Full-orbit studies of wave-particle interaction on the Mega Ampere Spherical Tokamak

Abstract

Energetic particles with super-Alfvenic speeds could potentially drive Alfvenic instabilities in a magnetically confined plasma.
The driven waves can influence the fast particle distribution function as energetic particles are redistributed or lost to the vessel wall leading to a reduction in energetic particle confinement and heating efficiency.
This thesis investigates the interaction between particles and waves via full orbit numerical simulations.
The work presented herein takes steps towards the development of a capability to assess whether future reactor scenarios will be susceptible to these adverse effects or not.

A full orbit particle tracking code has been developed to calculate particle trajectories and more importantly to compute particle orbital frequencies as they are followed in the simulation.
Based on the wave-particle resonance condition, resonant particles are identified using this code for realistic tokamak geometries.

Experimental observations of fast-ion driven waves on the MAST tokamak are presented.
Magnetic perturbations in the kilo-Hertz range are detected by a set of high resolution Mirnov coils during the neutral beam injection heating phase where the mode frequency is observed to chirp downwards over the course of a magneto-hydrodynamics (MHD) burst.
A decrease in fast-ion deuterium alpha signals is found to be correlated with the electromagnetic bursts indicating fast ion redistribution during the MHD activity.
Simulation results suggest that the increase in plasma pressure is disproportional to the increase in NBI heating power in the presence of MHD modes.
The effect of instabilities on energetic particle behaviour has been analysed by calculating resonance maps and resonant particle orbits.
Full orbit calculations show that the chirping frequency broadens the wave-particle resonance region which can result in enhanced particle transport.

Preliminary attempts have been made to evaluate fast particle transport induced by chirping modes using the non-linear full orbit code.
The chirping behaviour of the mode frequency is simulated by an ad-hoc function similar to experimental measurement.
Calculations are performed for a simple cylindrical tokamak geometry and a mocked-up alpha particle distribution.
An toroidal Alfven eigenmode (TAE) is found numerically for this equilibrium.
The results of the simulations show that fast particles are transported outwards from the plasma centre when chirping modes are present while no significant particle transport is seen when the mode frequency is constant.
The level of transport is affected by either mode amplitude or chirping rate.
These results suggest that the inclusion of a chirping effect is necessary to study particle redistribution in the presence of fast-ion modes when considering plasma scenarios in the future.