SHEAR ALFVEN MODES IN THE PRESENCE OF A MAGNETIC ISLAND

Shear Alfven wave dynamics is studied in the presence of a magnetic island in tokamak plasmas. Geometrical effects due to the shape of the flux surface’s cross section are retained to all orders, whereas tokamak curvature is considered only in the creation of the beta-induced Alfven eigenmode (BAE) gap in the low-frequency part of the continuous spectrum. A continuous spectrum very similar to that of the whole tokamak plasma is found inside the island, where the coupling between modes with different mode numbers creates frequency gaps. In particular, a very large ellipticity induced Alfven eigenmode (EAE) gap is found, due to the strong eccentricity of the island’s cross section. The nonlinearly modified frequency of the BAE continuum accumulation point is also calculated. The magnetic-island induced AE (MiAE) dispersion relation is computed in the limit of small eccentricity flux surfaces. Moreover, the BAE dispersion relation is also computed, in the limit of small magnetic islands.