Fast Particle Destabilization of TAE Modes
Authors: C.Z. Cheng, N.N. Gorelenkov, C.T. Hsu
High-h TAE modes are studied based on a kinetic model that includes full thermal ion finite Larmor radius effects, trapped electron collisions and fast particle instability drive. Lower KTAE modes are shown to be non-existent. Like TAE modes, upper KTAE modes are shown to exist due to thermal ion FLR effects in the dissipationless limit. Dissipation effects on the stability of both TAE and upper KTAE modes can be treated perturbatively. However, due to their extended mode structure in the ballooning space, upper KTAE modes usually remain stable or weakly unstable even with large fast particle free energy. On the other hand, TAE modes can be strongly destabilized. A new resonant TAE mode (RTAE) can be excited when the fast particle drive is signicantly large. The RTAE mode is a beam-like mode with its frequency determined mainly by the wave-particle resonance condition. The frequency of the RTAE mode can be much less than the TAE gap frequency and may be interpreted as the BAE observed in DIII-D experiments. As plasma b increases, the TAE, RTAE and kinetic ballooning modes strongly couple; the TAE mode changes into the RTAE mode and eventually connects to the kinetic ballooning mode. Numerical results and analytical analysis on the stability of the RTAE and KTAE modes will be presented and compared with the TAE mode stability.