BEAM SMOOTHING EFFECTS ON THE STIMULATED BRILLOUIN-SCATTERING (SBS) INSTABILITY IN NOVA EXPLODING FOIL PLASMAS

The effects of beam smoothing on the stimulated Brillouin and Raman ba ckscattering instabilities (SBBS and SRBS) in a long scale length expl oding 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 b ackscattered light. The results of this paper focus on the SBS scatter ing, where we find that random phase plate spatial beam smoothing lead s to a decrease in the total SBS reflectivity. Adding temporal smoothi ng to this spatial smoothing leads to an even greater decrease in the total reflectivity, as well as changes in the spectral nature of the b ackscattered light. Beam smoothing induced changes to the instabilitie s are limited to plasmas with a peak density below about 0.2n(cr); bea m smoothing causes little change in the backscattered emission for hig her densities. We describe the measurements and discuss models that ma y explain certain aspects of the results. (C) 1995 American Institute of Physics.