The role of absorption in Raman amplification in warm plasma

Raman backscattering in plasma is subject to-collisional and collisionless-absorption of the interacting waves. A model for studying its role over a wide parameter range is developed by coupling the envelope equations for pump, probe, and plasma waves with those describing heating of the plasma. The latter is treated as a warm fluid, making the model useful for moderate temperatures and field amplitudes. The main effect is the time-dependent Bohm-Gross shift of the Langmuir resonance frequency, which can either enhance or suppress amplification; this can be further controlled by varying the frequency of the pump. Anisotropy in the collisional processes for longitudinal and transverse waves leads to temperature anisotropy at high field amplitudes. Direct Landau damping of the plasma wave, as well as its contribution to the frequency shift, can be neglected due to rapid saturation.