Mitigation of MHD induced fast-ion redistribution in MAST and implications for MAST-Upgrade design
2015 (English)In: Nuclear Fusion, ISSN 0029-5515, E-ISSN 1741-4326, Vol. 55, no 1, 013021Article in journal (Refereed) Published
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. Fusion 53 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.
Place, publisher, year, edition, pages
2015. Vol. 55, no 1, 013021
neutral-beam injection, MAST, MHD, fast ions
IdentifiersURN: urn:nbn:se:uu:diva-259181DOI: 10.1088/0029-5515/55/1/013021ISI: 000357595900015OAI: oai:DiVA.org:uu-259181DiVA: diva2:843580