The Physics of Sawtooth Stabilisation

Long period sawteeth have been observed to result in low- triggering of neo-classical tearing modes, which can significantly degrade plasma confinement. Consequently, a detailed physical understanding of sawtooth behaviour is critical, especially for ITER where fusion-born particles are likely to lead to very long sawtooth periods. Many techniques have been developed to control, and in particular to destabilise the sawteeth. The application of counter-current Neutral Beam Injection (NBI) in JET has resulted in shorter sawtooth periods than in Ohmic plasmas. This result has been explained because, firstly, the counter-passing fast ions give a destabilising contribution to the n = 1 internal kink mode ­ which is accepted to be related to sawtooth oscillations ­ and secondly, the flow shear strongly influences the stabilising trapped particles. A similar experimental result has been observed in counter ­ NBI heated plasmas in MAST. However, the strong toroidal flows in spherical tokamaks mean that the sawtooth behaviour is determined by the gyroscopic flow stabilisation of the kink mode rather than kinetic effects. In NBI heated plasmas in smaller conventional aspect-ratio tokamaks, like TEXTOR, the flow and kinetic effects compete to give different sawtooth behaviour. Other techniques applied to destabilise sawteeth are the application of Electron Cyclotron Current Drive (ECCD) or Ion Cyclotron Resonance Heating (ICRH). In JET, it has been observed that localised ICRH is able to destabilise sawteeth which were otherwise stabilised by a co-existing population of energetic trapped ions in the core. This is explained through the dual role of the ICRH in reducing the critical magnetic shear required to trigger a sawtooth crash, and the increase of the local magnetic shear which results from driving current near the q = 1 rational surface. Sawtooth control in ITER could be provided by a combination of ECCD and co-passing off-axis Negative NBI fast ions.
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