Relationship between onset thresholds, trigger types and rotation shear for the m/n = 2/1 neoclassical tearing mode in a high-β spherical torus

The onset conditions for the m/n = 2/1 neoclassical tearing mode are studied in terms of neoclassical drive, triggering instabilities, and toroidal rotation or rotation shear, in the spherical torus NSTX (Ono M. et al 2000 Nucl. Fusion 40 557). There are three typical onset conditions for these modes, given in order of increasing neoclassical drive required for mode onset: triggering by energetic particle modes, triggering by edge localized modes and cases where the modes appear to grow without a trigger. In all cases, the required drive increases with toroidal rotation shear, implying a stabilizing effect from the shear. (letter)