Multiple Branches of Discrete Alfvén Eigenmodes in Tokamak Plasmas with Negative Magnetic Shear

Abstract

The multiple branches of discrete Alfvén eigenmodes trapped in the α-induced (α = −q ² Rdβ/dr) potential wells due to the pressure gradient and ballooning curvature effects are investigated in advanced tokamak operation regimes with negative magnetic shear. Here, q is the safety factor, β is the ratio of plasma to magnetic pressures, and R and r are, respectively, the major radius and the radial coordinate along the minor radius direction. Similarly to the case of positive, these eigenmodes experience small continuum damping due to wave energy tunneling that they could be readily destabilized by energetic particles. In the parameter regime of present tokamak experiments with negative magnetic shear, the discrete Alfvén eigenmodes are shown to be a possible candidate to understand the experimentally observed Alfvén modes.