Overview of ELM Control by Low n Magnetic Perturbations on JET

Abstract

A series of ELM control experiments have been performed on JET aiming at a better understanding of the plasma response to applied magnetic perturbations. The dynamics of the edge profiles with applied n = 1 fields have been studied in the type-I ELM H-mode plasmas. Typically, the pedestal density decreased by about 20 % when the n = 1 perturbation field was applied (So called pump-out effect). However, there is no influence of the n = 1 fields on the recovery rate of the pedestal pressure, but the ELM crash occurs earlier and at a lower level of the pedestal pressure and pressure gradient. The compensation of the density pump-out has been demonstrated using either gas fuelling or pellets injection in low triangularity H-mode plasmas. Strong toroidal rotation braking by more than 60 % has been observed, and found to be independent on the safety factor. The calculated Neoclassical Toroidal Viscosity (NTV) torque profile in the ν regime including the boundary effect is in agreement with the observed torque profile induced by the n = 1 fields on JET. No complete ELM suppression was observed by application of either n = 1 or n = 2 fields with a Chirikov parameter above one at √ Ψ ≥ 0.925, which is one of the important criteria for the design of ITER ELM control coils.

ELM control strategies and tools: status and potential for ITER

“…Instead, the frequency of type-I ELMs was observed to increase by more than a factor of 4 resulting in a corresponding decrease of W ELM /W. The largest type-I ELM frequency increases were obtained over narrow operational widows in terms of q 95, resembling a resonant-like behaviour as required in DIII-D for complete ELM suppression [64,65]. Applying resonant n = 3 perturbation allowed to introduce controlled but delayed (about 10 ms with respect to the pulsed coil current) ELM triggering in otherwise ELM-free lithiumenhanced phases at NSTX [66] or transition from low frequency type-I ELMs to smaller more frequent ELMs on MAST at n=3 [67].…”

“…The major issue that needs to be studied in detail for ITER is to which level ELM control by RMPs affects the divertor conditions for high density/high radiation conditions. Initial results from high density operation in DIII-D [65], NSTX [92], ASDEX-Upgrade [57] and JET [69] and high divertor radiation in DIII-D [93], indicate that a high density/highly radiating divertor can be obtained with controlled ELMs although elimination of type-I ELMs has only been demonstrated in ASDEX-Upgrade and DIII-D for high collisionality conditions so far. (d) Sustainment of ELM control/suppression by edge magnetic field perturbation in conditions with varying q 95 .…”

Pellet refuelling of particle loss due to ELM mitigation with RMPs in the ASDEX Upgrade tokamak at low collisionality

“…However the H-mode is typically accompanied by Edge Localised Modes (ELMs) which are not compatible with the long term divertor operation and therefore ELM control has to be considered [1]. One of the ELM control techniques is the application of Resonant Magnetic Perturbations (RMPs) produced by an external set of magnetic coils [2,3,4,5] and such a system will be installed on ITER.…”

Pellet fuelling with edge-localized modes controlled by external magnetic perturbations in MAST

“…Experiments with simultaneous pellet fuelling and ELM mitigation are rare and results are mixed. Some data are favourable and fuelling pellets do not affect the ELM mitigation [10,11,12], as it is assumed by ITER. In some experiments, however, fuelling pellets are followed by large ELMs [12,13] which can promptly remove pellet particles what would be highly unfavourable for ITER.…”