and analysis of measurements at the HESR of the future FAIR facility. For example, at the HESR the cross-over of the REC polarization degree to negative values could be studied. Moreover, when taking into account the shielding effects, by using the successive ionization approximation and neglecting the electron-electron correlation, the working domain of the calculator could be, in principle, extended to initially hydrogen- or helium-like projectiles.

Second, the data of Xe54+ ions colliding with neutral hydrogen gas at 150.5 MeV/u of energy, measured in 2008 using a 2D position sensitive Si(Li) detector, were analyzed with a sophisticated analyzing routine, which yielded results in good agreement with the currently available theory. The K-REC was found strongly polarized at the observation angle near 90° in the laboratory frame, which leads to the potential of tunable polarized hard X-ray source with energy (up to MeV) and degree of linear polarization tunability. The experimental uncertainty arose mainly from the indefiniteness of the quality factor (polarization sensitivity) of the polarimeter, which was estimated using a series of Monte Carlo simulations each requiring a day or more of computation time.

Last but not least, additional experimental work addressing the radiation yield arising from the interaction of high-power lasers with plasmas were performed using plastic scintillators (coupled to PMTs) and a fast multi-channel oscilloscope. It was possible to record the initial radiation burst and also subsequent events due to activation of the experimental setup. The results indicate that the radiation
ux in high-power laser environment is much too high to use large-volume, high-stopping power X-ray detectors like the 2D Si(Li) polarimeter.

G. A. Becker
Untersuchung des Einflusses von Targetmaterial, Foliendicken und Intensitätskontrast bei der Optimierung der Laser-Protonen-Beschleunigung
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2014)

Abstract: The present thesis reports on the results of a laser-driven ion acceleration experiment carried out at the POLARIS laser located in the Helmholtz-Institute Jena. In this experiment, the laser pulses of POLARIS were focused on thin metal foils. The dominant ion or proton acceleration mechanism in such an experiment is Target Normal Sheath Acceleration (TNSA). As a result of this acceleration process, quasi-thermal proton-spectra are generated with a cut-off energy in the range of MeV. The spectra and therefore the maximum proton energy depend on many experimental parameters. At POLARIS, we investigated the influence of foil thickness, material and the temporal intensity contrast on the maximum achievable proton energy. For this, we used copper, silver, gold, aluminium and tantalum foils with different thicknesses from a few 10’s of micrometers down to 100 nanometer. It was found, that the foil material exerts a strong influence on the maximum proton and an optimal foil thickness was found for most of the materials, where the proton energy attains its maximum. Furthermore the influence of pulse contrast improvement was investigated by using a fast Pockels cell and an alternative front-end based on XPW (cross-polarized wave generation). The contrast improvement resulted in a lower optimal foil thickness, but did not result in a higher maximum proton energy.

T. Kiefer
Investigation of the laser-based Target Normal Sheath Acceleration (TNSA) process for high-energy ions — an analytical and numerical study
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2014)

Abstract: The present work is dealing with the theoretical description of laser-driven ion acceleration in the Target Normal Sheath Acceleration (TNSA) process. Various, one-dimensional models describing the laser-heated plasma expansion into vacuum are studied to derive principal relations between the initial conditions of the laser-target interaction --- such as electron parameters, laser and target properties --- and the ion spectra and maximum ion energies which can be observed in experiments. In the first part of this work, two different approaches for the description of the hot electron population are compared when applied to these models. It turns out that a hydrodynamic ansatz for the electron density, which has widely been used in the literature, is contained in the general kinetic treatment of the electrons under the assumption of a particular class of electron energy distributions. Especially, this class contains a step-like electron energy. The impact of a step-like hot electron energy distribution on the ion acceleration process is described in the second part of this thesis. The application of the various adiabatic plasma expansion models to the data from ultrashort-pulse experiments convincingly shows that the analytic results of the expansion model assuming a step-like electron energy distribution reproduce the observed maximum ion energies and the corresponding ion spectra quite well, while this is not the case for the models assuming Maxwellian electron distributions. The third part of this work covers the impact of an initial density gradient at the rear surface of the target. The developed model is able to closely reproduce the experimentally observed relation between the maximum ion energy and the initial target thickness. By using the model prepulse effects in the plasma expansion process can be considered, explaining the experimental observation of an optimal target thickness.

