A test-bed for Langmuir wave turbulence modeling of stimulated Raman backscatter

Abstract

Stimulated Raman backscatter (SRS) may incorporate several, qualitatively different regimes of Langmuir wave dynamics, as it grows convectively in space. These typically include a strictly linear regime at the far end of the plasma from the laser, where SRS comes up from thermal Langmuir wave fluctuations; which may progress to a regime where the primary SRS daughter Langmuir wave is unstable to the Langmuir wave decay instability (LDI); and perhaps to a regime of strong Langmuir wave turbulence (SLT). The accurate description of the spatial transition between these regimes, which may involve large Langmuir wave correlation lengths, is a great challenge for turbulence modeling. In this paper a highly idealized model of SRS in periodic geometry is introduced which allows for the presence of a unique Langmuir wave regime for a given set of physical parameters, and therefore presents the minimal challenge for a turbulence model. One- and two-dimensional simulations of this SRS model, which allows for LDI and SLT as described by Zakharov’s model of nonlinear Langmuir wave dynamics, are compared with the predictions of a recently introduced turbulence model, and quantitative agreement is obtained, without the use of any ad hoc parameters, for the SRS reflectivity and correlation length, and Langmuir and acoustic wave energy densities, over an order of magnitude variation of SRS growth rate and ion acoustic damping rate.