RANDOM-PHASE PLATE HOT-SPOTS AND THEIR EFFECT ON STIMULATED BRILLOUIN BACKSCATTER AND SELF-FOCUSING

Laser hot spots, as determined by Random Phase Plate (RPP) hot spots, control the critical value of the average intensity, Ic, at which there is a rapid onset of stimulated scatter in the strongly damped convective regime of three wave parametric instabilities. For the case of stimulated Brillouin backscatter in a long scale length plasma, nascent hot spot ponderomotive self-focusing is shown to reduce the value of Ic in the regime of very strongly damped acoustic waves. RPP hot spots have two, intrinsically nonlinear, thresholds for ponderomotive self-focusing. Large intensity amplifications occur in the hot spot neighborhood when the hot spot power exceeds a certain critical power, Pc, which is independent of the optic's f number, F. When the second, F-dependent, hot spot power threshold is exceeded, a filament emerges from the far side of the hot spot, whose extent grows erratically in time. © 1995 American Institute of Physics.