Localized MHD activity near internal transport barriers in JT-60U and TFTR

Localized MHD activity observed in JT-60U and TFTR near transport barriers with their associated large pressure gradients is investigated. Stability analysis of equilibria modelling the experiments supports an identification of the MHD activity as being due to an ideal MHD n = 1 instability. Theoretical models are used to identify the dependence of the instability on the salient features of the plasma profiles: the local pressure gradient, local shear in the q profile and the proximity of rational surfaces where q » m/n, with m and n the poloidal and toroidal mode numbers, respectively. The mode width is shown to depend on the value of q where the pressure gradient is largest, and increases as q decreases. The edge current density is shown to play an important role in coupling the internal mode to the plasma edge. Energetic particles in these neutral beam heated discharges are shown to drive fishbone-like modes when they exist in sufficient numbers in the vicinity of the maximum perturbation owing to the ideal instability.