Nonlinear reversed shear Alfven eigenmode saturation due to spontaneous zonal current generation

TL;DR

This paper derives nonlinear gyrokinetic equations to study how reversed shear Alfven eigenmodes (RSAEs) self-modulate through spontaneous generation of zonal structures, revealing the roles of zonal flow and current in saturation.

Contribution

It introduces a new theoretical framework for RSAE nonlinear saturation involving both zonal flow and current, highlighting their dependence on plasma parameters.

Findings

Both electrostatic zonal flow and electromagnetic zonal current can be excited by RSAE.

ZC plays a significant role in RSAE saturation through scattering processes.

Modification of shear Alfven wave continuum is influenced by the saturation level of ZC.

Abstract

General nonlinear equations describing reversed shear Alfven eigenmode (RSAE) self-modulation via zero frequency zonal structure (ZFZS) generation are derived using nonlinear gyrokinetic theory, which are then applied to study the spontaneous ZFZS excitation as well as RSAE nonlinear saturation. It is found that both electrostatic zonal flow (ZF) and electromagnetic zonal current (ZC) can be preferentially excited by finite amplitude RSAE, depending on specific plasma parameters. The modification to local shear Alfven wave continuum is evaluated using the derived saturation level of ZC, which is shown to play a comparable role in saturating RSAE with the ZFZS scattering.