FILAMENTATION OF A LASER-BEAM INTERACTING WITH AN UNDERDENSE PLASMA AND ITS COUPLING TO STIMULATED BRILLOUIN-SCATTERING

The nonlinear behavior of a single laser beam with power above critica l is investigated in three spatial dimensions without the paraxial app roximation. In the case where stimulated Brillouin scattering (SBS) is suppressed by strong damping, it is found numerically that the hot sp ot contracts spatially in all directions, leading to transverse dimens ions smaller than the laser wavelength, even when the ion response is nonlinearly saturated. This contraction leads to the three dimensional collapse of the transverse wave in the absence of nonlinear saturatio n of the ion response. When the saturation of the ion response is prop erly retained, a narrow filament forms initially, This filament then a ppears to be unstable and exhibits violent modulations of the laser wa ve intensity along the direction of its propagation, leading to a comp licated dynamical behavior. In the case of moderate SBS damping, one s till observes the initial spatial contraction in all directions and th e formation of a narrow channel. SBS then occurs in violent bursts, an d does not stop the complex dynamical behavior of the filament.