On How Zonal Fields Suppress Reversed Shear Alfv\'en Eigenmode in Tokamak Plasmas

TL;DR

This study reveals that zonal fields can suppress reversed shear Alfvén eigenmodes in tokamak plasmas through nonlinear frequency chirping and mode conversion, confirmed by simulations and theoretical analysis.

Contribution

It demonstrates a novel nonlinear suppression mechanism of Alfvén eigenmodes via zonal fields and frequency chirping, combining simulations with theoretical insights.

Findings

Downward frequency chirping leads to mode suppression.

Enhanced conversion to kinetic Alfvén waves causes damping.

Theoretical results align with simulation data.

Abstract

Employing both nonlinear gyrokinetic simulations and theoretical analyses, we have discovered the novel result that, with energetic particle dynamics kept linear, the nonlinear suppression and eventual saturation of reversed-shear Alfv\'en eigenmode occur via the downward frequency chirping induced by the beat-driven zonal current. More specifically, as the mode frequency chirps downward, there is enhanced mode conversion to radially propagating electron Landau-damped kinetic Alfv\'en waves; resulting in enhanced convective (radiative) damping and, thereby, its suppression and saturation. Theoretical results are in good agreement with simulations both qualitatively and quantitatively.