The geodesic acoustic mode in strongly-shaped tight aspect ratio tokamaks

This thesis presents comparison between experimental measurements from the spherical tokamak MAST, two-fluid simulation data and theory of the Geodesic Acoustic Mode (GAM) in tight aspect ratio strongly shaped tokamak plasmas. The first identification of a strong ~10kHz mode detected in both potential and density fluctuations of the edge plasma in MAST using a reciprocating probe is given. The mode is radially localised, with outer limit ~ 2cm inside the separatrix, and is affected on application of resonant magnetic perturbations (RMP) generated by external coils. A shift in frequency with plasma rotation is found, and a suppression of the mode is observed above a certain threshold. Non-linear coupling to high wave number turbulence is evident, and an increase in power of turbulence fluctuations is seen after suppression. These observations are then interpreted in the context of known low frequency plasma modes present in the toroidal configuration. The supposition that the observed mode is a geodesic acoustic mode is considered and motivated by experimental observations and numerical simulations.