Generation of geodesic acoustic mode by nonlinear coupling of magnetic island and island-driven beta-induced Alfven eigenmode

Abstract

In a recent work, we have shown that the plasma flow around the magnetic island can excite the beta-induced Alfven eigenmode (BAE) (Marchenko et al 2016 Nucl. Fusion 56 106021). In the present communication, it is shown that coupling of this primary BAE and magnetic island generates secondary geodesic acoustic mode (GAM), which has the frequency and mode structure identical to those of the primary BAE. The fixed GAM/BAE amplitude ratio, determined by the plasma neutrality, is comparable with the plasma/magnetic pressure ratio. This nonlinear coupling can be responsible for axis-symmetric modes, which accompany islanddriven Alfvenic modes observed on HL-2A tokamak (Chen et al 2013 Nucl. Fusion 53 113010).