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Abstract

Substantial losses of plasma stored energy and toroidal ion momentum are observed during large amplitude oscillating or quasi-stationary MHD activity in JET when mode coupling effects become important. The reduction in the diamagnetic stored energy due to low m, n MHD modes increases with mode amplitude. Favourable comparisons are made with the degradation in incremental energy confinement time predicted by Chang and Callen during such MHD activity. The reduction in the plasma ion toroidal momentum, from charge exchange measurements on C6+ ions, depends on the extent of mode coupling within the plasma and on the oscillation frequency of the n=1 mode. When b(r)/B(0)> 0.1% for more than about 300 msec, toroidal coupling between low m, n modes together with a force coupling the plasma ions to the modes, equilibrates the toroidal ion rotation frequency with the MHD oscillation frequency over substantial regions of the plasma, depending on the radius of the rational q surface of the coupled MHD mode. This ion-mode coupling force becomes particularly apparent when the mode frequency drops to nearly zero and the ion toroidal rotation frequency also drops to zero within 100 - 300 msec, despite continued neutral beam injection. In such cases, the toroidal ion momentum appears to be lost electromagnetically via the MHD modes to the external structure or fixed stray fields of the tokamak, while the plasma stored energy losses must be explained by other processes.

Additional information

Authors: VON HELLERMANN M, JET Joint;CAMPBELL D J, JET Joint;HENDER T C, ;SNIPES J A, ;WEISEN H,
Bibliographic Reference: Report: JET-P(89)64 EN (1990) 31pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
Record Number: 199010999 / Last updated on: 1994-12-01
Category: PUBLICATION
Original language: en
Available languages: en