Low frequency plasma response to intense laser beams.

摘要

Numerical and theoretical studies of laser beam interaction with under-dense plasmas involving ion wave instabilities are presented. The theoretical model that is used involves a realistic distribution of laser intensity and a non-paraxial electromagnetic wave equation coupled to the ion acoustic wave equation in a two-dimensional geometry.;Three different cases of laser beams are considered, a single hot spot beam, a random phase plate (RPP) beam and two crossed RPP beams. In all the three cases, a close connection with the experiment is established.;For the case of a single hot spot beam the stability of laser light filaments in a homogeneous isothermal plasma with respect to coupled electromagnetic and density perturbations is considered. At early times, in addition to the known modulational instability of a guided electromagnetic mode, a new fast growing resonant instability is found. At later times, a weak correlation between backscattered stimulated Brillouin scattering (SBS) reflectivity and filamentation or self-focusing instabilities is established. It is demonstrated that the transmitted light angular spreading and frequency shifts are consistent with near-forward SBS.;In the case of a single RPP beam, the angular divergence and temporal bandwidth are shown to correspond to additional spatial and temporal incoherence in the regime where the average laser intensity exceeds the self-focusing threshold. The transverse and longitudinal sizes of the laser speckles inside the plasma are related in a way that makes it possible to define a local effective beam f-number as a measure of the effective speckle length and for the plasma induced temporal incoherence. The numerical simulations show that the effective f-number decreases as light propagates through plasma.;In the case of two crossed RPP beams a significant nonlinear enhancement of large angle forward scattering is observed. The spectral analysis of the transmitted light shows two components, one of which is unshifted with respect to the initial laser light frequency, and the other is red-shifted. The red-shifted component is found to be strongly enhanced in the case of crossed beam interaction in comparison with that of one beam illumination. The numerical simulations show that this enhancement is due to large angle forward stimulated Brillouin scattering.