Stimulated Brillouin and Raman scattering from a randomized laser beam in large inhomogeneous collisional plasmas. II. Model description and comparison with experiments

A model for stimulated Brillouin (SBS) and Raman (SRS) backscattering of a spatially smoothed laser beam interacting with a collisional, inhomogeneous , expanding plasma is presented. It is based on the independent hot spots d escription [H. A. Rose and D. F. DuBois, Phys. Rev. Lett. 72, 2883 (1994)], in which the overall plasma reflectivity is assumed to be a sum of the ind ividual speckle reflectivities. Self-focusing is taken into account in the computation of the speckle intensity profile and reflectivities. Two additi ons have been made to previous similar theories: (i) the thermal effects ar e retained along with the ponderomotive force for what concerns speckle sel f-focusing, and (ii) SRS (convective and absolute) is accounted for in calc ulations of the speckle reflectivity. The model is benchmarked against rece nt laser-plasma experiments at Laboratoire pour l'Utilisation des Lasers In tenses, at Ecole Polytechnique, France, with well-characterized interaction conditions. A good agreement is found between the experimental SBS levels and the model calculations using the measured plasma parameters. This agree ment applies for two types of beam smoothing techniques, random phase plate s, and polarization smoothing, various plasma densities, and laser energies . Self-focusing itself, and thermal effects in it, play both a fundamental role in defining the level of plasma backscattering. The absolute Raman ins tability in speckles dominates the SRS response. The model predictions for the SRS reflectivity are less satisfactory, although they demonstrate the s ame trends as the experimental data. It follows from model calculations and experimental data that the polarization smoothing technique provides an ef ficient method of control of parametric instabilities allowing a reduction of several times in the level of SBS and SRS reflectivities. (C) 2001 Ameri can Institute of Physics.