C. Rödel
Synthese von extrem ultravioletter Strahlung an relativistischen Plasmaoberflächen
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2014)

Abstract: In this thesis, high harmonic radiation is studied which is generated by the relativistic interaction of intense laser pulses with dense plasma surfaces. Laser plasma simulations are performed by the author and by colleagues from the University of Dusseldorf for interpreting the experimental results. At first glance, these simulations predict such a high generation efficiency of harmonics from relativistically oscillating mirrors (ROM) that they have been considered as the next generation attosecond light source for the last 15 years. The objective of this thesis is the spectral characterization of the harmonics' efficiency and the ROM process utilizing calibrated XUV diagnostics. The first step, that has been pursued in the thesis work, is the generation of ROM harmonics at the terawatt laser systems JETI and ARCTURUS operated by the University of Jena and the University of Düsseldorf, respectively.
According to the wide-spread belief, the efficient generation of ROM harmonics requires extremely short plasma density gradients which calls for high intensity laser pulses with excellent temporal contrast. For this reason, a plasma mirror system has been installed at both laser systems to improve the pulse contrast by two or three orders of magnitude depending of the target material. In experiments using contrast-enhanced laser pulses, a stable emission of ROM harmonics was observed. However, the highest yield has been measured for the intermediate pulse contrast which results in a plasma scale length L^ROM_P=lambda/5. Surprisingly, the overall efficiency of ROM harmonics decreases for shorter scale lengths < lambda/10 or high contrast, respectively. A strong signal of ROM harmonics could even be measured - indeed unstable - without any contrast improvement. Laser plasma simulations confirm the experimental observation of an optimum plasma scale length L^ROM_P=lambda/5. Two effects have been identified which lead to the reduction of the ROM harmonics' yield for short plasma scale lengths: First, the laser field at the plasma surface is reduced for very short plasma scale lengths. Second, the oscillating electron plasma at the plasma surface is held back by strong electrostatic fields due to the immobile ion background for short plasma density gradients. As a conclusion, the use of an intermediate plasma density gradient for generating ROM harmonics with highest efficiency has to be considered as a paradigm shift in this research field since previous work has called for the highest possible pulse contrast or the shortest plasma scale length, respectively, in order to generate ROM harmonics at all.
Using the optimized plasma scale length, a significant modulation and broadening of the ROM harmonic lines has been observed which is unfavorable for most of the potential applications of ROM harmonics. Laser plasma simulations reproduce the fine structure of the harmonic lines. They further reveal an unequally spaced attosecond pulse train and a positive chirp of the harmonics which is associated with the line broadening. This positive chirp is characteristic for ROM harmonics generated at expanded plasma density profiles and is explained by a temporal denting of the plasma surface due to radiation pressure. It is shown by simulations and experiments that the harmonics' linewidth can be minimized when the harmonics' chirp is compensated by chirped driving laser pulses.
For optimized preplasma conditions, the efficiency of the ROM harmonics was measured to be 10^-4 at 40nm and 10^-6 at 20nm per harmonic order and falls short of expectations nurtured by 1D PIC-simulations and plasma theory. Having a pulse energy in the order of a µJ per harmonic order, ROM harmonics are indeed suited, e. g., for seeding XUV free-electron lasers or coherent diffraction imaging. However, the efficiency of ROM harmonics of 10^-4 at 40nm is comparable to that of high harmonic generation in gaseous media which is state-of-the-art and technologically much less demanding. Considering the present results of the ROM harmonics' efficiency, the high expectations of a highly-efficient, next-generation attosecond source have not been met yet. The reason for the rather low efficiency of ROM harmonics has been investigated by means of 2D simulations. These simulations reveal surface plasma waves which can be generated in addition to the ROM oscillation and lead to a reduced harmonic emisson in the direction of reflection. Surface plasma waves could thus be responsible for the low efficiency of ROM harmonics measured in the experiments. The simulations suggest that shorter pulses with few-cycle pulse duration should be used in the future for a more efficient generation since surface plasma waves can not be built up at these time scales.
A prerequisite for most of the potential applications of ROM harmonics is the generation with a high repetition rate. Using fast-rotating targets and frequency-doubled laser pulses surface harmonics have been generated with the 10-Hz repetition rate of the JETI laser system. Due to the frequency-doubling process the pulse contrast is enhanced by several orders of magnitude such that extremely short plasma density gradients are obtained. Surprisingly, an effect was discovered which was not predicted by theory so far: The high harmonic spectra show a significant enhancement of particular harmonic orders located at twice the maximum plasma frequency 2 omega_P or 2 omega_P +- 2 omega_L. By using targets of different density we were able to tune the enhancement in a certain frequency range in the XUV. Moreover, the efficiency of the amplified harmonics is even higher than the one which is measured for the optimized plasma scale length. Laser plasma simulations confirm then experimental results and reveal the origin of the enhancement: The plasma surface oscillates relativistically with the laser frequency omega_L and the plasma frequency omega_P. The enhanced harmonics are due to a ROM-like oscillation at omega_P. A simple model based on the ROM model can explain the enhanced harmonics as a frequency-mixing process which utilizes the relativistic nonlinearity induced by retardation. This relativistic frequency synthesis at plasma surfaces can be regarded as a new regime of nonlinear optics in the XUV which employs plasma frequencies of dense surface plasmas in the order of several PHz.
At the end of the thesis, two selected applications of ROM harmonics are discussed: The generation of intense attosecond pulses by the ROM process would enable XUV-XUV pump-probe experiments providing attosecond time resolution. However, such experiments would require the determination of the attosecond time structure of ROM harmonics first. An apparatus has been constructed which allows the measurement of an attosecond pulse train by using a nonlinear autocorrelation technique. The second potential application of ROM harmonics is a non-invasive cross-sectional imaging technique which has been developed during the thesis work. This method provides a depth resolution of a few nanometers and employs broad-bandwidth XUV or soft x-ray radiation. ROM harmonics with a nearly continuous spectrum could be a suitable radiation source for this application of technical and industrial relevance.

