Ideal MHD stability analysis for equilibria with a pitch minimum

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Abstract
The ideal MHD stability of non-monotonic q-configurations has been studied for an ultra-low-q (ULQ) regime (0 < q < 1) by both analytical and computational methods. An ULQ configuration is characterized by a safety factor q-profile with dq/dr < 0 in a part of the plasma region and the existence of a pitch minimum. An ULQ equilibrium for single resonant surface modes with m = 1 is stable because of the wall stabilization effect, if the equilibrium has enough shear. Modes with m ≥ 2 are localized around the resonant surface and are stable. Double resonant surface modes are unstable, but the growth rates of these modes are smaller than those of internal kink modes. The stability of m ≥ 2 modes is strongly dependent on the depth of pitch minimum.

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