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The sawtooth instability in tokamak plasmas results in a periodic reorganisation of the plasma surrounding the magnetic axis. Plasmas with long sawteeth are more susceptible to neo-classical tearing modes (NTMs), which result in substantial confinement degradation. The stabilising effects of fusion-born alpha particles in burning plasmas are likely to exacerbate this. Thus, recent experiments have aimed to identify methods to deliberately destabilise sawteeth in an attempt to avoid seeding NTMs. This thesis first presents a series of experimental results from auxiliary heated tokamak plasmas in various devices, before showing how numerical modelling has explained the observed behaviour. Experimental results from MAST, JET and TEXTOR show that neutral beam injection (NBI) directed in either the opposite direction to the intrinsic rotation or off-axis can result in shorter sawtooth periods. In order to understand sawtooth stabilisation, the interaction of magnetohydrodynamic (MHD) stability and fast particle effects must be considered. A coherent physics explanation of the experimental results is proposed by modelling the' stability of the n=1 internal kink mode - which is thought to be associated with sawtooth oscillations - in the presence of toroidal rotation and anisotropic energetic ions resulting from NBt. In JET, th~re are relativety slow toroidal flows, so sawtooth stability is governed by the. energetic particle population. It is found that whilst the trapped particles are always'',. stabilising, the passing particles can be destabilising and the flow shear changes the. stabilisation from the trapped particles. In MAST, the plasma rotates at velocities·:. approaching the thermal deuterium sound speed. Such strong rotation can also stabilise the kink mode. In TEXTOR, the effects of toroidal rotation and energetic ions compete to determine the sawtooth behaviour. After explaining the different mechanisms that determine sawtooth stability, this model is then used to consider sawtooth stability in ITER plasmas. |
