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Title: Bi-directional Alfvén cyclotron instabilities in the mega-amp spherical tokamak

Alfvén cyclotron instabilities excited by velocity gradients of energetic beam ions were investigated in MAST experiments with super-Alfvénic neutral beam injection over a wide range of toroidal magnetic fields from ∼0.34 T 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 Alfvén 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 ∼250 kHz for n=1 to ∼3.5 MHz for n=15, well above the on-axis ion cyclotron frequency (∼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.
Authors:
; ; ;  [1] ;  [2] ;  [3] ; ;  [4]
  1. CCFE, Culham Science Centre, Abingdon OX14 3DB (United Kingdom)
  2. Physics Department, Imperial College, London SW7 2AZ (United Kingdom)
  3. Department of Physics and Astronomy, Uppsala University, SE-75105 Uppsala (Sweden)
  4. Centre for Fusion, Space and Astrophysics, Department of Physics, University of Warwick, Coventry CV4 7AL (United Kingdom)
Publication Date:
OSTI Identifier:
22303771
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 21; Journal Issue: 8; Other Information: (c) 2014 EURATOM; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; BEAM INJECTION; COMPRESSED AIR ENERGY STORAGE; CYCLOTRON FREQUENCY; CYCLOTRON INSTABILITY; EIGENFREQUENCY; ELECTRIC CURRENTS; IONS; MAGNETIC FIELDS; MAGNETOHYDRODYNAMICS; MAST TOKAMAK; MHZ RANGE 01-100; SPHERICAL CONFIGURATION