It is shown numerically that even a weak deviation from the axial symmetry of the magnetic configuration in tokamaks can drastically change the spatial structure of toroidicity-induced Alfvén eigenmodes (TAEs), turning them into modes characterized by strong dependence of the amplitude on the toroidal coordinate. The reason for this lies in the fact that the TAEs are close to degeneracy: TAEs with different toroidal mode numbers (n) may have almost the same frequency. A condition necessary for a certain steady-state non-axisymmetric harmonic of the magnetic field to couple TAEs and ellipticity-induced Alfvén eigenmodes (EAEs) with different n (the 'selection rule') is obtained. In particular, a coupling harmonic of significant amplitude can be found more easily in a stellarator with large rotational transform and a small number of field periods. In tokamaks, any magnetic island satisfies the selection rule and can thus couple TAEs. This may explain why multiple modes with different n often appear and disappear simultaneously in TAE bursts. This may also explain an observation of a TAE with the amplitude varying in phase with a quasi-steady-state magnetic perturbation in the spherical torus NSTX.
Instantaneous difference frequency locking observed during toroidicity-induced Alfvén eigenmode coupling in the DIII-D tokamak