2ND-ORDER TOROIDICITY-INDUCED ALFVEN EIGENMODES AND MODE SPLITTING IN A LOW-ASPECT-RATIO TOKAMAK

A variational ideal magnetohydrodynamic stability code is used to compute the toroidicity-induced Alfven eigenmode (TAE) for toroidal mode number n = 1 in full toroidal geometry with circular cross section. At finite aspect ratio, toroidicity also couples poloidal mode numbers m with m + 2 to produce a higher frequency gap in the shear Alfven spectrum, which is associated with a new, second-order TAE mode. It is also found that toroidal coupling between the TAE modes and continuum branches induces splitting of the TAE modes into two or more global modes at slightly different frequencies. Both the new modes and splitting of the TAE modes have important consequences for the identification of TAE modes in experiments.