Gyrokinetic simulations on zonal flow-turbulence spreading coupling

TL;DR

This paper uses gyrokinetic simulations to explore how turbulence spreading transports zonal flows into stable regions, revealing a coupling mechanism relevant for plasma confinement.

Contribution

It demonstrates the role of turbulence spreading in transporting zonal flows and extends a momentum theorem to explain the coupling in toroidal plasmas.

Findings

Turbulence spreading transports zonal flows into stable regions.

A theoretical framework links turbulence-driven enstrophy transport to momentum generation.

Simulation results support the extended momentum theorem.

Abstract

Zonal flows and turbulence spreading play important roles in magnetic fusion plasma confinement, yet their coupling mechanisms remain elusive. Using global nonlinear gyrokinetic simulations, we show that turbulence spreading transports zonal flow radially, extending into the linearly stable regions after local nonlinear saturation of turbulence. Theoretical understanding has been gained by analyzing the simulation results in the context of a momentum theorem in toroidal plasmas [T.S. Hahm \textit{et al.}, Phys. Plasmas \textbf{31}, 032310 (2024)] which is an extension of the Charney-Drazin non-acceleration theorem [J.G. Charney and P.G. Drazin, J. Geophys. Res. \textbf{66}, 83 (1961)]. It indicates a direct relation between turbulence-driven enstrophy transport and perpendicular momentum generation.