High-Intensity Laser-Target Interactions for Applications in Laboratory Astrophysics

Over the past decades, there has been a significant effort to study the interactions of high-intensity, short-pulse lasers with solid-density targets due to their numerous crossfield applications. Understanding the plasma phenomena taking place during the formation of these extreme states of matter in the laboratory ultimately will lead to significant advances in a plethora of fields, including laser-plasma driven particle accelerators, translational research for radiation oncology, laboratory astrophysics, inertial confinement fusion, and material degradation studies for future fission and fusion reactors. However, the microphysics of many relativistic laser-plasma processes, such as plasma heating, field generation, and shock formation, are still not understood. In particular, the role of kinetic plasma instabilities, such as the Weibel or current filamentation instability, in fundamental plasma processes, such as electron transport, magnetic field amplification, and ion acceleration, remains an open question. In astrophysical environments, these instabilities are thought to govern the seeding and amplification of magnetic fields, leading to the formation of collisionless shocks, which are capable of accelerating particles to extreme energies and thus form large parts of the cosmic ray spectrum that is measured on Earth. The formation of these collisionless shocks, however, is still a highly debated topic, to which laboratory experiments can offer particularly valuable insights into the involved instabilities. Yet, the direct experimental observation and detailed characterization of the current filamentation instability in solid density plasmas has been challenged due to the limitations of conventional diagnostics to probe the high plasma density with great precision. This thesis presents the results of an experiment conducted at the SLAC National Accelerator Laboratory Linac Coherent Light Source X-ray laser, during which the solid density region of the relativistic laser-plasma interaction was successfully imaged for the first time. The images show the development and growth of density filaments associated with the current filamentation instability and suggest the importance of ion motion in facilitating the instability growth. The filament structures are analyzed through a standardized method developed specifically for this thesis, which enables the extraction of the filament wavelength and integrated filament density. These observations are used to benchmark fully kinetic PIC simulations and analytical models, revealing details of a more intricate process of this instability growing in an asymmetric regime. Furthermore, at later times the hydrodynamic evolution of the high-intensity laser-solid interaction is revealed, featuring a laser-generated shock wave moving at an average velocity of 80 km/s. The preliminary analysis of these shocks is compared with basic magnetohydrodynamic models and shows the results to be consistent with a rarefaction wave attenuating an initially strong shock wave generated during the high-intensity shortpulse laser interaction. Finally, a cryogenic liquid jet target system is discussed as a key element for the ongoing effort to extend High Energy Density science experiments, such as the one described in this thesis, to high-repetition rate. As such, this thesis lays the foundation of the direct experimental observation and characterization of plasma instabilities at solid density and provides a guide for future studies of relativistic laser-solid interactions.