Nonlinear evolution of the toroidal Alfvén instability using a gyrofluid model*

Discrete shear Alfvén modes such as the TAE (toroidal Alfvén eigenmode) are susceptible to destabilization by energetic alpha populations and neutral beams; this can lead to enhanced fast ion losses and degraded heating efficiencies. A gyrofluid model with Landau closure has been developed for understanding both the linear and nonlinear phases of these instabilities. The linear wave–particle resonances necessary to excite Alfvén instabilities are included in a coupled set of fluid equations. This model is used to analyze several nonlinear saturation mechanisms that arise from mode coupling effects. The effects of shear flow velocity generation (through the Reynolds stress) and localized current generation (leading to modifications in the q profile) are specifically examined.