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A neoclassical tearing mode (NTM) requires a finite size seed island to become unstable. Usually the local pressure gradient is relatively large at the beta values needed for these seed islands to destabilise the NTMs. Therefore the island has a large growth rate at mode onset and grows rapidly to its saturated island width. This width is proportional to beta as long as it is well above the marginal beta limit below which the mode is stable. The marginal beta limit is independent of the seed island trigger mechanism and provides detailed information on the stabilising terms in the modified Rutherford equation, which are not unambiguously determined theoretically. It is shown that in JET the marginal normalised beta limit for the 3/2 mode, beta(N),(marg), is of the order of 0.5 to 1 for magnetic fields between 3.3T and 1T, with q(95) � 3.3, and near the H-L transition. Therefore all H-modes with typical q profiles (q(95) � 3.3) are metastable in JET to 3/2 NTMs. In addition the marginal island width is of the order of 2-4cm and the stabilising terms are such that they influence the saturated island width when it is smaller than 4-6cm in these H-mode discharges. It is also shown that detailed analyses of the time evolution of the island width with slow beta ramp-down suggest that the convective form of the stabilisation term due to the "chi(perpendicular) model" is more appropriate and can explain the island decay between 4-6cm to the marginal island width.

Additional information

Authors: SAUTER O, Centre de Recherches en Physique des Plasmas, Association EURATOM-Confédération Suisse, Ecole Polytechnique Fédérale de Lausanne (CH);BUTTERY R J, EURATOM/UKAEA Fusion Association, Abingdon (GB);FELTON R, EURATOM/UKAEA Fusion Association, Abingdon (GB);HENDER T C, EURATOM/UKAEA Fusion Association, Abingdon (GB);HOWELL D F, EURATOM/UKAEA Fusion Association, Abingdon (GB)
Bibliographic Reference: Internal report LRP 731/02, July 2002. pp.28.
Availability: Available free of charge from: École Polytechnique Fédérale de Lausanne (EPFL), Ecublens, CH-1015 Lausanne Fax +41-21-6934747
ISBN: ISSN: 0458-5895
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