Investigation and Characterization of Single Hot Spot Laser-Plasma Interactions

Abstract

Control of parametric laser-plasma interactions (LPI) is essential to the success of inertial confinement fusion (ICF). Through a research collaboration with the Los Alamos National Laboratory (LANL), we have had the opportunity to participate in world-class laser-plasma experiments. The goal of these experiments was to gain a fundamental understanding of LPI by studying the interaction of a single laser hot spot, or speckle, with a preformed, quasi-homogeneous, long scale-length plasma. Recent single hot spot experiments resulted in a wealth of data and the first definitive observation of two LPIs. Namely, the Langmuir decay instability (LDI) cascade and stimulated scattering off of an acoustic-like electron mode below the usual electron plasma wave frequency. The LDI is the result of the electron plasma wave (EPW) generated by stimulated Raman scattering (SRS) growing to a sufficient amplitude such that it exceeds a threshold (proportional to the damping of the LDI daughter waves) and undergoes parametric decay into another counter-propagating EPW and a co-propagating ion acoustic wave (IAW). Subsequent EPW decays due to LDI are possible and collectively more than one EPW generated by LDI is called LDI cascade. The LDI cascade can play a role in the saturation of SRS since wave energy from the SRS EPW couples into secondary waves that are nonresonant with the SRS process. Stimulated scattering from an electrostatic wave at a frequency and phase velocity ( ω ≈0.4 ωpe, v φ≈1.4ve) between that of an EPW and IAW was also observed. In this thesis, a Vlasov-Maxwell code is used to numerically predict the time evolution of the electron distribution function for the experimental parameters. The resultant distribution function is then modeled as a bi-Maxwellian (one background and one beaming) to show that it exhibits linear modes that include the observed electron acoustic wave. A quasimode analysis of laser scattering off of this linear mode is presented as one possible explanation of the experimental observation.