Frequency Sweeping Directivity of Bursting Alfv´en Waves on MAST

Abstract

In a burning plasma, such as the next generation tokamak experiment ITER, significant numbers of highly energetic alpha particles will be produced. The presence of energetic particles may excite kinetic instabilities and affect the transport and heating of particles in the plasma. Of particular interest is the so called toroidal Alfv´en eigenmode (TAE) which can be excited by the super Alfv´enic alpha particles through resonant wave-particle interaction. In this thesis we identfy the linear TAEs with corresponding bursting nonlinear evolution in a discharge on the spherical tokamak MAST using the equilibrium code HELENA and the linear codes CSCAS and MISHKA. If this mode is excited there is a consecutive exponential growth of the linearized perturbations when the linear growth rate due to the fast particles is slightly larger then the total damping in the plasma. Nonlinear theory is then needed to describe the evolving nonlinear modes, which are observed to sweep in frequency with a preferred frequency sweeping directivity that changes in time. We investigate a possible reason for the change in preferred frequency sweeping directivity and derive an expression for the radial motion of the mode during the frequency sweeping.