Towards understanding edge localised mode mitigation by resonant magnetic perturbations in MAST

Abstract

Type-I Edge Localised Modes (ELMs) have been mitigated in MAST through the application of n = 3, 4 and 6 resonant magnetic perturbations (RMPs). For each toroidal mode number of the non-axisymmetric applied fields, the frequency of the ELMs has been increased significantly, and the peak heat flux on the divertor plates reduced commensurately. This increase in ELM frequency occurs despite a significant drop in the edge pressure gradient, which would be expected to stabilise the peeling-ballooning modes thought to be responsible for type-I ELMs. Various mechanisms which could cause a destabilisation of the peeling-ballooning modes are presented, including pedestal widening, plasma rotation braking, three dimensional corrugation of the plasma boundary and the existence of radially extended lobe structures near to the X-point. This leads to a model aimed at resolving the apparent dichotomy of ELM control, that is to say ELM suppression occurring due to the pedestal pressure reduction below the peeling-ballooning stability boundary, whilst the reduction in pressure can also lead to ELM mitigation, which is ostensibly a destabilisation of peeling-ballooning modes. In the case of ELM mitigation, the pedestal broadening, 3d corrugation or lobes near the X-point degrade ballooning stability so much that the pedestal recovers rapidly to cross the new stability boundary at lower pressure more frequently, whilst in the case of suppression, the plasma parameters are such that the particle transport reduces the edge pressure below the stability boundary which is only mildly affected by negligible rotation braking, small edge corrugation or short, broad lobe structures.

23 pages, 12 figures. Copyright (2013) United Kingdom Atomic Energy Authority. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics

References in corpus (4)

• Observation of lobes near the X-point in resonant magnetic perturbation experiments on MAST
• Effect of resonant magnetic perturbations with toroidal mode numbers of 4 and 6 on ELMs in single null H-mode plasmas in MAST
• Inter-ELM pedestal evolution on MAST and impact of Resonant Magnetic Perturbations
• Effect of resonant magnetic perturbations on ELMs in connected double null plasmas in MAST

Cited by in corpus (9)

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• Non-linear modeling of the plasma response to RMPs in ASDEX Upgrade
• Field-line localized destabilization of ballooning modes in 3D tokamaks
• Non-linear modeling of the threshold between ELM mitigation and ELM suppression by Resonant Magnetic Perturbations in ASDEX Upgrade
• Understanding the effect resonant magnetic perturbations have on ELMs
• Three dimensional boundary displacement due to stable ideal kink modes excited by external n=2 magnetic perturbations
• Pellet refuelling of particle loss due to ELM mitigation with RMPs in the ASDEX Upgrade tokamak at low collisionality
• Modelling of three dimensional equilibrium and stability of MAST plasmas with magnetic perturbations using VMEC and COBRA
• Nuclear Fusion: Bringing a star down to Earth