Abschlussarbeiten

To be added

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.

Until recently, the nonlinear interaction between light and matter has been restricted to only low photon energies produced by optical lasers. However, about a decade ago, the rise of free-electron laser facilities revolutionized the field of nonlinear light-matter interaction by delivering intense high-energy light pulses. Today, such lasers are used for research in materials science, chemical technology, biophysical science, solid-state physics as well as fundamental research. It is the new experimental possibilities provided by free-electron lasers that motivated the work presented in this thesis. Two-photon ionization process is one of the simplest nonlinear interactions in which absorption of two photons by an atom (or a molecule) leads to promoting one of its bound electrons to continuum. This work presents studies of two-photon ionization of neutral atoms. After a brief historical introduction to the topic of nonlinear light-matter interaction, the density matrix describing the state of an atom and a photoelectron following two-photon ionization is derived. The density matrix contains the complete information about the overall system consisting of a photoion and a photoelectron. In each successive chapter, part of this density matrix is used to obtain characteristic quantities such as total two-photon ionization cross section, photoelectron angular distributions, ion polarization or even degree of polarization of fluorescence photon produced by subsequent decay of the photoion. Physical properties of these quantities are studied and intriguing phenomena, such as elliptical dichroism, polarization transfer as well as relativistic and screening effects are investigated. In one-photon ionization, the photon energy for which the dominant ionization channel vanishes is called the Cooper minimum. This concept is extended to nonlinear ionization of atoms and the effect of the minimum on all above mentioned quantities is studied. In this work it is shown, that the nonlinear Cooper minimum leads to strong variation in practically all observables of the two-photon ionization process. For example, the polarization transfer from the incident to fluorescence photon can be maximized and so can be the elliptical dichroism in photoelectron angular distributions. Furthermore, it is theorized, that detection of the energy position of the nonlinear Cooper minimum could lead to comparison of experimental measurements and theoretical calculations at hitherto unreachable accuracy.

High-harmonic generation is a versatile process, for one thing, useful to explore the structure of atoms or molecules during the generation itself and apart from that a source of bright, short, coherent extreme ultraviolet radiation. Thereby the harmonic radiation can be controlled by the shape of the driving laser with respect to its polarization or frequencies. Recent advances show that Laguerre-Gaussian beams, which carry in addition to their spin also orbital angular momentum, can be utilized for high-harmonic generation. In this thesis, we analyze high-harmonic generation with Laguerre-Gaussian beams in the framework of the strong-field approximation and show that this requires both the interaction of a single atom with the driving laser and the macroscopic superposition of all single atom contributions. We first investigate high-harmonic generation with linearly polarized Laguerre-Gaussian beams. There, we show how the orbital angular momentum of the driving laser is transferred to the generated harmonics. Here, we developed vivid photon diagrams to explain the conservation of orbital angular momentum. We then consider phase matching of the generated radiation in order to increase the conversion efficiency. In particular, we analyze the coherence length at different positions in the generating beam. Furthermore, we investigate high-harmonic generation with a pair of counter-rotating circularly polarized Laguerre-Gaussian beams. Here, we derive selection rules that take account of the conservation of energy, spin and orbital angular momentum. In addition, we show that the orbital angular momentum of the generated harmonics can be precisely controlled by the orbital angular momentum of the driving beam.

Advancements in high peak power laser development have resulted in laser systems capable of accelerating charged particles in a plasma to nearly the speed of light. For a comprehensive understanding and optimization of such interactions towards higher experimental yields, further enhancements in the laser system performance are required, along with a method that enables a direct view into the laser-induced plasma with a high spatial and temporal resolution. The work presented in this thesis details the results of multiple investigations regarding upgrades to the petawatt-class POLARIS laser and the development of a multi-beam ultrashort laser system for probing relativistic laser-plasma interactions at Friedrich Schiller University and Helmholtz Institute in Jena, Germany. As laser pulse intensities are improved worldwide, the spatial, temporal, and temporal intensity contrast profiles of the pulses become increasingly crucial to the experimental performance and future scalability of the laser system. Where possible, an optimization of these parameters should be accomplished using simple, robust methods to avoid large-scale changes to the operational petawatt-class system. To improve the fluence homogeneity of the POLARIS laser pulse, a comprehensive spatio-temporal model of the pump-induced wavefront aberrations was constructed and the results of the verified model were applied to correct the heavily aberrated amplified beam profile in a joule-class multi-pass amplifier through a precise adjustment the pump distribution. Furthermore, the pulse duration post-CPA could be further compressed by a factor of 3 after near field SPM in a highly nonlinear material. In parallel to the spatial and temporal profile improvements, the temporal intensity contrast of the POLARIS laser pulse was enhanced 1000-fold using a plasma mirror. An insight into the complex dynamics of relativistic laser-plasma interactions produced by the enhanced POLARIS laser can be achieved by employing an additional ultrashort laser system as an optical probe. For this purpose, a multi-beam ultrashort optical probing system, seeded by the POLARIS oscillator and pumped by a dedicated Yb:FP15-based CPA system, has been developed and installed within the petawatt-class laser system. The probing setup simultaneously offers two millijoule-level, nearly 100 fs laser pulses, along with a few-cycle laser pulse for high precision optical probing. Here, noncollinear optical parametric amplification (NOPA) is utilized to generate 20 µJ, 230 nm FWHM bandwidth pulses centered at 820 nm. The nonlinear BBO crystal is employed not only as the gain medium, but also as the pulse compressor, delivering near-FTL 11 fs pulses in a setup smaller than 40 cm × 40 cm. The temporal synchronization of the ultrashort probe pulses with the main POLARIS pulse are characterized using a live diagnostic system that monitors several orders of magnitude of delay. With the enhanced petawatt-class laser pulse, now equipped with a few-cycle optical probe, the intricate details of relativistic laser-plasma interactions can be revealed at the POLARIS laser system.

