Nonlinear transition from convective to absolute Raman instability with trapped electrons and inflationary growth of reflectivity

Abstract

We propose a nonlinear mechanism for transition from convective to absolute in stimulated Raman backscattering instability due to the effect of trapped particles in the plasma wave. Convective instability saturates at the low level, yet it is sufficient to trap electrons near the plasma phase velocity. The trapped electrons tend to flatten the distribution function. With spatial averaging over the trapped region, we find that the flattened distribution function reduces the damping rate due to bounce resonance of the plasma wave and then decreases the threshold for absolute instability. So the transition from a weak, convective instability to a strong, absolute instability can occur, leading to exponential growth everywhere and inflation of reflectivity of several orders of magnitude as observed in the experiment, once the threshold is exceeded.