Interferometric Analysis of Femtosecond Laser-Generated Plasma Expansion from Solid Targets

The paper presents the preliminary results of interferometric studies on plasma expansion induced by femtosecond laser pulses interacting with solid aluminum targets. These interactions offer great potential for the generation of energetic particle streams (electrons and ions) commonly known as laser ion (electron) acceleration, which are of particular interest in research on inertial confinement fusion, modeling of astrophysical phenomena, and other applications. Interferometry is an excellent tool for visualizing processes of plasma stream expansion and provides information about the spatial-temporal distributions of these streams with high spatial and temporal resolution. However, there is a limited body of research that focuses on interferometric measurements of plasma generated by femtosecond laser pulses on solid targets, making this study a significant contribution to the field. The presented research was conducted at the High Power Laser Laboratory, located in the Institute of Plasma Physics and Laser Microfusion, using a femtosecond laser system with an energy of approximately 280 mJ (after compression, on target) and a pulse duration (full-width at half-maximum) of similar to 260 fs. The studies involved flat aluminum solid targets and foils, illuminated by a laser pulse with different spot sizes depending on the target's position relative to the focal point. A one-frame interferometer with a tunable delay line was used to capture images at various stages of plasma expansion. These studies clearly demonstrate the significant impact of spot size on the character of plasma expansion, with spherical expansion dominating for small spot sizes and axial expansion dominating for larger spot sizes. A substantial part of the paper is devoted to the description of the quantitative analysis of the obtained data.