Details

IT01 · Alfvén cascades in JET discharges with NBI-heating

S.E. Sharapov¹, B. Alper¹, Yu.F. Baranov¹, H.L. Berk², D. Borba³, C. Boswell⁴, B.N. Breizman², C.D. Challis¹, M. de Baar⁵, E. De La Luna⁶, E.A. Evangelidis⁷,S.Hacquin³, N.C. Hawkes¹, V.G. Kiptily¹, S.D. Pinches⁸, P. Sandquist⁹, I. Voitsekhovich¹, N.P. Young^1,10, and JET-EFDA Contributors*

¹Euratom/UKAEA Fusion Association, Culham Science Centre, Abingdon OX14 3DB, UK
²Institute Fusion Studies, University of Texas at Austin, Austin, USA
³Euratom/IST Fusion Association, Centro de Fusao Nuclear, Lisboa, Portugal
⁴PSFC, Massachussets Institite of Technology, Cambridge, USA
⁵Association Euratom-FOM Institute Plasmaphysics ‘Rijnhuizen’, Nieuwegein, The Netherlands
⁶Association Euratom-CIEMAT, Lab. National de Fusion, Madrid, Spain
⁷Euratom/Helenic Fusion Assoc., Democritos University of Thrase, Xanthi, Greece
⁸Max Planck Institute fur Plasmaphysik, Euratom Association, Garching, Germany
⁹Euratom/VR Fusion Association, Chalmers Univ. of Technology, Goteborg, Sweden
¹⁰University of Warwick, Coventry, UK
* See the Appendix of J.Pamela et al., Proc. 20th IAEA Fusion Energy Conference 2004 (Vilamoura, 2004) (Vienna:IAEA)
e-mail contact of main author: sergei.sharapov@jet.uk

Abstract: Alfvén cascade (AC) eigenmodes excited by energetic ions accelerated with ion-cyclotron resonance heating in JET reversed-shear discharges are studied experimentally in high-density plasmas fuelled by neutral beam injection (NBI) and by deuterium pellets. The recently developed O-mode interferometry technique and Mirnov coils are employed for detecting ACs. The spontaneous improvements of plasma confinement (internal transport barrier (ITB)triggering events) and grand ACs are found to correlate within 0.2 sec in JET plasmas with densities up to ~ 5x10¹⁹ m^-3. Measurements with high time resolution show that ITB triggering events happen before “grand” ACs in the majority of JET discharges, indicating that this improvement of confinement is likely to be associated with the decrease in density of rational magnetic surfaces just before q (t ) min passes an integer value. Experimentally observed ACs excited by sub-Alfvénic NBI-produced ions with parallel velocities as low as V_{II NBI} ~ 0.2>V_A are found to be most likely associated with the geodesic acoustic effect that significantly modifies the shear-Alfvén dispersion relation at low frequency. Experiments were performed with a tritium NBI-blip (short time pulse) into JET plasmas with NBI-driven ACs. Although considerable NBI-driven AC activity was present, a good agreement was found both in the radial profile and in the time evolution of DT neutrons between the neutron measurements and TRANSP code modelling based on the Coulomb collision model, indicating the ACs have at most a small effect on fast particle confinement in this case.

Note: In special issue in Nuclear Fusion - volume 46, issue 10
http://www.iop.org/EJ/toc/0029-5515/46/10

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