The effect of kinetic processes on Langmuir turbulence

Kinetic processes are shown to be crucial in determining the saturation level of stimulated Raman scattering for regimes relevant to NOVA [Campbell et al., Fusion Technol. 21, 1344 (1992)] and the National Ignition Facility [Lindl, Phys. Plasmas 2, 3933 (1995)]. To investigate these kinetic effects, the Zakharov, quasilinear-Zakharov, and reduced-description particle-in-cell simulation models are compared in the test case of a uniformly driven plasma. Good agreement is observed between all three simulation methods for relatively low primary Langmuir wave numbers $(k1λDe∼0.1)$ in weakly driven regimes. In the strongly driven case, quasilinear diffusion provides an important correction to the Landau damping rate, producing saturation levels in agreement with reduced-description particle-in-cell simulations, in contrast to pure Zakharov simulations, which overestimate the saturation significantly. At higher $k1λDe∼0.25,$ both the quasilinear-Zakharov and pure Zakharov models fail. In this regime, the autocorrelation time of the Langmuir wave spectrum is much larger than the quasilinear diffusion time, causing the quasilinear diffusion approximation to break down.