Nonlinear modification of laser-plasma interaction processes under crossedlaser beams

Effects of multiple crossed laser beams on stimulated Brillouin and Raman s cattering (SBS and SRS) growth rates, as well as on the SBS and SRS competi tion, have been investigated in a preformed plasma using Thomson scattering of a short-wavelength probe beam. Reduction of the amplitude of ion acoust ic waves (IAW) associated with SBS and amplification of the amplitude of el ectron plasma waves associated with SRS in the case of irradiation by two o r three beams compared to a single beam are reported. Mechanisms by which c rossed laser beams can reduce SBS by detuning the interaction or by nonline arly enhancing the damping rate of the IAWs will be discussed. The same mec hanisms could explain the enhancement of SRS backscatter through modified s econdary decay processes of SRS. In a second experiment, nonlinear enhancem ent of extreme forward scattering of one laser beam going through a preform ed plasma has been observed when a second interaction beam was present. The role of ion wave instabilities on forward scattering will be discussed, ba sed on the spectral analysis of the forward scattered light. (C) 1999 Ameri can Institute of Physics. [S1070-664X(99)91205-3].