Stimulated Raman scattering from density cavities in laser‐produced plasmas

The occurrence of stimulated Raman scattering from within localized density cavities that may be present in many laser‐produced plasmas is studied. Then scattering in both the backward and forward directions from a single cavity becomes absolutely unstable, with a common growth rate and zero inhomogeneous plasma thresholds whenever electron plasma waves are totally trapped within the cavity. Both back‐ and forward scattering mechanisms contribute to this common growth rate. A cavity therefore equalizes the growth of emission in backward and forward directions. Forward scattering in its own right gives rise to absolute growth. The threshold for absolute growth is then set by damping, and by Landau damping of the plasma wave in particular. Only very modest density depressions are needed to trap the long‐wavelength weakly damped plasma waves. Absolutely unstable back‐ and forward scattering from within shallow cavities in the underdense coronal plasmas of laser fusion targets may prove to be commonplace rather than the exception, although growth rates are generally slow and sensitive to the cavity density and scale.