Theses

In this work, we demonstrate how new theoretical concepts enable measurements of the signature of the QED vacuum nonlinearity beyond the background in collision experiments of all-optical high-intensity laser pulses. Using the vacuum emission picture, we develop the method of channel analysis of the signal. Based on these findings, we study two different experimental scenarios and identify discernible signals. In the first case, we consider the collision of two high-intensity laser pulses that differ only in their focus waist sizes. We present a numerical method to identify the regions where the signal dominates the background. Furthermore, we use this to investigate the behavior of the discernible signal, particularly with respect to the effects of the waist size of the probe beam. Of particular note, maximization of the measurable signal photons is not achieved by minimal focusing. This can be explained by the interplay of intensity in the interaction volume and decay behavior of the background in the far field. With the help of an elliptical cross section of the probe pulse, the signal can be further enhanced. Moreover, we show that a discernible signature of vacuum birefringence is achievable in the all-optical regime. In a second setup, elastic and inelastic photon-photon scattering mediated by the nonlinearity of the quantum vacuum is investigated. Based on a collision of four laser pulses of different oscillation frequencies, we observe signals in regions beyond the forward direction of the driving lasers as well as with frequencies beyond the laser frequencies. These features allow us to measure the signal beyond the background. The preceding channel analysis not only helps in the interpretation of the results, but it also allows effective amplification of the signal while maintaining experimental constraints.

Quantum electrodynamics (QED) is the first quantum field theory that describes all phenomena associated with electrically charged particles. Despite its mathematical complexity, it is quite effective in describing and predicting experimental results. With the introduction of lasers, atomic spectroscopy is constantly evolving, contributing to QED testing and continuous improvements in the precision of physical constants determination. Atomic systems offer many opportunities for high-precision QED tests. In the present dissertation, we focus on the magnetic sector of QED: the hyperfine structure and the Zeeman effect in few-electron ions. We present the systematic QED treatment of the electron correlation effects in the ground-state hyperfine structure in lithiumlike ions for the wide range of nuclear charge numbers Z = 7 - 82. The one- and two-photon exchange corrections are evaluated rigorously within the QED formalism. The electron-correlation contributions due to the exchange by three and more photons are accounted for within the Breit approximation employing the recursive perturbation theory. The calculations are performed in the framework of the extended Furry picture, i.e., with the inclusion of the effective local screening potential in the zeroth-order approximation. In comparison to previous theoretical computations, we improve the accuracy of the interelectronic-interaction correction to ground-state hyperfine structure in lithiumlike ions. The g factor of a bound electron is a rigorous tool for verifying the Standard Model and searching for new physics. Recently, a measurement of the g factor for lithiumlike silicon was reported and it disagrees by 1.7! with theoretical prediction [D. A. Glazov et al., Phys. Rev. Lett. 123, 173001 (2019)]. Attempting to resolve this deviation another theoretical value for silicon has been delivered. It results in a disagreement with experimental value [V. A. Yerokhin et al., Phys. Rev. A 102, 022815 (2020)]. We perform large-scale high-precision computations of the interelectronic-interaction and many-electron QED corrections to determine the cause of this disagreement. Similar to the case of hyperfine splitting, we carry out the calculations within the extended Furry picture of QED. And we carefully analyze the final values’ dependence on the binding potential. As a result, the agreement between theory and experiment for the g factor of lithiumlike silicon improves significantly. We also present the most accurate theoretical prediction for lithiumlike calcium too, which perfectly agrees with the experimental value.

