Alfven stability properties of reversed regimes in DT tokamaks

The formation of transport barriers with reduced neoclassical transport and centrally reversed shear q-profiles can potentially lead to high fusion reactivities near the magnetic axis. The concomitantly high alpha production in this region can be a drive for Alfven instabilities. We have analyzed the stability thresholds of such modes and find that reversed shear can result in significant differences from the more normal tokamak q-profiles which have been analyzed previously. The larger values of q(0) and qmin which characterize the reversed shear profiles will lower the shear Alfven frequency gap. In principle, this should have no effect on the resonance velocity (vres = ω/kparallel) in the center of the gap since kparallel decreases by a similar amount as (o with increasing q). However, due to the fact that the mode structure has a finite radial width and poloidal couplings, it always will sample a range of kparallel's. As a result, we find TAIE destabilization in reversed shear profiles predominately occurs at sub-Alfvenic velocities (Vres/vAlfven ∼ 0.3 to 0.4). TAE thresholds for energies characteristic of the neutral beam species then can become quite low even for full field tokamaks. This can have consequences for the understanding and maintenance of the sheared flow (which is thought to cause the transport barriers) since sheared flows have also been shown to be a nonlinear consequence of unstable toroidal Alfven modes. Recent cases from DIII-D and TFTR will be analyzed