THEORY OF RAMAN-SCATTERING FOR A SHORT ULTRASTRONG LASER-PULSE IN A RAREFIED PLASMA

Direct forward and backward Raman scattering in an underdense plasma a re considered for relativistically strong laser pulses. Dispersion ana lysis and analytical solutions, taking account of the pulse shape, are presented. It is shown that the Raman instability has an absolute max imum of the growth rate for the laser field amplitude (eE0/momega0c) c ongruent-to 1, which corresponds approximately to the power density of 10(18) W/cm2 for 1 mum wavelength laser radiation. Analytical solutio ns are obtained for both backward- and forward-scattered light that sh ow the quite different way that the forward and backward scattering af fect the global pulse evolution. The forward scattering develops in th e whole body of the pulse and affects most considerably the trailing p art of the pulse. In contrast with it, the backward scattering can be expected to become saturated aleady in the leading part of the pulse a nd affects mainly the evolution of the leading pulse edge. The relatio n of the obtained results and recent three-dimensional studies of the short-pulse evolution are discussed.