Laser plasma accelerators (LPAs) have the potential to revolutionize research fields that rely on relativistic particle beams and secondary radiation sources thanks to their 10-100 GV/m accelerating fields. In the Laser Wakefield Acceleration (LWFA) scheme, a relativistically intense pump or driver laser is focused into a low-Z gas target, ionizing the gas and driving a relativistic, electron plasma wave. Under the proper conditions, such a plasma wave can be used to accelerate electrons to GeV kinetic energies in only centimeters of plasma propagation. As LPAs continue to be tested and refined, nondestructive measurement techniques must be developed to further investigate and understand the dynamic laser-plasma interaction as well as to help ensure reliable operation and measurement of future accelerator facilities based on plasma technology. In this thesis, experiment, theory and simulation are combined to investigate the magnetized, relativistic plasma coinciding with the pump laser at the front of the plasma wave. Experimentally, the Jeti 40 TW laser system was used at the Institute of Optics and Quantum Electronics in Jena, Germany to drive a LWFA in tenuous plasma. The plasma wave was then shadowgraphically imaged using a transverse, few-cycle probe pulse in the visible to near-infrared spectrum and an achromatic microscope using various polarizers and spectral interference filters. The resulting shadowgrams were sorted depending on the properties of the LWFA’s accelerated electron bunches, and subsequently stitched together based on the timing delay between the pump and probe beams. This allowed for the detailed investigation of the laser-plasma interaction’s propagation and evolution as imaged in different polarizations and spectral bands. The resulting data showed two primary signatures unique to the relativistic, magnetized plasma near the pump pulse. Firstly, a significant change in the brightness modulation of the shadowgrams, coinciding with the location of the pump pulse, is seen to have a strong dependence on the pump’s propagation length and the probe’s spectrum. Secondly, after ~1.5 mm of propagation through the plasma, diffraction rings, whose appearance is polarization dependent, appear in front of the plasma wave. A mathematical model using relativistic corrections to the Appleton-Hartree equation was developed to explain these signals. By combining the model with data from 2D PIC simulations using the VSim code, the plasma’s birefringent refractive index distribution was investigated. Furthermore, simulated shadowgrams of a 3D PIC simulation using the EPOCH code were analyzed with respect to the aforementioned signals from magnetized, relativistic plasma near the pump pulse. The results of the study present a compelling description of the pump-plasma interaction. The previously unknown signals arise from relativistic, electron-cyclotron motion originating in the 10s of kilotesla strong magnetic fields of the pump pulse. Advantageously, a VIS-NIR probe is resonant with the cyclotron frequencies at the peak of the pump. With further refinement, the measurement of this phenomenon could allow for the non-invasive experimental visualization of the pump laser’s spatiotemporal energy distribution and evolution during propagation through the plasma.

A good way to test the common theories in atomic physics and astronomy is to determine the degree of polarization of the emitted radiation. The short wavelengths in the X-ray range make a direct determination of the polarization impossible and the use of known interaction mechanisms necessary. A simple mechanism with significant anisotropy with respect to the polarization of the incident photons and a high effective cross section in the low to medium keV energy range is Compton scattering. Taking advantage of position- and energy-sensitive semiconductor detectors, this anisotropy provides the basis for Compton polarimetry. Double sided segmented semiconductor strip detectors have therefore been used for polarization determination for several years. Within the SPARC collaboration of FAIR, the design of a Si(Li) polarimeter has now been further developed. This novel Compton polarimeter with a cooled first preamplifier stage is characterized in detail in this work. Compared to the previous models, it allows for a better energy resolution and a more precise polarization determination, as well as for the first time a precise determination of the degree of polarization and the orientation of the polarization vector at photon energies well below 100 keV. This makes the emission properties of radiative transitions of heavy atoms accessible for polarization spectroscopy for the first time. Until now, the precision of the determination of the degree of polarization was largely limited by the statistics of the investigated data set. Studies based on simulations, which are presented in this thesis, show that for the sizes of experimental data sets available here, statistical uncertainty continues to dominate systematic sources of error. In particular, the improved detector setup allowed for the first time the determination of the degree of polarization for radiative electron capture into the K-shell (KREC) of ions for the previously inaccessible range of photon energies below 70 keV. Close to complete polarization has been demonstrated for this important electron capture process, which is very prominent in collisions of heavy ions and light targets. This demonstration was achieved for the interaction of a Xe54+ ion beam with an H2 gas target and a K-REC photon energy of 56 keV. In the present work, it has thus been shown that radiative electron capture (REC), in particular into the K-shell, is one of the most significant mechanisms of the production of strongly linearly polarized X-rays. In particular, with variation of projectile ion and energy and observation angle, it provides a well-defined source of polarized X-rays with tunable energy and, at the same time, variable polarization properties.

