Self-focusing of Gaussian laser beam in collisionless plasma and its effect on stimulated Brillouin scattering process

This paper presents an investigation of self-focusing of Gaussian laser beam in collisionless plasma and its effect on stimulated Brillouin scattering process. The pump beam interacts with a pre-excited ion-acoustic wave thereby generating a back-scattered wave. On account of Gaussian intensity distribution of laser beam, the time independent component of the ponderomotive force along a direction perpendicular to the beam propagation becomes finite, which modifies the background plasma density profile in a direction transverse to pump beam axis. This modification in density affects the incident laser beam, ion-acoustic wave and back-scattered beam. We have set up non-linear differential equations for the beam width parameters of the main beam, ion-acoustic wave, back-scattered wave and SBS-reflectivity with the help of moment and paraxial theory approach. Results of the moment theory approach have been compared with that of paraxial theory approach. It has been observed from the analysis that reflectivity of the back-scattered wave is less in moment theory approach as compared to paraxial theory approach. (C) 2012 Elsevier B.V. All rights reserved.