Edge Localised Modes (ELMs): Experiments and Theory
- EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxon, OX14 3DB (United Kingdom)
- University of York, Heslington, York, UK, YO10 5DD (United Kingdom)
Edge Localised Modes (ELMs) are periodic disturbances of the plasma periphery occurring in tokamaks with an H-mode edge transport barrier. As a result, a fraction of the plasma energy present in the confined hot edge plasma is transferred to the open field lines in the divertor region, ultimately appearing at the divertor target plates. These events can result in high transient heat loads being deposited on the divertor target plates in large tokamaks, potentially causing damage in devices such as ITER. Consequently it is important to find means to mitigate their effects, either avoiding them or, at least, controlling them. This in turn means it is essential to understand the physics causing ELMs so that appropriate steps can be taken. It is generally agreed that ELMs originate as MHD instability caused by the steep plasma pressure gradients or edge plasma current present in H-mode, the so-called 'peeling-ballooning' model. Normally this is considered to be an ideal MHD instability but resistivity may be involved. Much less clear is the non-linear evolution of these instabilities and the mechanisms by which the confined edge plasma is transferred to the divertor plasma. There is evidence for the non-linear development of 'filamentary' structures predicted by theory, but the reconnection processes by which these are detached from the plasma core remain uncertain. In this paper the experimental and theoretical evidence for the peeling-ballooning model is presented, drawing data from a number of tokamaks, e.g. JET, DIII-D, ASDEX-Upgrade, MAST etc. Some theoretical models for the non-linear evolution of ELMs are discussed; as well as ones related to the 'peeling-ballooning' model, other candidate models for the ELM cycle are mentioned. The consequential heat loads on divertor target plates are discussed. Based on our current understanding of the physics of ELMs, means to avoid them, or mitigate their consequences, are described, e.g. the use of plasma shaping or introducing resonant magnetic perturbation coils to reduce plasma gradients at the plasma edge.
- OSTI ID:
- 21143420
- Journal Information:
- AIP Conference Proceedings, Vol. 1013, Issue 1; Conference: 1. ITER international summer school on turbulent transport in fusion plasmas, Aix en Provence (France), 16-20 Jul 2007; Other Information: DOI: 10.1063/1.2939030; (c) 2008 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-243X
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ASDEX TOKAMAK
BALLOONING INSTABILITY
BOUNDARY LAYERS
DISTURBANCES
DIVERTORS
DOUBLET-3 DEVICE
EDGE LOCALIZED MODES
ELECTRIC CURRENTS
H-MODE PLASMA CONFINEMENT
HEATING LOAD
ITER TOKAMAK
JET TOKAMAK
MAGNETOHYDRODYNAMICS
MAST TOKAMAK
NONLINEAR PROBLEMS
PLASMA
PLASMA PRESSURE
RADIATION TRANSPORT