Alpha particle destabilization of shear Alfven waves in ignited tokamaks

The effects of alpha particles on the stability of the shear Alfven waves are investigated for realistic numerical toroidal equilibria with noncircular flux surfaces by employing a nonvarionational kinetic-MHD stability code (NOVA-K). Both the low-n and the high-n toroidicity-induced shear Alfven eigenmodes (TAE) were shown to exist with frequencies lying in the gaps of the shear Alfven continuum spectrum due to toroidal couplings of neighboring poloidal harmonics. The TAE modes can be strongly destabilized via transit resonance with alpha particles when the inverse Landau damping associated with the alpha particle pressure gradient becomes larger than the velocity space Landau damping. On the other hand, the global Alfven eigenmode (GAE), whose frequency lies just below the minimum of the continuum, are shown to remain stable. (author) 6 refs., 2 figs