Unified mesoscale picture of nonlinear generation of zonal flows in toroidal geometry

TL;DR

This paper uses global nonlinear gyrokinetic simulations to unify the understanding of how zonal flows are nonlinearly driven in toroidal geometry, clarifying the roles of turbulent energy flux and Reynolds stress.

Contribution

It presents a comprehensive mesoscale model showing that turbulent energy flux and Reynolds stress drive zonal flows differently depending on the timescale relative to the ion bounce period.

Findings

Turbulent energy flux drives zonal flows without being shielded by toroidal geometry.

Turbulent poloidal Reynolds stress is shielded by toroidal geometry on timescales longer than the ion bounce period.

The study resolves controversies about the nonlinear drivers of zonal flows in toroidal plasmas.

Abstract

Recent experimental findings on limit-cycle-oscillations indicate that the nonlinear driving of the turbulent poloidal Reynolds stress to zonal flows is not a significant factor in the toroidal geometry, sparking fundamental controversial issues within the fusion community. By using the global nonlinear gyrokinetic simulations, we propose a unified mesoscale picture of nonlinear driving of zonal flows in the ion-temperature-gradient turbulence. Zonal flows are nonlinearly driven by the turbulent energy flux and the turbulent poloidal Reynolds stress. The turbulent energy flux is not shielded by the toroidal geometry effect in nonlinearly driving zonal flows. The turbulent poloidal Reynolds stress is not shielded on the time scale shorter than the ion bounce period; however, on the time scale longer than the ion bounce period, the turbulent poloidal Reynolds stress is indeed shielded by the toroidal geometry effect.