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Shear Alfv'en wave continuous spectrum in the presence of a magnetic island

The continuous spectrum of shear Alfvén waves is calculated for a finite-beta tokamak equilibrium in the presence of a finite-size magnetic island. The beta-induced Alfvén Eigenmodes continuum accumulation point (BAE-CAP) is found to be positioned at the separatrix flux surface, while the frequency remains the same. The most remarkable novel feature is the presence of new CAPs at the island O-point, which give rise to gaps in the continuous spectrum. This could make the existence of new magnetic-island induced Alfvén Eigenmodes (MiAE) possible, excited via wave-particle resonances if the island is sufficiently wide with respect to the mode radial localization. Experimental data of modes observed in Frascati Tokamak Upgrade (FTU) are presented, which are consistent with theoretical scalings of the MiAE-CAPs. Due to the frequency dependence on the magnetic island size, the feasibility of utilizing this scaling as a novel magnetic island diagnostic is also discussed.