  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.