2013

P. Wustelt
Ionisation atomarer Ionen in intensiven Laserfeldern
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2013)

Abstract: In this work a momentum resolved study of strong field multiple ionization is presented. Atoms exposed to super-intense laser pulses can be ionized to high charge states. In the optical regime, the ionization probability depends highly nonlinear on the field strength. Therefore, for a pulsed field, ionization is concentrated in a narrow intensity and a correspondingly narrow time interval for each ionization step.

Using a fast ion beam, the multi-electron strong-field ionization dynamics of atomic ions is investigated as function of the laser polarization state and the laser intensity. In the experiment, a beam of Ne+ ions is produced in a hollow-cathode discharge duoplasmatron ion source and accelerated to an energy of 8 keV. Intensities of up to about 10^17 W/cm2 are achieved in the interaction region using 10-mJ laser pulses with a pulse duration of 35-fs pulses. The three-dimensional momentum distributions are reconstructed from the time and position information recorded for each ion by a delay-line detector.

In contrast to linear polarization, for elliptically polarized many cycle pulses, the final ion momentum distribution in single ionization provides direct and complete information on the ionizing field strength as well as the ionization time. A deconvolution method was developed, which allows the reconstruction of the electron momenta from the final ion momentum distributions after multiple ionization up to four sequential ionization steps and within a retrieval of the ionization field strength as well as on the release times for subsequent ionization steps. The results are compared to predictions from classical Monte-Carlo simulations based on quasistatic ionization rates. In addition, the subtle effects of the Coulomb interaction on the electron trajectory lead to a tilt in the observed momentum distribution. These effects can be used to study the kinematics and the initial conditions of the electron following tunnel ionization.

