  The phenomenon of redistribution of neutral beam fast-ions due to MHD
activity in plasma has been observed on many tokamaks and more recently has
been a focus of research on MAST (Turnyanskiy M. et al, 2011 Nucl. Fusion 53
053016). n=1 fishbone modes are observed to cause a large decrease in the
neutron emission rate corresponding to 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 density levels at two
neutral beam power levels to determine the region within this parameter space
in which MHD activity and consequent fast-ion redistribution is suppressed.
Analysis of these experiments shows complete suppression of MHD activity at
high density with increasing activity and fast-ion redistribution at lower
densities and higher NB power accompanied by strong evidence for localisation
of the redistribution effect to a specific region in the plasma core. The
results also indicate correlations between the form of 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 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.
