HYBRID MAGNETOHYDRODYNAMIC-GYROKINETIC SIMULATION OF TOROIDAL ALFVEN MODES

Resonant energetic particles play a major role in determining the stab ility of toroidal Alfven eigenmodes (TAE's) by yielding the well-known driving mechanism for the instability and by producing an effective d issipation, which removes the singular character of local oscillations of the shear-Alfven continuum and gives discrete kinetic Alfven waves (KAW's). Toroidal coupling of two counterpropagating KAW's generates the kinetic analog of the TAE, the KTAE (kinetic TAE). The nonperturba tive character of this phenomenon and of the coupling between TAE and KAW's, and the relevance of finite drift-orbit effects limit the effec tiveness of the analytical approach to asymptotic regimes, which are d ifficult to compare with realistic situations. A three-dimensional hyb rid fluid-particle initial-value code for the numerical simulation of the linear and nonlinear evolution of toroidal modes of the Alfven bra nch has been developed. It is shown that for typical parameters the KT AE is, indeed, more unstable than the TAE. (C) 1995 American Institute of Physics.