Alpha particle effect on global MHD modes

Stability criteria in terms of the α-particle beta βα, α-particle pressure gradient parameter (ω*/ωA) (ω* is the α-particle diamagnetic drift frequency), and (vα/vA) parameters for the high-frequency, low mode number TAE modes, will be presented for TFTR, BPX, and ITER tokamaks. The volume averaged α-particle beta threshold for TAE instability also depends sensitively on the core electron and ion temperature. Typically the volume averaged α-particle beta threshold is in the order of 10-4 if the continuum damping effect is absent. For BPX and ITER equilibria, we find that <ω*/ωd> will change from being positive near the magnetic axis to negative as the minor radius increases, i.e., magnetic drift reversal. Second, for resonant trapped α-particles with ω approx. ωd, their dynamics are no longer rigid with respect to the MHD perturbation and wave particle resonances can occur. Resonant ''fishbone'' internal kink instabilities can be excited by tapping the free energy of the α-particle pressure nonuniformity. Stability diagrams of the n=1 internal kink for the parameters β, βα, and q(0) will be presented for BPX and ITER equilibria. The numerical results from the NOVA-K code indicate that the trapped α-particles have destabilizing effects and can significantly lower the total beta threshold. 3 refs, figs