2D continuous spectrum of shear Alfven waves in the presence of a magnetic island

TL;DR

This paper analyzes how magnetic islands in tokamak plasmas alter the shear Alfven wave spectrum, revealing new accumulation points and nonlinear frequency modifications relevant for plasma stability.

Contribution

It introduces a detailed calculation of the continuous spectrum of shear Alfven modes considering magnetic islands, including nonlinear effects on accumulation point frequencies.

Findings

New continuum accumulation points at the magnetic island's O-point.

Beta-induced Alfven Eigenmodes (BAE) accumulation point at the separatrix.

Nonlinear modification of the BAE continuum accumulation point frequency.

Abstract

The radial structure of the continuous spectrum of shear Alfven modes is calculated in the presence of a magnetic island in tokamak plasmas. Modes with the same helicity of the magnetic island are considered in a slab model approximation. In this framework, with an appropriate rotation of the coordinates the problem reduces to 2 dimensions. Geometrical effects due to the shape of the flux surface's cross section are retained to all orders. On the other hand, we keep only curvature effects responsible of the beta induced gap in the low-frequency part of the continuous spectrum. New continuum accumulation points are found at the O-point of the magnetic island. The beta-induced Alfven Eigenmodes (BAE) continuum accumulation point is found to be positioned at the separatrix flux surface. The most remarkable result is the nonlinear modification of the BAE continuum accumulation point frequency.