Abschlussarbeiten

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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.

In collisions of highly charged ions with electrons or light, ions are usually excited to one of their excited states and, then, may stabilize radiatively under the emission of fluorescence photons. Detailed studies on the emitted photons can help understand the structure and collision dynamics of the ions. When compared with total decay rates, angle-resolved properties such as angular correlation and polarization of emitted photons were found more sensitive to various interactions and effects and, actually, have helped provide new insights into electron-electron and electron-photon interactions in the presence of strong Coulomb fields. For this reason, such kind of studies has attracted considerable interest in both theory and experiment. Until now, however, almost all studies of x-ray angular correlation and polarization were performed for photons emitted from well-isolated energy levels. Little attention was paid so far to photon emissions from two or more overlapping resonances of ions. In this thesis, we develop a novel theoretical formalism to study radiative decay from the overlapping resonances. Special attention is paid to the question of how the splitting of these resonances affect the angular and polarization properties of emitted photons. Calculations are performed based on the density matrix theory and multi-configuration Dirac-Fock method. The obtained results from several case studies show that the photon angular distribution and polarization are strongly affected by the splitting and sequence of the overlapping resonances. Therefore, we suggest that accurate angle-resolved measurements of photon emissions may serve as a tool to identify level splitting and sequence of overlapping resonances in excited highly charged ions, even if they cannot be spectroscopically resolved. When applied to the isotopes with non-zero nuclear spin, moreover, such a tool can also be used to determine hyperfine splitting and associated nuclear parameters.

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

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

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

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

Recent scientific endeavors in the realm of relativistic laser plasma physics benefit from the increase of the on-target intensity (~10^21 W/cm2) generated by a sufficiently powerful laser. With the POLARIS laser system in Jena, Germany, the processes of ion or electron acceleration, laser-based x-ray generation, high intensity laser physics, and laser-based proton radiography can be more adequately understood and improved towards higher particle energies. In order to further fuel the investigation of these interactions, the question remains as to how a state-of-the-art petawatt class laser system can be upgraded. Several paths can be taken to increase the intensity: through improved beam profiles via wavefront aberration corrections, higher energies, and shorter pulse durations. Although diode-pumped solid-state laser systems employing Yb3+-doped active materials, such as POLARIS, can achieve femtosecond pulses with energies in the Joule regime, the focal spot intensity is nevertheless limited by the quality of the laser beam, resulting from strong phase distortions within the beam profile. These phase aberrations are mainly a product of the optical pumping process of the active material, necessary in order to generate population inversion and optical gain. A percentage of the energy from the pump laser is translated into heat through non-radiative transitions, which results in a temperature increase and subsequent refractive index change, based on the dn/dT, photoelastic effect, and expansion of the material. A non-uniform change in the refractive index due to the intensity profile of the pump laser causes the incident wavefront of the seed laser to experience an additional, spatially and temporally varying, phase-shift. This effect, due to the temperature rise in the active material, is referred to as a "thermal lens". An additional type of aberration is also formed when a difference exists in the charge distribution of the dopant ions at different energy levels. Since this is based on the amount of population inversion encouraged by the pump, this spatiotemporal phase-shift effect is called a "population lens". Both of these aberrations affect the phase profile of the incident seed laser with comparable amplitudes, yet occur on difference time scales. Therefore, revealing the full behavior of the pump-induced phase aberrations in diode-pumped active materials requires a spatially and temporally resolved study. The investigation presented in this thesis accomplishes this through high-resolution interference measurements, gain measurements, and a thermal simulation with COMSOL, verified with a thermal imaging camera. A testbed amplifier was constructed afterwards, which can be used to further observe these aberrations within normal amplifier operation and test relay-imaging vs multi-pass amplification, multiple active materials, and additional accessories utilized within amplifier stages, through multiple diagnostics. The combination of the pump source, active material, and amplifier design topics in this thesis grants a well-rounded insight into the field of diode-pumped solid-state laser systems.

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

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

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