NONLINEAR MECHANISMS FOR DRIFT WAVE SATURATION AND INDUCED PARTICLE-TRANSPORT

In previous studies, it was shown that, in the nonlinearly saturated phase of the evolution of drift instabilities in gyrokinetic particle simulations, the saturation levels and especially the particle fluxes are significantly less than predicted by quasilinear theory, and have an unexpected dependence on collisionality. In this paper, a theory is developed that explains these phenomena. The key features of the theory are as follows: The saturation level is determined by a balance between the steady-state electron and ion fluxes. The ion flux is small for levels of the potential below an EXB-trapping threshold and increases sharply once this threshold is crossed. Because of the presence of resonant electrons, the electron flux has a much smoother dependence on the potential. In the 2 1/2-dimensional ("pseudo-3-D") geometry, the electrons are accelerated away from the resonance as they diffuse spatially, resulting in an inhibition of their diffusion, and thereby reducing the values of the potential and flux at which the fluxes can balance. Collisions and three-dimensional effects can repopulate the resonance thereby increasing the value of the particle flux.