Bi-directional Alfv\'en Cyclotron Instabilities in the Mega-Amp   Spherical Tokamak
  Alfv\'en cyclotron instabilities excited by velocity gradients of energetic beam ions were investigated in MAST experiments with super-Alfv\'enic NBI 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). 
S.E. Sharapov, M.K.Lilley, R.Akers, N. Ben Ayed, M.Cecconello,   J.W.C.Cook, G.Cunningham, E.Verwichte and the MAST Tea
physics.plasm-ph