NONLINEAR EVOLUTION OF THE TOROIDAL ALFVEN INSTABILITY USING A GYROFLUID MODEL

Discrete shear Alfven modes such as the TAE (toroidal Alfven eigenmode ) are susceptible to destabilization by energetic alpha populations an d neutral beams; this can lead to enhanced fast ion losses and degrade d heating efficiencies. A gyrofluid model with Landau closure has been developed for understanding both the linear and nonlinear phases of t hese instabilities. The linear wave-particle resonances necessary to e xcite Alfven instabilities are included in a coupled set of fluid equa tions. This model is used to analyze several nonlinear saturation mech anisms that arise from mode coupling effects. The effects of shear flo w velocity generation (through the Reynolds stress) and localized curr ent generation (leading to modifications in the q profile) are specifi cally examined.