Nonlinear electromagnetic plasma eigenmodes and their stability to stimulated Raman scattering

Transverse mode structure of nonlinear laser eigenmodes in underdense and overdense plasmas has been obtained by numerically solving the wave equation under relativistic and ponderomotive nonlinearities. The mode structure closely resembles a Lorentzian with half width scaling inversely as the axial intensity of the laser. The threshold condition for laser penetration in an overdense plasma turns out to be gamma(0)equivalent to(1+a(0)(2)/2)(1/2)>= 2n(0)/n(cr)-1, where n(0) is the equilibrium electron density, n(cr) is the critical density at laser frequency, and gamma(0) is the electron Lorentz factor due to the laser of normalized axial intensity a(0)(2). The nonlinear laser eigenmode, in a low density plasma, is unstable to stimulated Raman backscattering off a copropagating space charge reactive quasimode. The growth rate increases with laser intensity as a(0) rises up to a(0)similar to 1. Beyond this value, growth rate decreases with a(0), due to the enhancement of electron mass and depletion of electrons from the axial region. Geometrical effects also reduce the growth rate. (c) 2006 American Institute of Physics.