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Theses


2017

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

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

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

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

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

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

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

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

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

2016

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

2015

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

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

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

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

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

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

2014

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

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

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

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

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

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

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

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

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

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

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

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

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

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

2013

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

A. Blinne
Paarproduktion in rotierenden elektrischen Feldern
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2013)

Abstract: Schwinger pair production from the vacuum in rotating time-dependent electric fields is studied using the real-time DHW formalism. This formalism is shortly introduced in general and a specific equation of motion for the purpose of this thesis is derived. Using this equation the time evolution of the Wigner function as well as asymptotic particle distributions neglecting back-reactions on the electric field are determined. Whereas qualitative features can be understood in terms of effective Keldysh parameters, the field rotation leaves characteristic imprints in the momentum distribution that can be interpreted in terms of interference and multiphoton effects.

M. Kienel
Passive Coherent Beam Combining of Temporally Cascaded Pulses
Master thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2013)

Abstract: High-power ultrafast lasers are beneficial for a vast number of applications ranging from fundamental science all the way to industrial scale materials processing. Especially ytterbium-doped fiber lasers have proven to allow for high average power, high pulse energy and remarkable efficiency at the same time. Therefore, they are ideal candidates for most applications. Over the past decades, their output parameters have been scaled by orders of magnitude. However, further power increase is limited by nonlinear and thermal effects, which cause detrimental distortions of the pulses and beams. Promising approaches to overcome these limitations are spatial and temporal coherent beam combination. In this technique, the power and the scaling challenges are distributed among several pulses during the amplification process and afterwards the pulses are combined into a single output pulse. Thereby, the system efficiency is the most crucial parameter, which describes the quality of the pulse combination. Coherent beam combination can be implemented with an (active) or without (passive) a stabilization system.
In this work, simultaneous implementing of both spatial and temporal beam combining has been investigated in a passively stabilized setup. A cascaded Sagnac interferometer-type implementation has been used to generate and combine two pulse trains of up to four pulses each. An ytterbium-doped fiber amplifier was placed inside the Sagnac loop and was used as main amplification stage of a pre-existing chirped-pulse-amplification system. Temporal delays of 7 ns and 14 ns for the temporal division of 2 ns stretched pulses have been realized. Investigations at low pulse energy showed system efficiencies larger than 80% that decreased to >60% for high pulse energy. Based on simulations it was shown that this degradation is due to differences of the accumulated nonlinear phases of the divided pulses. An actively stabilized setup is proposed, which is able to compensate for the differences in nonlinearities.

2012

R. Lötzsch
Bent crystal X-ray optics for the diagnosis and applications of laser-produced plasmas
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2012)

Abstract: The present work deals with x-ray optics based on bent crystals. Such crystals are used for monochromatic imaging and high-resolution x-ray spectroscopy of laser-produced plasmas. In this thesis, the reflection properties of perfect, elastically bent crystals are investigated and it is shown, that the elastic deformations of these crystals depends not only on the depth in the crystal, as hitherto considered, but also on the lateral coordinates.
Beneath these fundamental investigations, the thesis presents a variety of x-ray optics, which demonstrate their application potential. This includes two optics, which are used in the field high repetition rate x-ray sources based on laser-produced plasmas.
Furthermore, a new application of toroidally bent crystals presented. These crystals allow for a scheme to measure crystal rocking curves with both great angular and spatial resolution. With this technique, it is possible to detect lateral variations of strain in the order of 10-5 and with lateral resolution better than 20 µm.
The last part of the thesis presents an experiment from the field of x-ray spectroscopy of laser-produced plasmas. X-ray emission of ions in high electric fields is analyzed. Therefor the emission of these ions has to be recorded at laser intensities of 1020 W/cm² with high dynamics. To this end, a new spectrometer is developed, which allows to detect the transient subtle changes in the spectra caused by electric fields in the order of TV/m, which are created in laser-plasma experiments.