R. Riedel
Pulse Metrology Tool and Burst-Mode Laser Amplifier for the Free-Electron Laser in Hamburg
Doctoral thesis
Universität Hamburg, Fakultät für Mathematik, Informatik und Naturwissenschaften (2013)

Abstract: The full scientific potential of high repetition rate free-electron lasers is still not exploited. The attainable resolution of time-resolved experiments is limited by fluctuating temporal pulse properties due to the self-amplified spontaneous emission process. To overcome this limitation, the temporal characterization of free-electron laser pulses was improved by the development of a single-shot temporal pulse metrology tool, based on a solid-state cross-correlation technique. The method is based on probing the optical transmission change of a transparent solid material pumped by a free-electron laser pulse. A comprehensive theoretical model allows the reconstruction of the free-electron laser pulse structure. Pulse duration measurements were performed at the Free-Electron Laser in Hamburg, FLASH, yielding 184 fs at 41.5 nm wavelength and sub-40 fs at 5.5 nm. Online measurements during a running experiment are possible with a residual soft-X-ray transmission of 10-45%. A resolution of sub-10 fs can be attained, provided that sufficiently short optical probe pulses are available.

Achieving the full performance of high repetition rate free-electron lasers, such as FLASH, requires also optical laser systems with a high repetition rate. A novel burst-mode optical parametric chirped-pulse amplifier is being developed for high-resolution pump-probe experiments and seeding of FLASH at its full repetition rate of 100 kHz-1 MHz. In this work, a first prototype was tested, delivering 1.4 mJ pulse energy and a spectral bandwidth supporting sub-7 fs pulse duration at 27.5 kHz intra-burst repetition rate. A passive pump-to-signal synchronization method was developed for long-term stability with sub-7 fs root mean square jitter between pump and signal pulses. The developed amplifier technology is scalable to high average powers for the future generation of kilowatt-pumped ultrashort laser amplifiers.

B. Ecker
Entwicklung kohärenter Lichtquellen im XUV-Regime
Doctoral thesis
Johannes Gutenberg-Universität Mainz, Fachbereich 08 Physik, Mathematik und Informatik (2013)

Abstract: Due to their short wavelength and very narrow spectral bandwidth, plasma-basedrnx-ray lasers present an interesting diagnostic tool for a variety of applications, amongst them spectroscopy, microscopy and EUV-lithography. However, up to date x-ray lasers ﬁnd only limited use in applications, which is related to low pulse energies and an insufﬁcient quality of the x-ray laser beam. Within this context, tremendous efforts have been achieved over the last few years. The simultaneous improvement of pump laser systems as well as pumping mechanisms lead to compact x-ray laser sources operated with up to 100 Hz. To achieve both, higher pulse energies and beam quality, including full spatial coherence, intense theoretical and experimental studies have been performed.rnIn the presented work, a new experimental design has been developed that allowsrnfor pumping two independent x-ray laser targets at the same time. Within the so-rncalled Butterﬂy conﬁguration, the x-ray laser pulse generated by the ﬁrst target is used as a seed pulse. It is injected into the second x-ray laser medium, which acts as an ampliﬁer. This results in the circumvention of undesirable effects, which are related to the ampliﬁcation of spontaneous emission and limit the beam quality of the x-ray laser. For the ﬁrst time, the Butterﬂy setup provides an efﬁcient pumping scheme for both the seed- and the ampliﬁer-target, including travelling wave excitation.rnA ﬁrst experimental campaign has succeeded in demonstrating a seeded and ampliﬁed silver x-ray laser at 13.9 nm and 1 µJ pulse energy. In addition, the measured data reveals the 3 ps lifetime of the population inversion within the silver plasma.rnIn a follow-up experiment, a molybdenum x-ray laser at 18.9 nm was characterized.rnIn addition to the regular pumping scheme used at GSI, a novel pumping strategyrnhas been deployed, which relies on an additional pumping pulse. Seeded x-ray laser operation has been demonstrated in both schemes, resulting in x-ray laser pulses of up to 240 nJ. The peak brilliance of the ampliﬁed x-ray laser was two orders of magnitude larger compared to the original seed pulses, and more than one order of magnitude larger compared to an x-ray laser based on a single target. The experimental setup developed and deployed in this work holds the promise to provide extremely brilliant plasma-based x-ray lasers with full temporal and spatial coherence.rnThus, the presented experimental concept presents a highly interesting alternative to the currently more common approach relying on high-order harmonic pulses as a seed source.rnThe results obtained and discussed in this work are a valuable contribution in the development of an x-ray laser for spectroscopy experiments on highly-charged heavy-ions. These experiments are scheduled at the experimental storage ring at GSI, as well as the high-energy storage ring of the future FAIR facility

