Excitation of TAE instability by velocity anisotropy of fast ions

The excitation of toroidal Alfven eigenmodes (TAE) by a beam of high energy ions is studied in the framework of local theory and the instability associated with the velocity space anisotropy of a fast ion distribution function is investigated. A stability criterion in terms of the injection angle and plasma parameters is obtained. Pitch angle scattering, plasma temperature and finite drift orbit width effects on the TAE stability are discussed. It is shown that the anisotropy instability drive can be stronger than the spatial gradient drive for typical values of the beam and background plasma parameters. The TAE instability observed in experiments with neutral beam injection on DIII-D is analysed and explanations of some experimental features are proposed. (author). Letter-to-the-editor. 11 refs, 3 figs