Sawtooth control and the interaction of energetic particles

Long-period sawteeth have been observed to result in the low-β triggering of neo-classical tearing modes (NTMs), which can significantly degrade plasma confinement. In ITER, the stabilizing effects of the fusion-born α particles are likely to exacerbate this. Consequently, in order to avoid triggering NTMs, many techniques have been proposed to control, and in particular, to destabilize the sawtooth oscillations. Here, sawtooth behaviour in off-axis NBI-heated plasmas in JET and ASDEX Upgrade is presented. It is found that the energetic particles born outside the q = 1 surface due to off-axis NBI can destabilize the sawteeth, even in the presence of stabilizing on-axis fast particles. In order to model the stability of the n = 1 internal kink mode, which is associated with the sawtooth oscillations, both a magnetohydrodynamic code including toroidal rotation and a drift kinetic code have been employed. The modelling highlights the significant role played by both the passing energetic particles and the toroidal flow shear in determining the kink mode stability in the presence of an energetic particle population.