C. Hahn
Energie- und polarisationssensitiver Nachweis harter Röntgenstrahlung an Hochintensitätslasern
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2013)

Abstract: The present thesis details the extensive calibration and characterization of two CdTe-based semiconductor detectors equipped with Timepix-class readout chips, and presents first results of polarimetric measurements conducted with these sensors. The readout's Time-over-Threshold mode provides a means to measure the energy deposited in each of the 65k sensor pixels, opening the way for a compact and versatile Compton polarimeter for the high-energy X-ray regime that is commonly encountered at, e.g., laser-generated plasmas.

Since each pixel features its own dedicated set of conversion electronics, an individual calibration of every pixel is mandatory if the full potential of the energy-sensitive detection mode is to be exploited. Exposures to both gamma and X-ray fluorescence radiation were used to generate the necessary data. In addition, a range of MATLAB programs and classes was created to facilitate the lengthy analyses. The final obtainable energy resolution is on the order of 9%, with higher bias voltages providing some potential for improvement while simultaneously increasing the observed detector noise. The fraction of charge-sharing events, i.e. those that encompass multiple pixels, was found to conform with expectations, increasing with the incident photon's energy while, for a given energy, being somewhat lower at higher bias voltages.

Furthermore, a two-detector Compton polarimeter was constructed where two Timepix detectors are arranged around a passive scattering target of approximately 1 cm diameter, covering azimuthal scattering angles that differ by 90°. This setup was first tested at DESY's PETRA III accelerator. The observed stark contrast between radiation scattered parallel and perpendicular to the incident photon electric field vector confirms the setups' fitness for Compton polarimetry in the energy range of some 100 keV. By adding a Tantalum plate collimator to further restrict the scattering angle of the incident photons, the contrast between both detectors was enhanced by an additional 18%. In this configuration, the setup almost reached the contrast theoretically expected for an ideal Compton polarimeter.

R. Geithner
Optimierung eines kryogenen Stromkomparators für den Einsatz als Strahlmonitor
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2013)

Abstract: The non-destructive, non-reactive monitoring of particle beams in the nA range is one of the challenges in the accelerator technology. One way of achieving this objective is the detection of the azimuthal magnetic field created by the particle beam. In the present work a detection system was optimized in terms of noise limited resolution which is based on the principle of the Cryogenic Current Comparator (CCC). In the case of the CCC, the measurement of the magnetic field is realized with a superconducting pick-up coil and a superconductor current sensor (DC-SQUID), which are surrounded by a superconducting shield. It can be shown that the noise-limited resolution of the detector is determined primarily by the low-temperature properties of the pick-up coil and therewith the ferromagnetic core material used in the coil. To this end, extensive temperature and frequency-dependent studies on amorphous and nanocrystalline core materials with respect to their permeability and their noise contribution were carried out. Based on the results obtained an optimized cryogenic current comparator was set up, its noise-limited resolution was significantly reduced compared to previous models already tested.

T. Rathje
Photodissoziation des Wasserstoffmolekülions durch Einzelzyklenlaserpulse
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2013)

Abstract: *

M. O. Herdrich
Ionisationsquerschnitte von Uranionen in Speicherringen
Bachelor thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2013)

Abstract: For many experiments at accelerator facilities high luminosities are necessary, which are only achievable with highest ion beam intensities. Some of the experiments planned for the FAIR project need beam intensities up to a few 10^11 heavy ions in order to observe effects having extremely low reaction cross-sections. Furthermore, applications like the ion-driven fusion require high-intensity beams with beam currents up to 200 Ampere in total. Low charged particles have to be used, because space charge effects limit the maximum expected intensity and phase space volume of the ion beams. However, in typical beam energy regimes above 1 MeV/u, these particles are far from their equilibrium state, resulting in charge changing events during interactions with the residual gas of the accelerator tubes occurring more frequently. In ring accelerators these effects lead to the loss of ions, which for high intensities and high repetition-rates can result in dynamic processes leading to a sudden loss of the whole beam. To minimize the impact of such charge changing effects, a good understanding and characterization of the underlying processes is crucial. The theoretical description of dynamical processes in many electron systems is challenging and can only be done in an approximate way. Therefore an experimental validation of the theoretical predications within a broad parameter range is needed. For this purpose, beam lifetime experiments with two typical uranium charge states, namely U^28+ and U^73+, at three beam energies (30,50 and 150 MeV/u) have been carried out at the ESR storage-ring of the GSI Helmholtz Center for Heavy Ion Research, to determine their ionization cross-section in interactions with several different target gases.

