Mitigation of MHD induced fast-ion redistribution in MAST and implications for MAST-Upgrade design

Abstract

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 fastion 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.

Measurements and modelling of fast-ion redistribution due to resonant MHD instabilities in MAST

“…The results presented in this paper demonstrate the advantages of employing several complementary diagnostics to study the behaviour of the FI population. Extending previous studies on MAST [8,28,29,36], a systematic analysis of NC and FIDA data has allowed the deleterious effects of chirping TAE and fishbones on the confined FI population to be identified with confidence. The velocity-space sensitivity of the FIDA measurements moreover permits losses of FI to be identified unambiguously, and these losses are seen to be strongly correlated with the MHD activity.…”

“…The additional flexibility offered by the NBI system on the forthcoming MAST-Upgrade device, which will have the capability to inject beams simultaneously on and off-axis, will allow a detailed investigation into the relationship between beam deposition profiles, MHD mode activity and FI transport [36].…”

Neutron rate estimates in MAST based on gyro-orbit modelling of fast ions

“…In particular, in MAST, a systematic discrepancy up to 40 % between predicted and measured fusion products (neutrons and protons) rates (with the predicted rates much higher than the measured ones) has been observed regardless of the plasma scenarios, i.e. both in MHD quiescent and non-quiescent plasma discharges the latter being characterized by large fast ion redistribution and losses [6][7][8]. The interested reader can find a detailed discussion of the observed fusion products discrepancy in presence of strong MHD activity in section 4.2 of [1].…”

Impact of pressure anisotropy on magnetic configuration and stability

“…In this work we apply these advances in theory and computational models to study mode activity in the UK Mega Ampere Spherical Tokamak (MAST). MAST discharge #29221, produced during a power-density scan set of experiments [16], was a 3.1 MW two-beam heated plasma (a southsouth (SS) 'supercusp' beam operating at 1.5 MW and a south-west (SW) 'chequerboard' beam operating at 1.6 MW) with a plasma current of 0.9 MA and normalised beta 3 n β ≈ . Figure 1 is time trace of the evolution of the discharge, and figure 2 a spectrogram of magnetic perturbation coil data.…”

“…The magnetics reveal a rich range of activity, including: suspected low-frequency (10 kHz) tearing modes at 170 ms; toroidal Alfvén eigenmodes (TAEs) from 150 ms to 220 ms with frequency ramping down from 100 kHz to 50 kHz; chirping-down fishbones from 250 ms to 300 ms; and beyond 280 ms, longlived mode activity. Keeling et al [16] focused on the fishbone phase just prior to long-lived mode activity. Our focus is the TAE mode activity at 190 ms, where a high fidelity TRANSP simulation is available.…”