The Physics of Sawtooth Stabilisation in Tokamak Plasmas
Long period sawteeth have been observed to result in the low-b triggering of Neo-classical Tearing Modes (NTMs), which can significantly degrade plasma confinement. In ITER, the stabilising effects of the fusion-born a particles are likely to exacerbate this. Consequently, it is important to develop a detailed physical understanding of sawtooth behaviour. Recent work on plasmas heated using Neutral Beam Injection (NBI) in JET, MAST, ASDEX Upgrade and TEXTOR has provided significant insight into the physical processes that determine sawtooth behaviour. The asymmetric dependence of sawtooth period upon neutral beam power injected in either the co- or counter-current direction exhibited in both JET and TEXTOR can be explained by including the effects of the passing energetic ions, the toroidal flow and the flow shear. In JET, the counter-passing fast ions destabilise the n = 1 internal kink mode ­ thought to be associated with sawteeth ­ whilst the flow shear strongly influences the stabilisation from the trapped particles. The sawtooth behaviour in TEXTOR has been explained through a combination of both kinetic effects and stabilisation from toroidal rotation. In order to avoid triggering NTMs, many techniques have been proposed to control, and in particular, to destabilise the sawtooth oscillations. Here, sawtooth behaviour in off-axis NBI-heated plasmas in JET, MAST and ASDEX Upgrade is presented. It is found that the energetic particles born outside the q = 1 surface due to the off-axis NBI can destabilise the sawteeth, even in the presence of stabilising on-axis fast particles. We also consider sawtooth control using Ion Cyclotron Resonance Heating (ICRH). Previously it had been assumed that the change in sawtooth behaviour in ICRH heated plasmas arose due to the change in the magnetic shear at q = 1. The energetic passing ions are found to influence the internal kink mode when the distribution of ions is asymmetric in vk, a natural feature of co or counter propagating ICRH waves.