Stimulated Raman scattering in inhomogeneous collisional laser-produced plasmas

Convective gain and absolute growth of stimulated Raman scattering (SRS) are determined for a hot inhomogeneous collisional plasma using an integrated global model that includes the effects of wave propagation and both collisional and Landau damping in a density ramp. This unifies the time asymptotic theory of SRS occurring near the quarter-critical density of the laser light and that occurring at lower densities. The level of collisions has a striking effect on the convective behavior of the instability. When collisions are weak, no significant regime of convective gain exists. In consequence, absolute resonances are very sensitive and susceptible to detuning, particularly so near the quarter-critical density. This is shown to be a key factor in understanding the Raman ''gap'' seen in experiments. No such gap appears in highly collisional regimes in which the absolute resonances are insensitive and significant convective gain occurs over the whole range of wavelengths