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In a cylindrical plasma, tearing modes can be calculated by asymptotic matching of ideal MHD solutions across a critical layer. This requires a quantity delta' which represents the 'discontinuity' in the ideal solution across the layer. In a torus, poloidal harmonics are coupled and there are many critical surfaces for each toroidal mode number, and correspondingly many discontinuities delta'(m). The ideal MHD solutions do not determine the delta'(m) but only a relation between them - described by an "E-matrix". The calculation of an E-matrix is discussed for a large aspect-ratio tokamak. In a weak-coupling approximation it is tri-diagonal and can be computed from integrals over the uncoupled eigenfunctions or from simple "basis-functions" comprising triplets of coupled poloidal harmonics. This weak coupling approximation fails if delta'(m) is already small for an uncoupled harmonic. An alternative strong-coupling approximation is developed for this case.

Additional information

Authors: CONNOR J W, Culham Laboratory, Abingdon, Oxon. (GB);HASTIE R J, Culham Laboratory, Abingdon, Oxon. (GB);TAYLOR J B, Institute for Fusion Studies, The University of Texas at Austin, Austin (US)
Bibliographic Reference: Report: CLM-P884 EN (1990) 14 pp.
Availability: Available from the Librarian, UKAEA, Culham Laboratory, Abingdon, Oxon. OX14 3DB (GB)
Record Number: 199011113 / Last updated on: 1994-12-01
Original language: en
Available languages: en