title: Spectral Analysis of Laser-Driven, Layered X-Ray Backlighter Targets
Creators: name: Sander, Steffen
Publisher: name: UNSPECIFIED
Year: 2023

description: In this thesis, a layered X-ray backlighter target with front side modifications for optimal laser target coupling is developed. The testing of this novel target design was done at the PHELIX laser system, located at the GSI Helmholtzzentrum für Schwerionenforschung GmbH in Darmstadt. This dissertation was carried out within the laser and plasma physics group of the Institut für Kernphysik at the Technische Universität Darmstadt. X-ray radiation is an invaluable tool for the diagnosis of high energy density (HED) experiments. Strong X-ray emission can be generated, among other things, through the irradiation of solid-state targets by high-energy short-pulse lasers with intensities above 10^18 W cm^−2. The resulting photon flux is in the order of 10^10 - 10^11 ph J^−1 within a narrow spectral bandwidth. The possible applications of such a source include radiography and scattering studies of dense plasmas in order to determine key thermodynamic variables like temperature or density. Two types of X-ray backlighter sources are commonly used, which can be differentiated by the degree of ionization of the emitting material. For weak ionization, the spectrum is very similar to the characteristic X-ray line emission and is called Kα radiation. With rising ionization the energy of the X-rays increases as well, leading to thermal line emission for highly ionized sources. However, thermal line emission cannot be produced efficiently with laser driven backlighters for photon energies beyond 10 keV, as more and more electrons have to be ionized from the heavier elements. Therefore, for high energy line emission, a laser-driven Kα source is preferred. Within the scope of this thesis, a novel laser driven Kα source was developed and successfully tested. The goal was two-fold. Primarily, an improvement of the energy transfer from laser into target to increase the number of emitted Kα photons should be achieved. In order to fulfil this goal, front side modifications in form of cone-like microstructures were used. Secondly, a thermal isolation of the emitting material from the laser plasma interaction region should be achieved. This produces a clean spectrum without any additional thermal line emission. This was done by constructing the target from two layers, namely the aforementioned front side modifications and a back side layer of different material for the X-ray emission. The performance of the developed target was studied in an experiment with the PHELIX laser system. The observed spectra from the layered target displayed a successful suppression of thermal line emission, displaying the effective thermal isolation. This result is further supported with a spectroscopic line shape analysis to determine the mean electron temperature within the emitting material. Temperatures of 31 - 42 eV were found, indicating a weak ionization without involvement of the two inner-most electron shells. The total number of Kα photons were measured to be in the order of 10^10 ph J^−1. A clear increase of factor 2 - 3 in Kα photons due to the microstructures on the front side was demonstrated. The source size of the targets were determined to be 140 - 200 μm. The orientation of the cone-like microstructures with respect to the laser were varied as well and displayed a clear influence on the Kα yield and source size. For a parallel alignment, the highest photon flux of 3.9 × 10^11 ph J^−1 mm^−2 was found. Atomic radiative simulations were successfully used to determine the electron temperature in the backside layer to 31 - 42 eV. Another unique feature of the layered target design is the possibility to influence the source size through the shape of the back side layer. The source size was successfully reduced to 100 μm using a circular shaped back side layers of the same dimensions. With these targets it was also possible to demonstrate the possibility of generating two spatially separated, distinct sources from one target.