Langmuir wave turbulence transition in a model of stimulated Raman scatter

Abstract

In a one-dimensional stationary slab model, it is found that once the stimulated Raman scatter (SRS) homogeneous growth rate, γ0, exceeds a threshold value, γT, there exists a local, finite amplitude instability, which leads to Langmuir wave turbulence (LWT). Given energetic enough initial conditions, this allows forward SRS, a linearly convective instability, to be nonlinearly self-sustaining for γ0>γT. Levels of forward scatter, much larger than predicted by the linear amplification of thermal fluctuations, are then accessible. The Stochastic quasilinear Markovian (SQM) model of SRS interacting with LWT predicts a jump in the value of 〈ε〉, the mean energy injection rate from the laser to the plasma, across this threshold, while one-dimensional plasma slab simulations reveal large fluctuations in ε, and a smooth variation of 〈ε〉 with γ0. Away from γT, 〈ε〉 is well predicted by the SQM. If a background density ramp is imposed, LWT may lead to loss of SRS gradient stabilization for γ0≪γT.