Modeling of the competition of stimulated Raman and Brillouin scatter in multiple beam experiments

Multiple laser beam experiments with plastic target foils at the Laboratoir e pour L'Utilisation des Lasers Intenses (LULI) facility [Baldis , Phys. Re v. Lett. 77, 2957 (1996)] demonstrated anticorrelation of stimulated Brillo uin and Raman backscatter (SBS and SRS). Detailed Thomson scattering diagno stics showed that SBS always precedes SRS, that secondary electron plasma w aves sometimes accompanied SRS appropriate to the Langmuir Decay Instabilit y (LDI), and that, with multiple interaction laser beams, the SBS direct ba ckscatter signal in the primary laser beam was reduced while the SRS backsc atter signal was enhanced and occurred earlier in time. Analysis and numeri cal calculations are presented here that evaluate the influences on the com petition of SBS and SRS, of local pump depletion in laser hot spots due to SBS, of mode coupling of SBS and LDI ion waves, and of optical mixing of se condary and primary laser beams. These influences can be significant. The c alculations take into account simple models of the laser beam hot-spot inte nsity probability distributions and assess whether ponderomotive and therma l self-focusing are significant. Within the limits of the model, which omit s several other potentially important nonlinearities, the calculations sugg est the effectiveness of local pump depletion, ion wave mode coupling, and optical mixing in affecting the LULI observations. (C) 2001 American Instit ute of Physics.