Destabilization of TAE modes by particle anisotropy

Plasmas heated by ICRF produce energetic particle distribution functions wh ich are sharply peaked in pitch-angle. At moderate toroidal mode numbers, t his anisotropy is the dominant instability drive when compared with the uni versal instability drive due to the spatial gradient. The universal drive, acting alone, destabilizes only co-propagating waves (i.e., waves propagati ng in the same toroidal direction as the diamagnetic flow of the energetic particles), but stabilizes counterpropagating waves (i.e., waves propagatin g in the toroidal direction opposite to that of the diamagnetic Bow of the energetic particles). Nonetheless, in a tokamak, it is possible that partic le anisotropy can produce a larger linear growth rate for counter-propagati ng waves, and provide a mechanism for preferred destabilization of the coun ter-propagating TAE modes that are sometimes experimentally observed. (C) 1 999 Elsevier Science B.V.