Recent Trident single hot spot experiments: Evidence for kinetic effects, and observation of Langmuir decay instability cascade

Single hot spot experiments offer several unique opportunities for developing a quantitative understanding of laser-plasma instabilities. These include the ability to perform direct numerical simulations of the experiment due to the finite interaction volume, isolation of instabilities due to the nearly ideal laser intensity distribution, and observation of fine structure due to the homogeneous plasma initial conditions. Experiments performed at Trident in the single hot spot regime have focused on the following issues. First, the intensity scaling of stimulated Raman scattering (SRS) for classically large damping regimes (kλD=0.35) was examined, and compared to classical SRS theory. SRS onset was observed at intensities much lower than expected (2x1015 W/cm2), from which nonclassical damping is inferred. Second, Thomson scattering was used to probe plasma waves driven by SRS, and structure was observed in the scattered spectra consistent with multiple steps of the Langmuir decay instability. Finally, scattering from a plasma wave was observed whose frequency and phase velocity are between an ion acoustic wave and an electron plasma wave. The presence of this wave cannot be explained by linear Landau theory, and it is shown to be consistent with a BGK-type mode due to electron trapping