Experimental Studies of Stimulated Brillouin Scattering from Krypton Fluoride Laser-Produced Plasma

A comprehensive study of stimulated Brillouin scattering (SBS) in nanosecond KrF laser-produced plasma at intensity I_{rm L} ~eq 10^{14}W/cm^2 has been performed. Measurements of reflectivity and time -resolved backscattered spectra were done for CH, Al, and Au targets at various angles of incidence theta. The measured backscatter reflectivity ranged from 0.1% to 1.0% for Al and C plasma, with higher reflectivity being observed for larger theta. Au plasma showed significantly lower reflectivity ( ~eq0.005%). The time-resolved spectra of backscattered light were found to be correlated with laser intensity and pulse shape. Numerical codes developed to model the SBS process in homogeneous and inhomogeneous plasmas dominated by strong inverse bremsstrahlung absorption predicted an optimum density for SBS amplification. Two -dimensional hydrodynamic simulations predicted locally planar density and spherical pressure profiles as a consequence of the large mass ablation rate and localized plasma heating. Models for SBS in inhomogeneous plasma and non-uniform laser irradiation were proposed and used to explain the experimental data. Analytically and experimentally, we find evidence for the convective nature of the SBS instability. Supplementary experiments using thin foil targets showed strong temporal and spatial modulation in transmitted light. It was found that the laser beam non-uniformity was not smoothed out by the plasma but remained throughout the laser pulse.