The scaling of absolutely unstable, stimulated Raman scattering from planar, laser-produced plasmas

Abstract

A study of the scaling of the fraction, fSRS, of the laser energy scattered by stimulated Raman scattering (SRS) is reported. A simple model of SRS as an absolute instability is presented, leading to the hypothesis that fSRS will be determined by the depth of the plasma producing SRS. The results of a number of experiments are used to test this hypothesis. The experiments analyzed used irradiating wavelengths of 1.06, 0.53, and 0.35 μm. In all cases, the plasmas were planar and the electron–ion collision rate was small compared to the homogeneous SRS growth rate. The depth of the plasma, as opposed to the laser intensity or laser wavelength, is found to be the primary determinant of fSRS. In addition, as the depth of the plasma increases, the angular distribution of the scattering is observed to broaden. The implications of these results for analysis of SRS as an absolute instability and for laser fusion are discussed.