title: Enhancing laser-induced X-ray emission and ion acceleration with microstructured targets
Creators: name: Ebert, Tina
Publisher: name: UNSPECIFIED
Year: 2021

description: The irradiation of thin, solid foils with a high intensity laser leads to the heating of electrons, which in turn, among other things, generate characteristic X-ray line emission and strong electrostatic fields thatcan accelerate ions. The induced radiation and particle yield can be enhanced by increasing the energy transfer from the laser to the hot electrons. Potential applications of these laser-induced secondarysources range from non-destructive imaging via proton or X-ray radiography, to the ignition of inertial confinement capsules by focused ion beams, to the generation of directed neutron pulses. This thesis investigates targets with microstructured front surfaces to enhance the laser matter coupling. In previous studies, targets with micro-engineered surfaces have shown great potential in improving the energy transfer, as they can affect the relevant heating mechanisms, introduce guiding effects and increase the interaction volume and time. Much effort has been spent on investigating fragile structures such as nanowires, however, since these structures are only suited to laser systems with ultrahigh contrast and pulse lengths in the tens of femtoseconds range, high energy lasers with longer pulse lengths call for more durable microstructures. Therefore, in this thesis the surfaces of flat silicon wafers are modified with conical, spike-like microstructures in a randomly distributed, dense array. The structures are produced via ultrashort laser pulse processing. For this, an experimental setup was designed, which allows to create a wide range of spike geometries by changing the processing parameters, including the parameters of the femtosecond laser, the processing medium as well as the target material. In addition, a replication procedure based on molding with polydimethylsiloxane (PDMS) was developed. It enables the fabrication of a master target mold, which is subsequently used to create identical microstructures out of other materials such as polystyrene and copper. The fabricated targets were studied in experimental campaigns at the PHELIX laser at the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany, and the Omega EP laser at the Laboratory for Laser Energetics (LLE), Rochester, NY, USA, with varied laser contrasts, intensities, and pulse lengths. Both laser systems deliver pulse energies above 100 J at pulse lengths exceeding 0.5 ps, thereby fulfilling a main requisite for the generation of powerful radiation and particle sources. The laser-induced X-ray generation and ion acceleration with the microstructured targets was evaluated in comparison to flat foils. In addition to the experiments, complementary 2D particle-in-cell (PIC) simulations were performed to study the impact of the microspikes on the electron heating processes, optimize the target parameters and validate the experimental findings. The microstructured targets showed both an enhancement of the accelerated proton numbers and their maximum energies. In particular, matching the structures tilt angle to the laser incidence angle resulted in a 40% increase of the maximum energy compared to structures with a relative angle of 30°, which was observed both experimentally and in corresponding simulations. Furthermore, the silicon targets with a microstructured surface generated an up to 13 times stronger Heα emission and a 12 times stronger Lyα line compared to a flat target at a laser pulse length of 1 ps. The benefit decreases with increasing pulse length, but even at the longest investigated pulse length of 20 ps they showed an enhancement for both lines. In contrast to alternative target types such as foam targets or targets with an attached shield, the presented microspikes generate a well-defined X-ray pulse with a rapidly decaying slope. Using PIC simulations, the importance of target geometry optimization to further enhance the performance is demonstrated and the spiked targets are compared to another microstructure type, namely micropillars, in terms of their ion acceleration efficiency. The results presented in this thesis highlight the potential of microstructured targets for enhancing laser-induced secondary sources.