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This work explores the experimental observation of the Breit-Wheeler process, first described by Gregory Breit and John A. Wheeler in 1934 [1], where two photons collide to form an electron positron pair from the quantum vacuum. The specific challenge thereby is the low cross section of a few 10e 29 m2 or 0.1 b combined with the requirement of photon energies in the range of mega electronvolt. Such beams can be provided by particle accelerators, for instance LCLS at SLAC or the European XFEL at DESY. Experiments exploring photon photon collisions with conventional accelerators were done in the past, for example E144 at SLAC in 1997 [2], however the two photon process described by Breit and Wheeler has not yet been observed. Over the last few decades, novel laser driven plasma based particle accelerators (LWFA) made significant progress [3, 4, 5, 6], allowing the production of the required photon beams to study the Breit-Wheeler process at pure laser facilities [7, 8, 9]. The work in hand explores the challenges related to such an experiment specifically at high power laser facilities using the example of Astra Gemini, a multi 100TW dual beam system at the CLF in England. In an experiment, multi 100MeV γ-rays from LWFA electron bremsstrahlung and 1-2 keV x-rays from Germanium M-L shell transition radiation are collided to produce pairs through the Breit-Wheeler process. A detection system to measure those pairs composed of a permanent magnet beam line and shielded single particle detectors is developed and tested within this thesis. The acquired data allows an estimate of the requirements for future experiments to measure the two-photon Breit-Wheeler process.

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Thin aluminum foils (0.4-8µm) have been irradiated by laser pulses at relativistic intensities. Hot electrons, which are periodically accelerated in the laser field at the foil front side, emit coherent optical radiation (COR) at the foil rear side. COR has been investigated spaceand polarization-resolved to study hot electron transport through dense matter. This is important for further progress in laser-driven ion acceleration and fast ignition inertial confinement fusion. The COR source size increased from 1.2 µm to 2.3 µm with foil thickness. This is significantly smaller than the laser focal width of 4 µm and therefore indicates that pinching or filamentation influenced the propagation of the diverging hot electron current. The strong increase of the COR energy at the laser wavelength λ = 1030nm and λ/2 with laser intensity I_L has been explained by considering an intensity dependent hot electron number N and temperature T_h in a coherent transition radiation (CTR) model. Fitting this CTR model to the experimental data allowed to determine Th which increases with I_L but slower than expected. The CTR model fits also showed that about 40% of the hot electrons have been accelerated at the laser frequency 60% at SHG, without significant changes with I_L. Hence, hole boring must have deformed the plasma surface. The COR polarization, measured at SHG, shows strong spatial changes along the COR emission region and varies with I_L, foil thickness and the COR source size at the foil rear surface.

In this thesis the generation of high order harmonics of ultrashort and high intensity laser pulses from solid density plasmas, so called surface high harmonic generation (SHHG), is studied. With SHHG, a compact source of coherent XUV and X-Ray radiation becomes possible. The results are obtained numerically using 1D and 2D Particle-In-Cell (PIC) computer simulations, which are supported by analytical models. This work focusses on two main issues of SHHG to date, pulse isolation and generation efficiency. It is shown that a single attosecond pulse (AP) can be obtained from a few-cycle incident laser pulse by choosing a suitable carrier-envelope phase (CEP), depending on the density and shape of density gradient of the target. An analytical model providing an interpretation of the results obtained from PIC simulations is presented. Spatial isolation of APs can be achieved using the attosecond lighthouse effect, but surface denting is detrimental to the separation of APs. PIC simulations are used to explain an experimental result, where a separation of pulses was not possible due to surface denting. Furthermore it is shown that the angular spectral chirp corresponds to the depth of the surface denting. The efficiency of SHHG can be enhanced greatly by reflecting the beam coming from a first target off a second target. Of major importance for the efficiency is the relative phase between harmonics on the surface of the second target. The relative phase changes even when propagating in free space due to the Gouy phase. To maximize the efficiency gain, a parametric study using PIC simulations has been performed to find the optimal distance between two targets.

