Localized MHD activity observed in JT-60U and TFTR near transportbarriers with their associated large pressure gradients is investigated. Stability analysis ofequilibria modelling the experiments supports an identification ofthe MHD activity as being due to an ideal MHD n = 1 instability. Theoreticalmodels are used to identify the dependence of the instability on thesalient features of the plasma profiles: the local pressuregradient, 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 isshown to depend on the value of q where the pressure gradient is largest, and increases as qdecreases. The edge current density is shown to play an importantrole in coupling the internal mode to the plasma edge. Energeticparticles in these neutral beam heated discharges are shown to drivefishbone-like modes when they exist in sufficient numbers in thevicinity of the maximum perturbation owing to the ideal instability.
Shear reversal and MHD activity during pellet enhanced performance pulses in JET