Abschlussarbeiten

2014

T. Kiefer
Investigation of the laser-based Target Normal Sheath Acceleration (TNSA) process for high-energy ions — an analytical and numerical study
Dissertation
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (January 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
Dissertation
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (January 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
Masterarbeit
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (December 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
Dissertation
Universität Hamburg; Fakultät für Mathematik, Informatik und Naturwissenschaften (December 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
Dissertation
Johannes Gutenberg-Universität Mainz; Fachbereich 08 Physik, Mathematik und Informatik (November 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
Masterarbeit
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (October 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
Dissertation
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (October 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.
T. Rathje
Photodissoziation des Wasserstoffmolekülions durch Einzelzyklenlaserpulse
Dissertation
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (October 2013)
Abstract:
*
M. O. Herdrich
Ionisationsquerschnitte von Uranionen in Speicherringen
Bachelorarbeit
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (September 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
Masterarbeit
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (September 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
Bachelorarbeit
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (August 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
Masterarbeit
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (March 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
Masterarbeit
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (January 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
Dissertation
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (November 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
Dissertation
Johannes Gutenberg-Universität Mainz; Fachbereich 08 Physik, Mathematik und Informatik (June 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
Masterarbeit
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (April 2012)
Abstract:
n/a

2011

C. Hahn
Charakterisierung ortsauflösender Halbleiterdetektoren für harte Röntgenstrahlung
Bachelorarbeit
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (September 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
Dissertation
Ruprecht-Karls-Universität; Fakultät für Physik und Astronomie (September 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
Dissertation
Friedrich-Schiller-Universität Jena; Physikalisch-Astronomische Fakultät (July 2011)