Theses

Die lasergetriebene Beschleunigung von Protonen mittels TNSA hat ein erhebliches Potential, die physikalische Grundlagenforschung um ein weiteres Instrument zur Untersuchung hochenergetischer Wechselwirkungen zu ergänzen. Um die erreichten Protonenenergien und die Stabilität für derartige Anwendungen weiter zu steigern, ist ein grundlegendes Verständnis der innerhalb weniger Pikosekunden ablaufenden Prozesse nötig. Im Rahmen dieser Masterarbeit wurde ein Teil der Diagnostik für die Entstehung solcher Protonenstrahlen untersucht. Dadurch stehen in Zukunft weitere Instrumente zur Charakterisierung von Protonen am POLARIS-System zur Verfügung.

If atoms or molecules are exposed to strong laser fields, various processes can occur after ionization, and the dynamics of these processes depend on the trajectory of the emitted electrons. Both the ionization rates and the electrons trajectory depend strongly on the shape of the laser field. Thus, tailoring strong laser fields on the sub-cycle and sub-femtosecond time scale, the insight and control of the underlying dynamics of these processes has been significantly increased in the last two decades. Here, orthogonal and parallel two-color laser fields represent an effective approach to manipulate the ionization rates and the subsequent electron movement in the laser-dressed continuum. This is achieved by varying the relative phase, ϕrel , between both field components ( ω and 2 ω ). In this thesis orthogonal and parallel two-color laser fields are used to study the ionization and scattering dynamics of noble gases. Further, phases-dependent photoelectron spectra- captured by a velocity map imaging spectrometer, are studied by applying the recently introduced phase-of-the-phase analysis [1]. The measured results are compared with three dimensional semi-classical calculations, which can be performed for arbitrarily polarized laser fields, while taking higher order scattering events into account. These simulation also allows for the separation and investigation of different classes of photoelectrons (e.q. direct and scattered electrons), which alows for analysis of the underlying dynamics. In one vmi measurement in this thesis, an orthogonal two-color laser field ( λω = 800 nm, λ2ω = 400 nm)with an unconventional orientation, i.e. with the polarization of the ionizing laser field perpendicular to the detector surface and the steering field parallel to it, is used. This allows for the investigation of the phase-dependent photoelectron spectra, as the deflections of photoelectrons due to the 2 ω -field are directly mapped onto the detector. The phase dependence of the photoelectron spectra of neon and xenon shows clear phase shifts between scattered and direct electrons. When comparing the phase dependency of neon and xenon, a strong target dependency is observed. Namely xenon show vastly more complex phase dependence then neon. Further investigations of xenon where perfomed using parallel two-color field within the short-wave infrared range ( λω = 1800 nm, λ2ω = 900 nm). To measure electrons with high energy, which are created during ionization with these long wavelengths, a high-energy VMI spectrometer was developed based on the design presented in [2]. Using this device, electron energies up to 320 eV can be detected. The intention of this measurement is to retrieve the ionization time of the photoelectrons contributing to the characteristic fork structure [3] based on the phase dependencies of the contributing photoelectrons. Using these wavelengths, the fork structure can be easily detected and provides a well-suited benchmark for this study. Based on the semi-classical model it is shown that phase-dependent photoelectron signal, which encodes information about the contributing ionization times, is convoluted with the phase dependencies resulting from perturbation of the electron trajectories propagating in the laser-dressed continuum. Independent on the degree of the perturbation this can mislead assignment of the ionization time by up to 80 as.

In atomic physics, nuclei are often described as a point-like charges with an infinite mass that binds the electrons. With more and more precise experimental techniques, however, this approximation is no longer sufficient and it is necessary to develop a better theoretical understanding of the ways atomic nuclei interact with the electron shell. We do observe for example small shifts in the lines of spectra of different isotopes of the same atomic species. In this thesis, we present calculations for these isotope shifts and use them to derive the difference between the nuclear charge radii of two thorium isotopes, ²³²Th and ²²⁹Th as well as ²²⁹Th and the isomeric state ²²⁹mTh. These results are of particular interest for the development of a future nuclear clock and coherent high-energy light sources. Moreover, we discuss precise isotope shift calculations for singly charged barium and compare them with a recent experiment. We motivate the relevance of such studies for the search for physics beyond the Standard Model. Spectral lines, however, do not only shift but also split due to the non-point-like nature of atomic nuclei. From the spectroscopy of this hyperfine splitting, it is possible to extract the multipole moments of the nuclear electromagnetic field. As a part of this thesis, we present the first value of the nuclear magnetic dipole moment of the ²²⁹mTh nuclear isomer that does not rely on previous calculations or measurements. Having extracted several important properties of the ²²⁹Th nucleus and the isomer ²²⁹mTh using atomic theory we invert our view in the second part of this thesis. Namely, we want to use processes in the electron shell to populate the ²²⁹mTh isomeric state. Preparatory to our calculations for the actual excitation of the isomer, we discuss the atomic structure of thorium. Of particular experimental interest is the level structure of singly charged thorium. In a recent study, we show the results of atomic structure calculations that help to interpret measured thorium spectra and can be used to estimate the probability of a nuclear excitation via the electron shell in this system. A deeper and more accurate discussion is performed for the comparably simple triply charged thorium ion. This study helps to test the various approximations necessary to discuss systems with a more complicated electronic structure. Bringing everything together the final publication presented in this thesis proposes an experimental setup to excite the ²²⁹Th nucleus in a controlled way depending on the yet to be found energy of the nuclear isomeric state. This method is currently applied in an experiment at the German National Metrology Institute.