The investigation of light-matter interactions is based on the description of the `photoelectric effect' in the early 20th century. The development of the first laser systems, especially of systems with high intensities and/or high photon energies, allowed to study previously unknown, non-linear effects like multiphoton or tunnel ionisation processes, which became subject of theoretical descriptions and experimental studies. Independently, the storage techniques for charged particles (electrons and ions) developed in parallel and different kind of devices, like Paul and Penning traps, had been invented in the 1950s and 1960s to study fundamental parameters of matter (for instance g-factor, mass etc.) with previously unknown accuracy. The HILITE experiment, presented within this thesis, is designed to combine and use for the first time the advantageous properties of target preparation a Penning trap can provide, like ensemble temperature, purity and localizability, in order to investigate laser-ion interactions at high intensities. Particular attention was paid to the compactness of the setup in order to be capable to transport the experiment to different laser facilities and perform experiments on site. In the frame of this thesis, the experimental setup was built and put into operation in terms of its dedicated ion source, ion selection, beam transport, deceleration and capture inside the Penning trap at the GSI Helmholtzzentrum für Schwerionenforschung GmbH. During commissioning, the storage and non-destructive detection of pure ion ensembles within the trap was demonstrated. The individual components have been characterised and their operation was checked. Additionally, a proposal was handed in for the first beamtime at an external laser facility (FLASH at DESY), which was granted and carried out. The interaction between the laser and low charged ions could be verified.

Magnetism, superconductivity, and other macroscopic quantum effects are based on symmetry breaking in solids. Their atomic and molecular structure can be studied using linearly polarized X-rays, where a change of the polarization state of the transmitted beam enables conclusions about electronic anisotropies in the material. Responsible for a change of the polarization state are the optical effects dichroism and birefringence. While X-ray absorption spectroscopy is a well-established method for the detection of dichroism, the effect of birefringence in the vicinity of an X-ray absorption edge is little studied. This work presents the first comprehensive experimental and theoretical investigation of X-ray birefringence and dichroism at the Cu K-absorption edge for two different model substances, CuO and La2CuO4. For this purpose, high-precision X-ray polarimetry, which detects changes of the polarization state with utmost sensitivity, was further developed into a spectroscopic method.

The aim of this work was to develop a new diagnostic method to probe preplasma properties in laser-plasma interaction experiments, using the time-resolved measurement of the laser pulse reflected by the plasma. Its spectral change over time can be attributed to the motion of the critical-density position of the plasma, which can be correlated with the preplasma properties that are present at the beginning of the interaction. 2-D particle-in-cell (PIC) simulations showed a correlation between the blue shift of the spectrum at the temporal beginning of the laser pulse and the expansion velocity of the preplasma, which can be used to derive the corresponding electron temperature. In addition, a correlation between the acceleration of the reflection point into the plasma and the density scale length has been observed. This has also been confirmed by an analytical description of the holeboring velocity and acceleration, which has been developed to include the effect of the preplasma scale length. To verify this method, two experimental campaigns were performed at the PHELIX laser system, while employing different temporal contrasts using so-called plasma mirrors. The experimental observations matched the predictions made by the numerical simulations. By comparing the maximum red shift of the spectrum with the results of the analytical description, the scale length of the preplasma was determined to be (0.18+-0.11) m and (0.83+-0.39) m with and without plasma mirror, respectively. At last, two further experimental campaigns to improve laser-ion acceleration at PHELIX were carried out. First, by increasing the laser absorption during the interaction using a p-polarized laser pulse and second, by increasing the laser intensity. The latter led to the generation of protons with a maximum energy of up to 93 MeV, for a laser intensity in the range of 8e20 W/cm^2, resulting in a new record for the laser system PHELIX.

