Collective stimulated Brillouin backscatter

TL;DR

This paper develops a statistical theory for stimulated Brillouin backscatter (BSBS) in incoherent laser beams relevant to fusion, revealing a new regime with higher thresholds and collective behavior independent of speckle dynamics.

Contribution

It introduces a novel theoretical framework for BSBS in incoherent beams, identifying a new high-threshold collective instability regime applicable to fusion plasmas.

Findings

New BSBS regime with higher threshold than classical coherent case

Instability is collective, independent of speckle dynamics

Thresholds within NIF parameters, but current bandwidth may be insufficient

Abstract

We develop the statistical theory of the stimulated Brillouin backscatter (BSBS) instability of a spatially and temporally partially incoherent laser beam for laser fusion relevant plasma. We find a new regime of BSBS which has a much larger threshold than the classical threshold of a coherent beam in long-scale-length laser fusion plasma. Instability is collective because it does not depend on the dynamics of isolated speckles of laser intensity, but rather depends on averaged beam intensity. We identify convective and absolute instability regimes. Well above the incoherent threshold the coherent instability growth rate is recovered. The threshold of convective instability is inside the typical parameter region of National Ignition Facility (NIF) designs although current NIF bandwidth is not large enough to insure dominance of collective instability and suggests lower instability threshold due to speckle contribution. In contrast, we estimate that the bandwidth of KrF-laser-based fusion systems would be large enough.