Optical probing of laser-induced expansion of levitating microspheres

Speicher, Martin (2022): Optical probing of laser-induced expansion of levitating microspheres. Dissertation, LMU München: Fakultät für Physik

Abstract

In the present work, the expansion dynamics of levitating microspheres following the interaction with a fs-short laser pulse in the intensity regime of 1015 −1016 W/cm2 is investigated in a pump-probe experiment. The study comprises two plasma diagnostics: an intrinsic probing along the laser axis via the pump pulse, fixed at t = 0 ps, and a time-variable lateral probing on a separate probe pulse. In both cases, the transmitted light is recorded via a scatter screen, providing a very simple diagnostic tool that can be implemented in most high-power laser experiments. In order to extract a plasma density distribution from the recorded inline holograms, the experiment is reproduced via numerical simulations using the Python package LightPipes. The simulation setup is calibrated by comparison to experimental conditions such as focus size, beam profiles and holograms of defined polystyrene spheres. Several radial symmetric models are investigated for modeling the density distribution of the plasma at different times during its evolution by comparing simulation results against recorded experimental images. The best agreement is found for a Gaussian density distribution with an additional, decentralized Gaussian component. The validity of this empirically determined model is further strengthened by simulations using the hydrodynamic code RALEF, where experimentally obtained values for the spatial and temporal intensity distribution are used as input. The temporal course of the expanding density distribution is compared to a simple model assuming hydrodynamic expansion of the plasma. The good agreement between experimental data and the model allows determining physical quantities such as laser absorption and relate them to experimental conditions of the plasma. The findings of this work are a first step towards studying the expansion of micrometer spherical targets at intensities well above the plasma generation threshold and are particularly relevant for future experiments investigating the interaction of relativistically intense laser pulses with density-tailored, sub-focus sized microplasmas, e.g. in the field of laser-ion acceleration.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 30. Juni 2022
• 1. Bericht­erstatter:in: Schreiber, Jörg