EXCITATION OF TAE INSTABILITY BY VELOCITY ANISOTROPY OF FAST IONS

The excitation of toroidal Alfven eigenmodes (TAE) by a beam of high e nergy ions is studied in the framework of local theory and the instabi lity associated with the velocity space anisotropy of a fast ion distr ibution function is investigated. A stability criterion in terms of th e injection angle and plasma parameters is obtained. Pitch angle scatt ering, plasma temperature and finite drift orbit width effects on the TAE stability are discussed. It is shown that the anisotropy instabili ty drive can be stronger than the spatial gradient drive for typical v alues of the beam and background plasma para meters. The TAE instabili ty observed in experiments with neutral beam injection on DIII-D is an alysed and explanations of some experimental features are proposed.