GYROKINETIC SIMULATIONS OF EXB VELOCITY-SHEAR EFFECTS ON ION-TEMPERATURE-GRADIENT MODES

Data from several current tokamak experiments indicate that the equili brium perpendicular velocity field can become strongly sheared accompa nying the transition from the L mode to the H mode, i.e. improved, con finement, and that fluctuation levels are reduced. Linear theory sugge sts that velocity shear can stabilize ion-temperature-gradient (ITG) m odes when the frequency shift experienced by the mode due to the radia l dependence of the Doppler shift is comparable to the growth rate. To confirm the predictions of linear theory and to explore nonlinear iss ues, e.g., self-generated shear flows, saturation amplitudes, and the concomitant energy transport levels, two- and three-dimensional gyroki netic simulations of ITG modes have been performed. The simulations we re done with and without magnetic shear in a slab configuration using the partially linearized (deltaf) algorithm to reduce statistical nois e. The simulations confirm theoretical analyses of the stabilizing and destabilizing effects of imposed perpendicular velocity fields. The i on energy transport levels at saturation follow the trends of the line ar growth rates and the mixing-length estimates. The gyrokinetic simul ations are in qualitative agreement with the results of gyrofluid simu lations, and exhibit saturation amplitudes and energy transport simila r to those in gyrofluid simulations. These transport levels are genera lly lower than those typically reported in the laboratory experiments; including toroidal driving terms significantly increases the transpor t levels in the simulations.