Experimental validation of coil phase parametrisation on ASDEX Upgrade, and extension to ITER

David Anthony Ryan, Yueqiang Liu, A. Kirk, W Suttrop, Benjamin Daniel Dudson, M Dunne, M Willensdorfer

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It has been previously demonstrated in (Li et al 2016 Nuclear Fusion 56 126007)
that the optimum upper/lower coil phase shift ∆Φopt for alignment of RMP coils for ELM
mitigation depends sensitively on q95, and other equilibrium plasma parameters. Therefore,
∆Φopt is expected to vary widely during the current ramp of ITER plasmas, with negative
implications for ELM mitigation during this period. A previously derived and numerically
benchmarked parametrisation of the coil phase for optimal ELM mitigation on ASDEX
Upgrade (Ryan et al 2017 Plas. Phys. Cont. Fus. 59 024005) is validated against experimental
measurements of ∆Φopt, made by observing the changes to the ELM frequency as the coil
phase is scanned. It is shown that the parametrisation may predict the optimal coil phase
to within 32 degrees of the experimental measurement for n = 2 applied perturbations.
It is explained that this agreement is sufficient to ensure that the ELM mitigation is not
compromised by poor coil alignment. It is also found that the phase which maximises
ELM mitigation is shifted from the phase which maximizes density pump-out, in contrast
to theoretical expectations that ELM mitigation and density pump out have the same ∆Φul
dependence. A time lag between the ELM frequency response and density response to the
RMP is suggested as the cause. The method for numerically deriving the parametrisation is
repeated for the ITER coil set, using the baseline scenario as a reference equilibrium, and the
parametrisation coefficients given for future use in a feedback coil alignment system. The
relative merits of square or sinusoidal toroidal current waveforms for ELM mitigation are
briefly discussed.
Original languageEnglish
Article number065005
Number of pages15
JournalPlasma Physics and Controlled Fusion
Issue number6
Publication statusPublished - 13 Apr 2018

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