EFFECT OF DIFFRACTION ON STIMULATED BRILLOUIN-SCATTERING FROM A SINGLE LASER HOT-SPOT

A single laser hot spot in an underdense plasma is represented as a fo cused Gaussian laser-beam. Stimulated Brillouin scattering (SBS) from such a Gaussian beam with small f/numbers 2-4 has been studied in a th ree-dimensional slab geometry. It is shown that the SBS reflectivity f rom a single laser hot spot is much lower than that predicted by a sim ple three wave coupling model because of the diffraction of the scatte red light from the spatially localized ion acoustic wave. SBS gain per one Rayleigh length of the incident laser beam is proposed as a quant itative measure of this effect. Diffraction-limited SBS from a randomi zed laser beam is also discussed. (C) 1996 American Institute of Physi cs.