Discrete-time quantum walks are among the branches of quantum information and computation. They are platforms for developing quantum algorithms for quantum computers. In addition, due to their universal primitive nature, discrete-time quantum walks have been used to simulate other quantum systems and phenomena that are observed in physics and chemistry. To fully utilize the potentials that the discrete-time quantum walks hold in their applications, control over the discrete-time quantum walks and their properties becomes essential. In this dissertation, we propose two models for attaining a high level of control over the discrete-time quantum walks. In the first one, we incorporate a dynamical nature for the unitary operator performing the quantum walks. This enables us to readily control the properties of the walker and produce diverse behaviors for it. We show that with our proposal, the important properties of the discrete-time quantum walks such as variance would indeed improve. To explore the potential of this proposal, we apply it in the simulations of topological phases in condensed matter physics. With our proposal, we can control the simulations and determine the type of topological phenomena that should be simulated. In addition, we confirm simulations of topological phases and boundary states that can be observed in one-, two- and three-dimensional systems. Finally, we report the emergence of exotic phase structures in form of cell-like structures that contain all types of topological phases and boundary states of certain classes. In our second proposal, we take advantage of resources available in quantum mechanics, namely quantum entanglement and entangled qubits. In this proposal, we use entangled qubits in the structure of a quantum walk and show that by tuning the initial entanglement between these qubits and how these qubits are modified through the walk, one is able to produce diverse behaviors for the quantum walk and control its behavior.

Optically generated, narrowband multi-cycle terahertz (MC-THz) radiation has the potential to revolutionize electron acceleration, X-ray free-electron lasers, advanced electron beam diagnostics and related research areas. However, the currently demonstrated THz generation efficiencies are too low to reach the requirements for many of these applications. In this project, a MC-THz generation approach via difference frequency generation (DFG) driven by a laser with a multi-line optical spectrum was investigated with the aim of increasing the conversion efficiency. For this purpose, a home-built, Yb-based laser source with a multi-line optical spectrum was developed. This laser source was amplified to tens-of-millijoule using a regenerative and a four-pass amplifier; it was used to generate MC-THz in magnesiumoxid-doped periodically poled lithium niobate (MgO:PPLN) and rubidium-doped periodically poled potassium titanyl phosphate (Rb:PPKTP). With this laser system, the highest optical-to-THz conversion efficiencies (CE) of 0.49% with a pulse energy of 30 mJ at 0.29 THz, and 0.89% with a pulse energy of 45 mJ at 0.53 THz in MgO:PPLN were achieved. These results compare well with 2-dimensional numerical simulations. In addition, Rb:PPKTP, which has a promising figure-of-merit compared to MgO:PPLN, achieved a CE of 0.16% with a pulse energy of 3 mJ at 0.5 THz. Next, to scale this laser system to tens of millijoule MC-THz output, large aperture crystals for both MgO:PPLN and Rb:PPKTP were investigated using a commercial laser, producing 200 mJ with a pulse duration of 500 fs at 1030 nm; although in this case an older method of optical rectification (OR) was used, achieving less efficiency than the multi-line source. With MgO:PPLN crystals of aperture size 10x15mm2, a CE of 0.29% at 0.35 THz was achieved with a pulse energy of 260 mJ. This is the highest known CE value using OR. In addition, wafer-stacks with alternating crystal-axis orientation of aperture size of 1” for LN and 10x10mm2 for KTP were successfully tested. Two novel experiments were performed with LN wafers: multi-stage wafer-stacks in a serial configuration with multi-output THz radiation and back-reflected seeded MC-THz generation. Both methods improved the efficiency of the MC-THz generation, compared to a single stack. In particular, for the backreflected seeded MC-THz generation, pulse energies of 280 mJ with a CE of 0.29% was achieved; thus demonstrating the potential of seeded MC-THz generation. These achievements are an important step for the realization of next-generation, THz-driven electron accelerators.