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.

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.

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.

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.

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.

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.

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.

-

Im Rahmen dieser Arbeit wurde ein Anrege-Abfrage-System mit zwei Probepulsen entwickelt. Mithilfe dieses Systems kann ein Plasma, das dem Vorplasma des Polaris-Lasers gleicht, erzeugt und untersucht werden. Das Vorplasma besitzt einen wichtigen Einfluss auf die Effizienz der TNSA Laser- Protonenbeschleunigung. Da das diese Prozesse auf sehr kurzen Zeitskalen von ca. 1 ps stattfinden, muss der Aufbau eine vergleichbare zeitliche Auflösung bieten. Dies ist Elektronisch nicht möglich. Dafür wurde eine rein optisches System zur zeitlichen Separation eines Pulses in mehrere Einzelpuse entwickelt, das Pulse mit einer Pulsdauer von 400 fs und einen zeitlichen Versatz zwischen 0 ps und 333 ps mit einer Genauigkeit von 67ps erzeugt.

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.

TNSA process is an important means to generate energetic ion beams. The understanding of the pre-plasma is an important step towards optimizing the TNSA process. In this work, a complete system to generate and characterize different kinds of plasma was assembled. Generating two pulses and using them to probe the plasma in a single shot increases the utility of such a step since it eliminates the shot to shot variations. Different absorption mechanisms were considered while investigating the plasma and their dominance evaluated in the context of current work. Two devices named temporal separation and spatial separation devices were used to generate the probe pulses. An imaging system to focus, collect and relay the pulses at a large distance was built and optimized to generate near diffraction limited spatial resolution (≈3.5μm). The pulses also give a sufficient temporal resolution with 330 fs pulses to study the hydrodynamic evolution of the plasma. The plasma was created with pulses ranging in intensity from 0.67E16 to 3E16 W/cm^2 with a pulse duration of 120 fs at a central wavelength of 1.03 μm. An intensity as well as a time scan was done to evaluate the plasma based on the scale lengths and plasma electron temperature. Both linear and circular polarization of pump pulse was used to create the plasma. A custom LABVIEW program was used to analyze the phase and generate scale lengths from it by Fourier transform. To gain access to the 3-D information, a cylindrical symmetry was assumed, and Abel inversion was applied on the 2-D chord phase integrals. From this, plasma scale lengths were calculated, and utilizing the single-shot pulses at different time steps, plasma velocity and plasma electron temperatures were calculated. Both the linear and circular polarized pump pulses generated plasma scale lengths in the range of 4-10 μm with an electron temperature of 50-280 eV. This data was also compared with MULTI-fs simulation data and possible reasons of deviations discussed. Dominant absorption mechanisms identified are the Normal and Anomalous Skin Effects under normal incidence. The similarity in the plasma scale lengths and the plasma electron temperature for both polarization implies the absence of vacuum heating and resonance absorption. This is also confirmed by underlying physics of these two absorption processes, which require an electric field component in the direction of the plasma electron gradients.

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.

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.

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.

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.

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.

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.

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.

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.

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.

As an alternative to established laser systems, directly diode-pumped petawatt systems based on Yb3+-doped laser materials are being developed, which can generate pulse energies of > 100 J as well as pulse durations of < 100 fs. The use of narrow-band high-power laser diodes as pump light sources allows an efficient excitation of the laser material, which significantly increases the repetition rate. For the successful application of these laser systems in experiments, however, they must be optimized both spatially and temporally with regard to the required experimental parameters. A maximum focused peak intensity and the highest possible temporal intensity contrast are of particular importance here. In the context of this thesis the possibilities for the spatio-temporal optimization of the pulses of Yb³⁺-based laser systems are investigated. Firstly, the effect of the spectral properties of Yb³⁺-doped materials on the amplified spectrum of laser pulses is investigated and optimized by the development of special spectral transmission filters, which results in an increased bandwidth and thus a reduction of the pulse duration. On the other hand, the spatial optimization of the laser pulse amplification is presented, whereby first the influences of the spatial amplification profile and the pump-induced phase aberrations are investigated. The optimization is then demonstrated by the development of a novel imaging amplifier architecture. Finally, the optimization of the temporal intensity contrast is presented. Newly developed methods have made it possible to completely avoid intensive pre-pulses. A detailed analysis of the generation of spontaneous amplified emissions in high-power laser systems is also derived. For the first time, the analytical model enables a comprehensive conceptual design of high-contrast laser systems with high peak powers.

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.