Influence of a finite level of ion acoustic waves on Raman gain in inhomogeneous plasma

A five-wave Raman scattering process is examined in inhomogeneous plasma in the presence of a finite level of ion acoustic waves. This five-wave inter action is a composite of conventional three-wave stimulated Raman scatterin g (SRS) followed by the decay of the Raman Langmuir wave into either a seco nd Langmuir wave (LD) or a second scattered light wave (ED) and an ion acou stic wave. The presence of a suprathermal level of ion acoustic waves detun es SRS and inhibits the temporal growth. In inhomogeneous plasma these deca ys take place locally so that phase matching for both decays is assumed to be coincident. The net Raman convective gain is shown to be sensitive to th e source levels of the amplifying waves. For LD the gain may be suppressed or enhanced depending on the relative intensities of the two Langmuir waves . For ED the gain is invariably suppressed. In either case, this occurs for very modest levels of ion acoustic waves. (C) 2000 American Institute of P hysics. [S1070-664X(00)04709-1].