The effect of kinetic processes on Langmuir turbulence

Kinetic processes are shown to be crucial in determining the saturation lev el 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-ce ll simulation models are compared in the test case of a uniformly driven pl asma. Good agreement is observed between all three simulation methods for r elatively low primary Langmuir wave numbers (k(1)lambda(De) similar to 0.1) in weakly driven regimes. In the strongly driven case, quasilinear diffusi on provides an important correction to the Landau damping rate, producing s aturation levels in agreement with reduced-description particle-in-cell sim ulations, in contrast to pure Zakharov simulations, which overestimate the saturation significantly. At higher k(1)lambda(De) similar to 0.25, both th e quasilinear-Zakharov and pure Zakharov models fail. In this regime, the a utocorrelation time of the Langmuir wave spectrum is much larger than the q uasilinear diffusion time, causing the quasilinear diffusion approximation to break down. (C) 2000 American Institute of Physics. [S1070-664X(00)94305 -2].