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Intrinsic ELMing in ASDEX Upgrade and global control system-plasma self-entrainment

Chapman, S. C., Lang, P. T., Dendy, R. O., Watkins, N. W., Dunne, M. and Giannone, L. (2018) Intrinsic ELMing in ASDEX Upgrade and global control system-plasma self-entrainment. Nuclear Fusion, 58 (12). p. 126003. ISSN 0029-5515

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Identification Number: 10.1088/1741-4326/aadd1c

Abstract

It is well established that edge localized modes can be entrained to the frequency of applied global magnetic perturbations. These perturbations are delivered to the plasma using the vertical control system field coil currents. These field coils are part of an active control system that is required to maintain the plasma in a steady state. We perform time domain timeseries analysis of natural ELMing when there are no applied perturbations in the ASDEX Upgrade tokamak. We find that the plasma can transition into a state in which the control system field coil currents continually oscillate and are synchronized with oscillations in characteristic plasma parameters such as plasma edge position and total MHD energy. These synchronous oscillations have a one-to-one correlation with the naturally occurring ELMs; the ELMs all occur when the control system coil current is around a specific temporal phase. Large and small ELMs may be distinguished by the amplitude of inward movement of the edge following an ELM. Large ELMs are then found to occur preferentially around a specific temporal phase of the vertical position control coil current. Small ELMs are most likely in antiphase to this. The large and small natural ELMs occur at the opposite extrema of the oscillations in the control system vertical position control coil current. The control system coil current phase may thus provide a useful parameter to order the observed ELM dynamics. We have identified a class of natural ELMing which is a self-entrained state, in which there is a continual non-linear feedback between the global plasma dynamics and the active control system that is intrinsic to the cyclic dynamics of naturally occurring ELMs. Control system-plasma feedback thus becomes an essential component for integration into future models of natural ELM dynamics.

Item Type: Article
Official URL: http://iopscience.iop.org/journal/0029-5515
Additional Information: © 2018 EURATOM © CC BY 3.0
Divisions: Centre for Analysis of Time Series
Subjects: Q Science > QC Physics
Sets: Research centres and groups > Centre for the Analysis of Time Series (CATS)
Date Deposited: 01 Nov 2018 09:52
Last Modified: 20 Feb 2019 06:55
Projects: 633053, EP/P012450/1, FA9550-17-1-0054
Funders: Euratom research and training programme, RCUK Energy Programme, Air Force Office of Scientific Research
URI: http://eprints.lse.ac.uk/id/eprint/90529

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