N. Seegert
Quantum Reflection at Strong Magnetic Fields
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2013)

Abstract: The zero-point energy of the quantum electrodynamical vacuum manifests itself through the existence of virtual electron-positron fluctuations. Real electromagnetic fields now have the ability to couple to these fluctuations, and the quantum vacuum hence facilitates a variety of nonlinear interactions between electromagnetic fields. The present work aims at introducing and investigating the effect of quantum reflection as a new means of probing the quantum vacuum nonlinearity. The term quantum reflection is commonly employed to describe the reflection of atoms, quantum mechanically regarded as matter waves, from attractive potentials. This effect can be used to investigate the surface of condensed matter by shining probe particles onto it at grazing incident angles. The reflected particles are then a superposition of both atoms reflected classically at the repulsive surface of the condensed matter as well as atoms subjected to quantum reflection due to the attractive long range potential.

This work now suggests to carry over this mechanism to the purely optical case by employing a highly sensitive pump-probe'' setup. A strong magnetic background field, created by a pump laser, modifies the QED vacuum to act as an effective potential for traversing probe photons. Since the magnetic field exhibits a spatial (as well as temporal) inhomogeneity, we expect the incoming probe photons to be partially reflected from the region of the inhomogeneity. In our analogy the probe photons play the role of the atoms, while the magnetized quantum vacuum plays the role of the attractive potential created by the condensed matter surface. However, probe photons unaffected by the vacuum fluctuations simply pass the entire region of inhomogeneity. This is in contrast to quantum reflection in the atomic case, where the repulsive potential of the condensed matter gives rise to a large background. We therefore end up with a highly sensitive setup possessing an inherent signal-background separation, which should prove to be an important advantage compared to other experiments aiming to probe fluctuation-induced nonlinearities of the quantum vacuum. Owing to the smallness of the nonlinear effects, one of the biggest challenges for such standard experiments is usually given by the separation of photons carrying the optical signatures from such photons which were unaffected by the fluctuations.

First, we lay down the theoretical foundations to describe quantum reflection, and investigate the effect for time-independent magnetic background fields varying in one spatial dimension. We then analyze various background profiles and give estimates for the number of reflected photons employing the design parameters of typical high-intensity laser facilities. The last part deals with a possible extension of the formalism to time-dependent fields.

R. A. Müller
Angular and Polarisation Properties of Bremsstrahlung Radiation in the Short-Wavelength Limit
Bachelor thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2013)

Abstract: In this work two methods for the description of atomic bremsstrahlung are discussed. The density matrix of the system after the scattering process is derived using a Rayleigh expansion of the photon interaction operator and a partial wave expansion of the free dirac electron. These derivations were done following Yerokhin and Surzhykov as well as Tseng and Pratt. From these results a new parametrisation of two observables of electron-atom bremsstrahlung is presented which expresses the angular distribution and the degree of linear polarisation in terms of spherical harmonics. That means once the coefficients are calculated the calculation of the bremsstrahlung properties is orders of magnitude faster than the calculations after Yerokhin and Surzhykov [6]. Also almost real-time calculations are possible when the tabulated coefficients are used. The coefficients yield a couple of symmetry relations and converge very fast against zero which reduces the needed expansion order remarkably. Also they behave very smooth when the other parameters are changed so we can get the coefficients for arbitrary parameter sets from an interpolation on a two dimensional grid. The number of coefficients needed increases with the photon energy but does not exceed 50 for energies up to several hundred keV while for energies less than 100keV for most applications a monadic number of coefficients is enough. Additionally the distance between the nodes on the grid can be increased for higher energies because the coefficients vary less for higher energies so less sets of coefficients are necessary to achieve the same accuracy.