Gyrokinetic δf particle simulation of trapped electron mode driven turbulence

The linear instabilities and nonlinear transport driven by collisionless trapped electron modes (CTEM) are systematically investigated using three-dimensional gyrokinetic delta f particle-in-cell simulations. Scalings with local plasma parameters are presented. Simulation results are compared with previous simulations and theoretical predictions. The magnetic shear is found to be linearly stabilizing, but nonlinearly the transport level increases with increasing magnetic shear. This is explained by the changes in radial eddy correlation lengths caused by toroidal coupling. The effect of zonal flows in suppressing the nonlinear CTEM transport is demonstrated to depend on electron temperature gradient and electron to ion temperature ratio. Zonal flow suppression is consistent with the rate of ExB shearing of the ambient turbulence and radial spectra broadening. Strong geodesic acoustic modes (GAM) are generated along with zonal flows. (C) 2007 American Institute of Physics.