FINITE ORBIT WIDTH STABILIZING EFFECT ON TOROIDAL ALFVEN EIGENMODES EXCITED BY PASSING AND TRAPPED ENERGETIC IONS

A general expression for the growth rate of the toroidal Alfven eigenm ode (TAE) instability is derived by employing a Hamiltonian action-ang le approach. It takes into account the finite orbit width effects, the finite Larmor radius (FLR) effects of energetic particles and the eff ect of mode localization, as well as the possible mode excitation by b oth passing and trapped energetic ions. In particular, the stabilizing effect of the large particle orbit widths on the TAE modes excited by fast passing and trapped ions is investigated. In the limiting cases when Delta(m)((i)) much less than Delta(b)(t) much less than Delta(m)( (o)) and Delta(m)((o)), the instability drive is reduced by factors of Delta(m)((i))/Delta(b) and Delta(m)((i)) (Delta(m)((o)))(2)/Delta(b)( 3), respectively, as compared with the prediction of the narrow orbit theory, when Delta(b)(t) much less than Delta(m)((i),) where Delta(b) is the particle orbit width and Delta(m)((i)) (Delta(m)((o))) is the i nner (outer) width of the mode structure. For the TFTR parameters stab ilization effect is much stronger in the case of TAE excitation by fus ion-produced alpha particles than that in connection with neutral beam injection (NBI)-generated energetic ions.