Self-organization and control in stimulated Raman backscattering

摘要

A stimulated Raman scattering (SRS) on electron plasma waves in underdense plasmas is of a big concern in laser fusion due to an energy loss and target preheating. Complex features of large Backward-SRS (BRS) in experiments and simulations with laser fusion targets are found. Recently, to reach ultra-high intensities at multi-exawatts and beyond, relevant to high-energy physics, Raman amplification based on BRS was proposed; still, with high sensitivity and a narrow operational window. Firstly, we revisit a standard three-coupled mode model of BRS to show that the condition for an absolute instability is readily satisfied in uniform plasmas which excites large Raman signals from a background noise. It sets in for interaction length L0 shorter than, both, the plasma length L and absorption length La. Further, we point out a generic BRS feature, which due to a nonlinear frequency shift in large electron plasma wave (relativistic/trapping effects), instead to a steady state, saturates via intermittent pulsations with incoherent spectral broadening. A 'break up' of Manley–Rowe invariants is shown to predict non-stationary BRS. Finally, an intermediate intensity regime is originally proposed for coherent femtosecond pulse generation in a thin exploding foil plasma, with scalings investigated by theory and particle simulations.