Towards understanding ELM mitigation: the effect of axisymmetric lobe structures near the X-point on ELM stability

The application of resonant magnetic perturbations (RMPs) with toroidal mode number n = 4 or n = 6 is observed to result in a significant increase in ELM frequency, despite a reduction in the edge pressure gradient. A picture for how type-I ELMs are mitigated, or destabilized, when magnetic perturbations are applied is proposed. Despite the magnetic perturbation incurring a drop in the pedestal pressure gradient, the critical pressure for an ELM to be triggered is dropped even further. Various mechanisms which could cause this degradation of the stability boundary are outlined. The stability of the edge of MAST H-mode plasmas has been tested when lobe structures are present in the separatrix after application of RMPs. The axisymmetric stability analysis presented here shows that when the plasma boundary is simulated with a model for the observed lobe structures included, the ballooning stability is degraded. This degradation in ballooning stability originates from the perturbed field lines dwelling in the region of unfavourable curvature due to the presence of lobe structures rather than the change in the plasma boundary shape.