The growing numerical development of Coherent Diffraction Imaging (CDI) towards ptychography allows for the first time the separate reconstruction of object and the wavefront illuminating it. This work is dedicated to the investigation of further possibilities resulting from the complex reconstruction of object and illumination. In this thesis, gold structures buried in silicon are reconstructed and examined with regard to their surface morphology in reflection geometry. This completely non-destructive method allows metrology on structures of embedded circuits and otherwise hidden defects. The increasing demand for easily accessible and compact high-performance light sources around the silicon and water window opens the question regarding their suitability for lensless imaging. In the following chapters a method is introduced which allows an almost complete source analysis by means of a single long time exposed diffraction pattern. The knowledge gained in this way allows an improvement of the source with respect to water window CDI and provides insight into dynamic processes within the source. The complex-valued reconstruction of the wavefront allows an insight into the plasma and the ionization states prevailing there. The XUV seed pulse of a seeded Soft-X-Ray laser (SXRL), which passes the pumped plasma and changes its properties with respect to the states in the plasma, is reconstructed ptychographically. Adapted Maxwell-Bloch simulations allow by comparison with the measurement to restore the ionization states during the passage of the seed pulse. Previous experiments showed artifacts during reconstruction, which were directly related to the periodicity of the objects. Simulation of periodic objects of different sizes and with the addition of intentional defects showed a dependence of the reconstruction of the object on the illumination function. Various criteria were derived from this simulation and are presented in this thesis.

The fundamentals of ultrafast optics based on Maxwell’s equations are presented, Gaussian beams, optical pulses and their propagation in dispersive media are introduced. The method of spectral interferometry (SI) is fundamentally introduced and explained in section 3, different possibilities for characterizing the spectral phase are presented. The experimental setup for the characterization and a referencing measurement to well characterized materials is done in section 4. It is also investigated in section 4 which experimental issues can occur, how large their influences on the measurement are and how they can be resolved. The derived methods of spectral phase characterization are used in section 5 to specify the optical components of an amplifier in a CPA laser system. The components of the laser amplifier are categorized and their effects on the spectral phase are compared and discussed. It is then summarized why dispersion measurements are important and how the method of SI can be utilized to select suitable components for a laser amplifier.

In dieser Arbeit werden zunächst die physikalischen Grundlagen vorgestellt, welche für das Verständnis der Simulationen und deren Auswertung notwendig sind. Das Plasma, welches sich vor dem Erreichen der maximalen Laserintensität ausgebreitet hat, kann den TNSA-Prozess und dessen Effektivität beeinflussen. Es ist daher wichtig die genaue Form und den Zustand des Targets zum Zeitpunkt des Eintreffens des Hauptpulses zu charakterisieren. Dafür wird der Computercode MULTI-fs verwendet, welcher noch einmal genauer in Abschnitt 3 diskutiert wird, um die Interaktion eines relativistischen Laserpulses mit einem dünnen Target zu simulieren. Die zeitliche Struktur des in der Simulation verwendeten Laserpulses wurde bei Experimenten am POLARIS-Laser in Jena gemessen. Betrachtet wird dabei die ansteigende Flanke der Laserintensit¨at bis zu dem Zeit-punkt, an dem die Laserintensität 10^17W/cm^2 ¨uberschreitet, für verschiedene Targetdicken und verschiedene maximale Laserintensitäten. Aus diesen Simulationen wird die Verteilung der Elektronendichte gewonnen und parametrisiert, um die Form der Plasmaverteilung systematisch beschreiben zu können. Die aus der Simulation gewonnenen Ergebnisse werden vorgestellt und diskutiert.

This thesis studies extreme nonlinear optical phenomena in highly excited ZnO semiconductor samples. ZnO with a band gap of 3.2 eV, in the near-ultraviolet spectral range, is irradiated with far-off resonance strong light fields in the near (0.8 µm, 1.5 eV) to the far-infrared (10 µm, 0.13 eV). Specifically, the coherent conversion of laser light into high orders of the fundamental frequency, also known as high harmonic generation (HHG) and optically pumped lasing were investigated.

