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Edge localised modes (ELMs) are observed during H-mode discharges in JET with widely varying amplitudes and repetition rates. They can be small and very frequent when an H-mode builds up, but as the edge electron temperature increases, they become larger and much less frequent. The larger, singular ELMs, are similar to the type III ELMs in DIII-D and ASDEX. Prior to these singular ELMs in JET, pressure gradients near the edge plasma are found to be well below the ideal ballooning stability limit, while resistive ballooning modes with medium to large toroidal mode numbers, greater than or equal to ten, are unstable. In the first part of this paper this analysis is extended to the fast events which sometimes terminate high-beta(p) or hot-ion phases in JET discharges, varying from large ELMs to global events in which central m = 1 activity and fast edge losses occur simultaneously. Asymptotic solutions of the ballooning equation obtained from a generalisation of the two-scale expansion method to flux surfaces with finite aspect ratio are then presented.

Additional information

Authors: DE BLANK H J, JET Joint Undertaking, Abingdon, Oxon. (GB);KERNER W, JET Joint Undertaking, Abingdon, Oxon. (GB);NAVE M F F, LNETI Sacavém (PT);HUYSMANS G T A, FOM Instituut voor Plasmafysica, Rijnhuizen, Nieuwegein (NL)
Bibliographic Reference: Report: JET-P(93)23 EN (1993) 8 pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
Record Number: 199310658 / Last updated on: 1994-11-29
Original language: en
Available languages: en
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