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Localised and intense Ion Cyclotron Radio Frequency Heating (ICRH) of two-ion species plasmas in the JET tokamak produces minority ion distributions having characteristic energies in the multi-MeV range, similar to that of fusion alpha-particles. For typical values of the poloidal magnetic field, the size of the particle orbits can exceed significantly the width of the RF deposition profile. In the presence of a negative radial gradient in the ion-electron slowing-down time, the finite orbit-width effect gives rise to a significant reduction in the calculated energy content of the minority particles. A model is described which has been compared with RF heating data from the JET tokamak with both hydrogen and 3He minority ion species. The model is capable of explaining previously observed discrepancies between measured fast ion energy contents and those which have been calculated using a zero orbit-width model. The largest corrections to the global energy content of the fast ions are about 56% and are found for low-current hydrogen minority cases in which the axial ion-electron slowing-down time is long (about 1.5 s). Within the experimental scatter, the upper limit of D(fast) is 0.18 m2/s on the fast-ion diffusion coefficient for any non-classical energy-loss processes.

Additional information

Authors: COTTRELL G A, JET Joint Undertaking, Abingdon, Oxon. (GB);START D F H, JET Joint Undertaking, Abingdon, Oxon. (GB)
Bibliographic Reference: Report: JET-P(90)17 EN (1990) 22 pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
Record Number: 199011338 / Last updated on: 1994-12-01
Original language: en
Available languages: en