Strong laser fields are a valuable tool to study the electron dynamics in atoms and molecules. A prominent strong-field process is the above-threshold ionization (ATI), where the momentum distributions of emitted photoelectrons encode not only details about the laser-atom interaction, but also properties of the driving laser field. Recent advances in the generation of intense laser beams at mid-infrared wavelengths enable the investigation of ATI in a new parameter range. Moreover, laser beams with a sophisticated spatial structure as a result of an orbital angular momentum (twisted light) have found applications in the strong-field regime. In this dissertation, we theoretically investigate ATI driven by mid-infrared and twisted light beams. We show that not only the temporal but also the spatial dependence of such beams has a pronounced impact on the ionization dynamics due to nondipole interactions. Therefore, we develop a quite general theoretical approach to ATI that incorporates this spatial structure: in order to extend the widely used strong-field approximation (SFA), we construct nondipole Volkov states which describe the photoelectron continuum dressed by the laser field. The resulting nondipole SFA allows the treatment of ATI and other strong-field processes driven by spatially structured laser fields and is not restricted to plane-wave beams. We apply this nondipole SFA to the ATI driven by mid-infrared plane-wave laser beams and show that peak shifts in the photoelectron momentum distributions can be computed in good agreement with experiments. As a second application, we consider the ATI driven by standing light waves, known as high-intensity Kapitza-Dirac effect. Here, we calculate the momentum transfer to photoelectrons for elliptically polarized standing waves and demonstrate that low- and high-energy photoelectrons exhibit markedly different angular distributions, which were not observed previously. Finally, we investigate the ATI of localized atomic targets driven by intense few-cycle Bessel pulses. Based on a local dipole approximation, we demonstrate that the photoelectrons can also be emitted along the propagation direction of the pulse owing to longitudinal electric field components. Moreover, when measured in propagation direction, the ATI spectra depend on both the opening angle and the orbital angular momentum of the Bessel pulse. To conclude, we also discuss the extension of this work towards long pulses, which can be treated within the above nondipole SFA.

In this thesis the spectra of light reflected back from a laser plasma are analyzed with respect to the surface dynamics in the region of the increasing density gradient. In addition, the indentation movement as a function of energy, polarization and foil thickness was investigated.

Quantitative studies of the interaction of atomic and molecular ions with laser radiationat high laser intensities and/or high photon energies are a novel area in the field of laser-matter-interaction. They are facilitated by precise knowledge of the properties of the ions as a target for the laser. This refers to the location, composition, density and shape of the ion cloud as a target, as well as to the capability of characterising the ion target before and after the laser interaction. Ion traps are versatile instruments when it comes to localising ions with a defined particle composition, density and state within a specific and small volume in space. They allow in particular the combination of ions in well-defined quantum states with intense photon fields. The present thesis contains the detailed description of the setup and commissioning of the HILITE (High-Intensity Laser Ion-Trap Experiment) Penning trap, which is dedicated to providing a well-defined cloud of highly charged ions for a number of different experiments with intense lasers. Various experimental procedures are necessary to create such an ion cloud, starting with the production of highly charged ions, their transport, selection, capture, storage, cooling, compression and detection. In the present thesis, the experimental setup is described in detail and the components required for ion target preparation, characterisation and non-destructive ion detection inside the trap are characterised. Special attention is paid to the counting limits of the detection electronics, because knowledge of the exact number of stored ions is essential for the planned experiments. Highly charged ions are produced in an electron-beam ion trap (EBIT), selected with respect to their mass-to-charge ratio, decelerated, and injected into the trap, where they are dynamically captured and stored. For the preparation of a well-defined ion cloud, the initially high energetic ions must be slowed and cooled to an energy of less than 1 meV. This thesis describes the applied methods of active-feedback cooling and resistive cooling and examines their potential cooling efficiencies.