B. Aurand
Untersuchungen zu Mechanismen der Laser-Teilchenbeschleunigung
Doctoral thesis
Johannes Gutenberg-Universität Mainz, Fachbereich 08 Physik, Mathematik und Informatik (2012)

Abstract: Within the last decade, many developments towards higher energies and particle numbers paved the way of particle acceleration performed by high intensity laser systems. Up to now, the process of a field-induced acceleration process (Target-Normal-Sheath-Acceleration (TNSA)) is investigated the most. Acceleration occurs as a consequence of separation of charges on a surface potential. Here, the broad energy spectrum is a problem not yet overcome although many improvements were achieved. Calculations for intensities higher than 10^(20..21) W/cm^2 give hint that Radiation-Pressure-Acceleration (RPA) may lead to a sharper, monoenergetic energy spectrum. Within the framework of this thesis, the investigation of the acceleration mechanism is studied experimentally in the intensity range of 10^19 W/cm^2. Suitable targets were developed and applied for patent. A broad range of parameters was scanned by means of high repetition rates together with an adequate laser system to provide high statistics of several thousands of shots, and the dependence of target material, intensity, laser polarisation and pre plasma-conditions was verified. Comparisons with 2-d numeric simulations lead to a model of the acceleration process which was analyzed by several diagnostic methods, giving clear evidence for a new, not field-induced acceleration process. In addition, a system for a continuous variation of the polarization based on reflective optics was developed in order to overcome the disadvantages of retardation plates, and their practicability of high laser energies can be achieved.

S. Fuchs
Optische Kohärenztomografie mit kurzen Wellenlängen
Thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2012)

Abstract: dummy

2011

C. Hahn
Charakterisierung ortsauflösender Halbleiterdetektoren für harte Röntgenstrahlung
Bachelor thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2011)

Abstract: The present thesis details the properties of position-sensitive detectors suited for the Compton polarimetry of high-energy X-ray radiation, investigating double-sided Ge(i) and Si(Li) strip detectors as well as a CdTe-based sensor equipped with the novel Timepix detection chip.

In the case of the strip detectors, special concern was dedicated to the so-called charge sharing effect, which denotes an incident photon's charge cloud being distributed across several segments. This obviously hampers the determination of the actual interaction position, a crucial parameter for the Compton-polarimetric analysis. A set of sophisticated software routines to recover such events was developed. Using this modified analysis, the number of multiplicity-1 events reported by the Si(Li) detector was increased by about 30%, while the Ge(i) system, having a much smaller strip width, saw an increase by up to a factor of 30 at energies above 200 keV. Consequently, the number of events usable for the Compton analysis, which has to be restricted to exposures with exactly two discernible interactions, grew in a similar fashion, such that, for the Ge(i) sensor, the computed degree of polarization deviated by as much as 9% from previously obtained values. The wide-stripped Si(Li) detector, on the other hand, proved to be much less susceptible to distortions induced through charge splitting.

In addition, Timepix acquisitions made during an electron acceleration experiment conducted at the JETI laser system were evaluated. To avoid saturation of the sensor, extensive shielding and indirect exposure, utilizing a plastic body for scattering, were necessary. While the employed Time-over-Threshold mode returns a pixelwise measure for the energy deposition, neither an absolute nor inter-pixel calibration was possible. However, a clear correlation between the total output signal and the intensity reported by an electron spectrometer could be observed, confirming the Timepix sensor's general capability of energy-resolved measurements. Furthermore, electron tracks that were visible in the obtained data were used to calculate a rough estimate of the primary photon energy by comparing their extent to the predictions of the Continuous Slowing Down Approximation. This yielded initial electron energies in the MeV range, which are in good agreement with the achieved energies reported for the JETI experiment in question.

R. Märtin
Röntgenpolarimetrie angewandt zur Untersuchung der Bremsstrahlung spinpolarisierter Elektronen
Doctoral thesis
Ruprecht-Karls-Universität, Fakultät für Physik und Astronomie (2011)

Abstract: The present thesis reports on the study of the linear polarization properties of bremsstrahlung produced in polarized electron atom collisions. The experimental investigation of bremsstrahlung photons has been performed using the polarized electron source (SPIN) at the TU Darmstadt. Gold and carbon targets were bombarded with 100 keV electrons whose spin was oriented parallel, anti-parallel and transverse with respect of the beam axis. In addition, an unpolarized electron beam was used for a reference measurement. For the detection of the bremsstrahlung photons, a novel Si(Li) Compton polarimeter has been employed at two different observation angles. This detector enabled the determination of the degree of linear polarization as well as the orientation of the polarization vector for various energies of the bremsstrahlung photons. The emphasis of the work was on the so-called polarization transfer where the polarization of the incoming electron spin influences the polarization of the emitted x-rays. This gives rise to an enhanced degree of linear polarization and to a rotation of the photon polarization vector with respect to the unpolarized case. Both effects were observed by comparing data for the unpolarized and the transversely polarized electron beam. The experimental results are in qualitative agreement with fully relativistic calculations.

M. Reuter
Interferometric Probing of Laser Generated Plasmas for Electron Acceleration
Thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2011)

Abstract: dummy