Energetic particle stabilization of ballooning modes in a finite aspect ratio tokamak

Abstract

The effect of energetic trapped particles on the stabilization of high toroidal mode number (n → ∞) ballooning modes in tokamaks is investigated numerically in the low frequency limit, for a realistic anisotropic equilibrium with a circular cross-section and a moderate aspect ratio of 3. In the case when qm (safety factor at the magnetic axis) is close to unity, energetic ions can double the ballooning first stability beta limit. This enhanced beta value is limited by the drift non-reversal condition used here as a theoretical assumption. In this case, second stability is not achieved because, with an aspect ratio of 3, the second stability limit is either very high or does not exist. However, if qm is increased somewhat above unity, there exists a second stability region on a large fraction of the flux surfaces for moderate beta values, i.e. there is an unstable region between the first and the second stability without energetic particles. It is shown that the energetic trapped particles can partially or completely stabilize this unstable gap between first and second stability. In summary, second stability can be attained by the introduction of energetic particles, in combination with current profile control to increase qm above unity, when the parameters that determine the energetic pressure profile are properly chosen.