Description
Edge Localised Modes (ELMs) - a periodic MHD instability ubiquitous in H-mode tokamak plasmas - are projected to degrade the plasma facing components of reactor scale devices, and so pose a threat to the viability of tokamak based commercial fusion power[2]. By applying Resonant Magnetic Perturbations (RMPs) ELMs can be mitigated, in which the ELM frequency is increased and energy content decreased, or suppressed, in which the ELMs vanish entirely and the peaked heat loads of ELMs are replaced by continuous transport[3]. ELM suppression has been demonstrated on many conventional aspect ratio tokamaks, but full suppression was not achieved in any spherical tokamak despite concerted efforts[4], posing a challenge in extrapolating suppression access from current tokamaks to the power plant scale. This work reports the first instances of RMP induced ELM suppression observed on a spherical tokamak – MAST Upgrade – and its interpretation supported by linear MHD modelling of the plasma response to the applied field using the MARS-F code. The application of a dominantly n=3 RMP caused a global deceleration of the toroidal rotation, density pump out and the disappearance of ELM activity, in a stationary ELM suppressed regime maintained for over 300ms. The core electron temperature was unaffected, and plasma pressure stabilised at βN~2.0. While the ne and Te pedestal heights were reduced, H mode was maintained. The evolutions of these and further experiments to explore this regime, and modelling results, will be reported.
[1] J R Harrison et al, 2019, Nuclear Fusion, 59, 112011
[2] A Loarte et al, 2003, Plasma Physics and Controlled Fusion, 45, 1549–1569
[3] T E Evans et al, 2004, Physical Review Letters, 92, 235003
[4] A Kirk et al, 2013, Plasma Physics and Controlled Fusion, 55, 124003