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Two-ion ICR (Ion Cyclotron Resonance) heating in a tokamak can produce superthermal minority ions having orbital radii comparable with the plasma radius. The drift motion causes the trapped fast ions to precess around the torus, thus forming a fast ion current. A semi-analytical model and a Monte-Carlo RF orbit code have been used to calculate the direction, magnitude and profile of this current. The effect of the fast ion current found is to flatten the q-profile within the region of the inversion radius of MHD sawteeth, where sawtooth stability is expected theoretically to be sensitive to the radial profile of current density. The fast ion current could explain the observed sawtooth stabilisation during ICRH. RF heated JET data have been examined for evidence of a correlation between the natural duration of sawtooth-free periods and the expected magnitude of the off-axis fast ion current density. The data show a threshold current density below which sawteeth are not stabilised. Above this threshold, the duration of sawtooth-stable periods increases monotonically with the magnitude of the off-axis RF fast ion current density.

Additional information

Authors: CORE W G, JET Joint Undertaking, Abingdon, Oxon. (GB);COTTRELL G A, JET Joint Undertaking, Abingdon, Oxon. (GB)
Bibliographic Reference: Report: JET-P(92)25 EN (1992) 12 pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
Record Number: 199211294 / Last updated on: 1994-11-29
Original language: en
Available languages: en
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