The phenomenon of the redistribution of neutral beam fast ions due to magnetohydrodynamic (MHD) activity in plasma has been observed on many tokamaks and more recently has been a focus of research on MAST (Turnyanskiy et al 2013 Nucl. Fusion53 053016). n = 1 fishbone modes are observed to cause a large decrease in the neutron emission rate indicating the existence of a significant perturbation of the fast-ion population in the plasma. Theoretical work on fishbone modes states that the fast-ion distribution itself acts as the source of free energy driving the modes that cause the redistribution. Therefore a series of experiments have been carried out on MAST to investigate a range of plasma densities at two neutral-beam power levels to determine the region within this parameter space in which fishbone activity and consequent fast-ion redistribution is suppressed. Analysis of these experiments shows complete suppression of fishbone activity at high densities with increasing activity and fast-ion redistribution at lower densities and higher neutral-beam power, accompanied by strong evidence that the redistribution effect primarily affects a specific region in the plasma core with a weaker effect over a wider region of the plasma. The results also indicate the existence of correlations between gradients in the modelled fast-ion distribution function, the amplitude and growth rate of the fishbone modes, and the magnitude of the redistribution effect. The same analysis has been carried out on models of MAST-Upgrade baseline plasma scenarios to determine whether significant fast-ion redistribution due to fishbone modes is likely to occur in that device. A simple change to the neutral-beam injector geometry is proposed which is shown to have a significant mitigating effect in terms of the fishbone mode drive and is therefore expected to allow effective plasma heating and current drive over a wider range of plasma conditions in MAST-Upgrade.
The behaviour of fast ions in tokamak experiments