Bi-directional Alfven cyclotron instabilities in the mega-amp spherical tokamak
2014 (English)In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 21, no 8, 082501- p.Article in journal (Refereed) Published
Alfven cyclotron instabilities excited by velocity gradients of energetic beam ions were investigated in MAST experiments with super-Alfvenic neutral beam injection over a wide range of toroidal magnetic fields from similar to 0.34 T to similar to 0.585 T. In MAST discharges with high magnetic field, a discrete spectrum of modes in the sub-cyclotron frequency range is excited toroidally propagating counter to the beam and plasma current (toroidal mode numbers n < 0). At lower magnetic field <= 0.45 T, a discrete spectrum of Compressional Alfven Eigenmodes (CAEs) with n > 0 arises, in addition to the modes with n < 0. At lowest magnetic fields, the CAEs with n > 0 become dominant, they are observed in frequency range from similar to 250 kHz for n = 1 to similar to 3.5 MHz for n = 15, well above the on-axis ion cyclotron frequency (similar to 2.5 MHz). The data is interpreted in terms of normal and anomalous Doppler resonances modified by magnetic drift terms due to inhomogeneity and curvature of the magnetic field. A Hall MHD model is applied for computing the eigenfrequencies and the spatial mode structure of CAEs and a good agreement with the experimental frequencies is found.
Place, publisher, year, edition, pages
2014. Vol. 21, no 8, 082501- p.
IdentifiersURN: urn:nbn:se:uu:diva-236102DOI: 10.1063/1.4891322ISI: 000342760600029OAI: oai:DiVA.org:uu-236102DiVA: diva2:763375
Correction in: Physics of Plasmas, 22, 049901 (2015); http://dx.doi.org/10.1063/1.49169422014-11-142014-11-122015-06-30Bibliographically approved