Assessing the merits of resonant magnetic perturbations with different toroidal mode numbers for controlling edge localised modes

Abstract

Document VersionPublisher's PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publicationCitation for published version (APA): Chapman, I. T., Kirk, A., Akers, R. J., Ham, C. J., Harrison, J. R., Hawke, J. N., ... Thorton, A. J. (2014). Assessing the merits of resonant magnetic perturbations with different toroidal mode numbers for controlling edge localised modes. Nuclear Fusion, 54(12), 123003-1/15. [123003]. DOI: 10.1088/0029-5515/54/12/123003 General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. AbstractAn increase in ELM frequency has been demonstrated in MAST by applying resonant magnetic perturbations (RMPs) with toroidal mode number, n RMP = 2, 3, 4, 6. It has been observed that the mitigated ELM frequency increases with the amplitude of the applied field provided it is above a critical threshold. This threshold value depends on the mode number of the RMP, with higher n RMP having a larger critical value. For the same ELM frequency, the reduction in the peak heat load on the divertor plates is approximately the same for all RMP configurations. The RMPs give rise to perturbations to the plasma shape, with lobe structures occurring due to the tangled magnetic fields near the X-point, and corrugations of the plasma boundary at the midplane. The X-point lobe length increases linearly with the applied field when above a threshold, with RMPs of higher toroidal mode number giving rise to longer lobes for the same applied resonant field. Similarly, the midplane displacements increase with the applied field strength, though the corrugation amplitude is less dependent upon the RMP configuration. For all n RMP , the RMPs ...

Overview of MAST results

“…Whilst n RMP = 3 cause less radially extended lobe structures near the X-point [12-14, 29, 30] as seen in figure 2, they are more poloidally extended, so may interact more with the first wall. n RMP = 3 fields also cause the smallest non-axisymmetric corrugation of the boundary [21,[25][26][27], as seen in figure 3. This corrugation is found to have a destabilizing effect on infiniten ballooning mode growth rate in the pedestal region for different RMP configurations, as shown in figure 4.…”

“…Consequently, for cases with ELM mitigation rather than complete ELM suppression, a configuration which maximizes the ELM frequency will mitigate the peak heat flux most effectively. Above a threshold value, the ELM frequency scales linearly with the applied field [21]. The fact that the threshold value for an increase in the ELM frequency increases with n RMP in MAST suggests that n RMP = 3 is more likely to achieve a requisite level of ELM mitigation in ITER than higher n RMP .…”

Effect of resonant magnetic perturbations on low collisionality discharges in MAST and a comparison with ASDEX Upgrade

“…For the n RMP =2 and 3 perturbations the rotation braking and density pump out is so large that it leads to a back transition to L-mode. Studies on the effect that RMPs applied before the L-H transition have on the power required to access H-mode have shown a non-monotonic increase with n RMP [12], with the least impact on the power required to access H-mode occurring for n RMP = 4 [13].…”

Edge plasma properties with 3D magnetic perturbations in RFX-mod

“…With this diagnostic it is possible to measure the time evolution of edge n e and T e at / ≈ r a 0.98, observing the line intensities of three lines of He I [26][27][28]. In RFX-mod the THB is only used during RFP discharges, since tokamak ones, being at a low current (≈150 kA), are also at a low density ( < − n 10 m e 18 3 in the edge), with too low a signal-to-noise ratio. In order to complete the poloidal characterization of the edge of the plasma, the link between the electron pressure P e and the helical angle u is studied for the two poloidal positions.…”

“…In particular, MPs in tokamaks and stellarators are used to control the plasma-wall interaction (PWI), to reduce power fluxes to the divertor plates, to mitigate or suppress edge-localized modes (ELMs) and for runaway electron control [1,2]. Despite this, the action of MPs on the edge plasma is far from being well understood: the effect on the PWI and ELM mitigation/suppression depends on the applied helicity of the MP, on the phase between the coils and on the shape of the plasma [3][4][5], and a complete physical interpretation has not been developed up to now. Moreover the applied magnetic field couples with the plasma, that reacts by generating a current and a magnetic field itself (plasma response), and resonant modes can couple to the kink ones, generating a complex spectrum of δB r , as shown in [6].…”