Throughout the current century, compact, high-energy radiation sources have become critically important for many advanced applications in medicine, industry, education, and scientific research. In contrast to conventional radiation sources mainly produced in huge facilities, plasma-based radiation sources with centimetre lengths can provide great flexibility and drive science forward. In this thesis, several plasma wakefield-based undulator schemes have been developed in parallel. First, the guiding of laser beams, including a single Gaussian pulse, Hermite-Gaussian (HG) modes, and Laguerre-Gaussian (LG) modes, is studied through the Schrödinger-like wave equation for a harmonic oscillator with paraxial and quasistatic approximations in a parabolic plasma density channel. If the laser pulse is injected into the plasma channel with a transverse offset or an angle with respect to the propagation axis, it will undergo centroid oscillation. Special conditions are found to control the interesting properties of such oscillation: frequency, amplitude, and polarisation. Second, wakefield excitation driven by the oscillating laser pulse is theoretically and numerically studied in the linear/nonlinear regime. The specific field structure of each scheme is demonstrated. For a short, wide laser pulse, the wakefield provides a linear focusing force near the propagation axis that drives the betatron oscillation of the injected electrons. The extra driving force is introduced by the centroid oscillation of the laser pulse. Surprisingly, the undulator field generated by beating several different HG modes becomes monochromatically sinusoidal when the strength of laser pulses matches a special condition. This is very beneficial for the generation of a narrow radiation spectrum. Third, the dynamics of both a single electron and an electron beam are studied in these generated undulator fields. Generally, an electron undergoes the combined motion of betatron and undulator oscillations. However, the weak betatron oscillation could be totally removed if certain injection conditions for an electron can be satisfied. Further theoretical work on the dynamics of an accelerated electron indicates that there is a resonance between the betatron oscillation of the electrons and centroid oscillation of the laser pulse. This resonance can be used to increase the oscillation amplitude and strength for the electron rapidly within the first several Rayleigh lengths of propagation. While being accelerated in the wakefield, the resonance is broken and results in a semi-steady oscillation with large amplitude and strength, which enables the generation of strong γ-ray radiation. Ultimately, the radiation spectrum from the oscillation of an electron beam is calculated. The proposed schemes are capable of generating an x-ray radiation spectrum with a narrow bandwidth or synchrotron-like x/γ-ray radiation of high energy. The energy and brightness are comparable with currently available conventional radiation sources. It is also demonstrated that these flexible schemes can be tuned to generate radiation carrying well-defined angular momentum.

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.

Imaging using sources in the XUV and X-ray spectral range combines high resolution with longer penetration depth (compared to electron/ion microscopy) and found applications in many areas of science and technology. Coherent diffractive imaging (CDI) techniques, in addition, lift the performance limitation of conventional XUV/X-ray microscopes imposed by image forming optics and enable diffraction limited resolutions. Until recently, CDI techniques were mainly confined to large scale facilities e.g. synchrotrons and X-ray free electron lasers due to unavailability of suitable table-top XUV/X-ray sources. Tabletop sources based on high-order harmonic generation (HHG) nowadays offer high and coherent photon flux which widened the accessibility of CDI techniques. First imaging experiments already showed the potential of HHG-based setups albeit with limited resolution on features much larger than the illuminating wavelength. So far, table-top CDI systems were not able to resolve sub-100 nm features using performance metrics that can qualify these systems for real world applications. The huge progress in scaling the coherent flux of HHG sources driven by high power femtosecond fiber laser systems presented unique opportunities for reaching new regimes in imaging performance. However, experimental issues with power handling and the onset of so-far-unexplored resolution limits for wavelength-scale features were some of the challenges that needed to be addressed. In this work, CDI experiments with the highest resolutions in different modalities using a highnflux fiber laser driven HHG source are presented. In conventional CDI, a record-high resolution of 13 nm is demonstrated together with the possibility of high speed acquisition with sub-30 nm resolution. In a holographic implementation of CDI, features with a half-distance of 23 nm are resolved which are the smallest features to ever be resolved with a table-top XUV/X-ray imaging system. In addition, waveguiding effects are shown to affect image quality and limit the achievable resolution in these wavelength-sized features. Ptychographic imaging of extended samples is also performed using a reliable Rayleigh-like resolution metric and resolving of features as small as 2.5 λ (sub-50 nm) is demonstrated. Together with the significant reduction in measurement times, the imaging results presented push the performance of table-top CDI systems a step closer to that required for real world applications. The scalability of the HHG flux at higher photon energies (soft X-rays) with the power of the driving fiber laser system promises to deliver imaging setups with few nanometer resolutions in the near future. These systems can find applications in material and biological sciences, study of ultrafast dynamics, imaging of semiconductor structures and EUV lithographic mask inspection.

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.