Beam smoothing effects on the stimulated Brillouin scattering (SBS) instability in Nova exploding foil plasmas

Abstract

The effects of beam smoothing on the stimulated Brillouin and Raman backscattering instabilities (SBBS and SRBS) in a long scale length exploding foil plasma are experimentally investigated using the Nova laser. We observe strongly time‐dependent and blueshifted SBS emission and broadband SRS emission in time‐resolved spectral measurements of the backscattered light. The results of this paper focus on the SBS scattering, where we find that random phase plate spatial beam smoothing leads to a decrease in the total SBS reflectivity. Adding temporal smoothing to this spatial smoothing leads to an even greater decrease in the total reflectivity, as well as changes in the spectral nature of the backscattered light. Beam smoothing induced changes to the instabilities are limited to plasmas with a peak density below about 0.2n_cr; beam smoothing causes little change in the backscattered emission for higher densities. We describe the measurements and discuss models that may explain certain aspects of the results.