Fast particle destabilization of toroidal Alfven eigenmodes

Abstract

High-n, toroidal Alfven eigenmodes (TAEs) are studied on the basis of a kinetic model that includes full thermal ion finite Larmor radius effects, trapped electron collisions and fast particle instability drive. Lower kinetic TAEs (KTAEs) are shown to be non-existent in the nearly collisionless limit. Like TAEs, upper KTAEs are shown to exist because of thermal ion finite Larmor radius effects in the dissipationless limit. Dissipation effects on the stability of both TAEs and upper KTAEs can be treated perturbatively. However, owing to their extended mode structure in the ballooning space, upper KTAEs usually remain stable or weakly unstable even with large fast particle free energy. On the other hand, TAEs can be strongly destabilized. A new resonant TAE (RTAE) can be excited when the fast particle drive is significantly large. The RTAE is a beam-like mode, with its frequency determined mainly by the wave-particle resonance condition. The frequency of the RTAE can be much less than the TAE gap frequency and may be interpreted as the BAE observed in DIII-D experiments. As plasma beta increases, the TAE, RTAE and kinetic ballooning modes strongly couple; the TAE changes into the RTAE and eventually connects to the kinetic ballooning mode. Numerical results and analytical analysis on the stability of the RTAEs and KTAEs will be presented and compared with the TAE stability