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Neoclassical magnetic islands are observed to limit the achievable beta in COMPASS-D low collisionality single-null divertor tokamak plasmas with ITER-like geometry. The limiting beta is typically well below that expected from ideal instabilities with maximum beta(N) in the range of 1.6-2.1. The plasma is heated with up to 1.8MW of 60GHz electron cyclotron resonance heating (ECRH) at 2nd harmonic with X-mode polarization. The time history of the measured island width is compared to the predictions of neoclassical tearing mode theory. Applied resonant helical error fields are used to occur naturally, allowing the study of the behaviour of neoclassical tearing modes in this regime. The critical beta for the onset of neoclassical tearing modes is seen to be approximately 3 times higher in the naturally stable region. A simple expression for the q-scaling of the maximum achievable beta(N) in the presence of neoclassical tearing modes is derived. The predicted q-scaling of this beta-limit is compared to data from a q-scan and the results are in good agreement.

Additional information

Authors: GATES D A ET AL, AEA Fusion, Culham Laboratory, Abingdon, Oxon (GB)
Bibliographic Reference: Report: UKAEA FUS 362 EN (1997) 16pp.
Availability: Available from the Librarian, UKAEA, Culham Laboratory, Abingdon, Oxon, OX14 3DB (GB)
Record Number: 199710927 / Last updated on: 1997-07-23
Original language: en
Available languages: en
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