Nonlinear excitation of a geodesic acoustic mode by toroidal Alfvén eigenmodes and the impact on plasma performance

Abstract

Spontaneous nonlinear excitation of a geodesic acoustic mode (GAM) by a toroidal Alfvén eigenmode (TAE) is investigated using nonlinear gyrokinetic theory. It is found that the nonlinear decay process depends on the thermal ion βi value. Here, β is the plasma thermal to magnetic pressure ratio. In the low-β limit, a TAE decays into a GAM and a lower TAE sideband in the toroidicity induced shear Alfvén wave continuous spectrum gap; while in the high-βi limit, a TAE decays into a GAM and a propagating kinetic TAE in the continuum. Both cases are investigated for the spontaneous decay conditions. The nonlinear saturation levels of both the GAM and daughter wave are derived. The corresponding power balance and wave particle power transfer to thermal plasma are computed. Implications for thermal plasma heating are also discussed.