Stimulated Raman backscattering in the presence of ion‐acoustic fluctuations

Threshold conditions are analyzed for stimulated Raman backscattering (SRS) from an underdense plasma in the presence of ion‐acoustic fluctuations. For typical ion‐acoustic spectra which could result from a Brillouin instability occurring simultaneously with SRS, a reduced collisional threshold due to induced bremsstrahlung is found for the Raman instability. Similarly, for inhomogeneous plasma the phase mismatch created by a density gradient can be compensated for by an inhomogeneity in the ion‐acoustic spectrum; as a consequence, the inhomogeneity threshold for absolute instability can be substantially reduced. Comparisons are made with experimental observations of stimulated Raman scattering in the presence of strong stimulated Brillouin scattering for CO2 laser‐plasma interaction. It is found theoretically that for fluctuation levels typical of that accompanying SBS, induced bremsstrahlung growth rates for Langmuir waves can be of the order of collisional damping rates.