Enhanced Raman scattering of a rippled laser beam in laser-plasma interaction

In the laser-plasma interaction experiments, self-focusing and filamentation affect quite a large number of other parametric processes including stimulated scattering processes. It has also been experimentally observed that SRS is more prone to the nonlinearly refraction-induced enhancement in intensity of the filaments. This situation is qualitatively well described by ripple model [Sodha et al. Phys. Fluids Vol. 24, 1981] in which we investigate the growing interaction of the rippled laser beam with electron plasma wave leading to enhanced Raman scattering. The rippled laser beam with initial Gaussian intensity is assumed to propagate along the externally applied static magnetic field in one of the possible modes. We consider here the non-linearity resulting from non-uniformity in heating, which leads to redistribution of carrier. Because of modified background density, the ripple is coupled to the main beam. Nonlinear differential equations for the beam width parameters of the pump and ripple are set up and solved numerically using Range-Kutta method. An expression for the growth rate of ripple is also derived. Further, the effect of growth of ripple on electron plasma wave excitation is also studied. The coupling between main beam, ripple and excitation is so strong that when appropriate conditions are satisfied, the excited electron plasma wave further interact with the rippled laser beam leading to the enhanced Raman scattering. From the computational results, it is observed that the effect of increased intensity leads to suppression of power associated with Raman scattered wave. Lastly, it is also observed that increase of externally applied magnetic field almost suppresses the scattered field.