Zonal Flow Excitation in Electron-Scale Tokamak Turbulence

TL;DR

This paper presents gyrokinetic simulations demonstrating how electron-scale turbulence in tokamaks excites zonal flows, which in turn regulate heat flux, confirming theoretical predictions about intermediate-scale ETG-ZF coupling.

Contribution

It provides the first comprehensive simulation validation of the intermediate-scale ETG-ZF coupling theory in nonuniform tokamak plasmas.

Findings

Zonal flows are generated by electron-temperature-gradient modes.

Full-spectrum ETG turbulence regulates electron heat flux.

Simulation results agree with theoretical predictions.

Abstract

The derivation of an intermediate-scale gyrokinetic-electron theory in nonuniform tokamak plasmas [Chen H. et al 2021 Nucl. Fusion 61 066017] has shown that a Navier-Stokes type nonlinearity couples electron-temperature-gradient (ETG) modes and zonal flow (ZF) modes with wavelengths much shorter than the ion gyroradius but much longer than the electron gyroradius. This intermediate-scale ETG-ZF coupling is typically stronger than the Hasegawa-Mima type nonlinearity characteristic of the fluid approximation and is predicted to lead to relevant zonal flow generation and ETG mode regulation. Electron-scale, continuum, gyrokinetic simulation results are presented here which include both single-mode ETG and full-spectrum ETG turbulence. The zonal flow generation due to single ETG modes is investigated and the single-mode intermediate-scale results are found to be in agreement with theory. The full-spectrum results are then presented and explained qualitatively in terms of the single-mode results. It is found that the ETG-driven zonal flows regulate intermediate-scale electron heat flux transport to levels in the predicted range.