Consequences of fast ion driven modes in MAST

As we enter the era of burning plasmas in next step devices such as ITER, the confinement of fusion born a-particles for sufficient duration that they impart their energy to the bulk fuel ions in order to maintain the thermonuclear burn is an important challenge in magnetically confined fusion. Fast ion driven plasma instabilities can cause significant redistribution and loss of the suprathermal energetic particle (EP) population, degrading performance. With dimensionless parameters such as the ratio of fast ion to thermal ion beta (Bfi/Bth ~50%) and the relative fast ion velocity to the Alfvén velocity (vfi/vA ~2) similar to those anticipated in ITER, the Mega Ampere Spherical Tokamak (MAST) provides the ideal place to study such instabilities. During periods of Neutral Beam Injection (NBI) heating, 'fishbone' instabilities are observed that coincide with a reduction to the fusion rate measured by drops in the neutron emission. Via experimental observations, fishbones are identified to be low frequency internal kink modes that burst in amplitude and chirp downwards in frequency and are synonymous with high power tokamak discharges on a wide range of devices around the world. This thesis provides a detailed analysis of what occurs during a single fishbone event. Experiments have been performed on MAST that have been interpreted using fast ion plasma physics codes. Modelling of the instability shows a resulting flux of fast ions away from the core, providing evidence at a fundamental level that they drive sufficient levels of anomalous fast ion transport to explain experimental observations. The diffusivity is shown to scale with mode amplitude, and the effect of altering other fishbone parameters within the scope of the experimental observations have been explained by identifying the extent of the fast ion population that is resonant with the mode. Resonant surfaces that sweep through phase space during the chirp are presented that coincide with populous domains of the EP distribution function; it is the gradients in this distribution function that define the drive and or damping of the instability. Via the use of synthetic diagnostics, changes to the radial profiles of neutron emissivity caused by a fishbone are shown to match those measured experimentally.