Studies of edge localized mode mitigation with new active in-vessel saddle coils in ASDEX Upgrade

Abstract

The ASDEX Upgrade tokamak is currently being enhanced with a set of invessel saddle coils for non-axisymmetric perturbations aiming at mitigation or suppression of edge localized modes (ELMs). Results obtained during the first experimental campaign are reported. With n = 2 magnetic perturbations, it is observed that type-I ELMs can be replaced by benign small ELM activity with strongly reduced energy loss from the confined plasma and power load to the divertor. During these phases with ELM mitigation, no density reduction (density 'pump-out') is observed. ELM mitigation has, so far, been observed in plasmas with different shape and different heating mixes and, therefore, different momentum input. The ELM mitigation regime can be accessed with resonant and non-resonant perturbation field configurations. The main threshold requirement appears to be a critical minimum plasma edge density which depends on plasma current. So far it is not possible to distinguish whether this is an edge collisionality threshold or a critical fraction of the Greenwald density limit.