Gyrokinetic Simulations of Zonal Flow Generation by Intermediate-Scale Electron Temperature Gradient Turbulence in Tokamak Plasmas

The mechanism of anomalous electron heat transport in tokamaks is currently not well-understood. In fluid models, the zonal flow generation by electron-temperature-gradient (ETG) turbulence has shown to be much weaker than that of similar ion (ITG) turbulence, leading to the expectation of a saturated state characterized by radially elongated streamers at electron-gyroradius scales. However, gyrokinetic electron simulations have shown that zonal flow (ZF) modes can contribute to long-time-scale behavior by breaking up these streamers into isotropic eddies. A recent toroidal, gyrokinetic-electron theory [1] has shown that as the ETG spectrum cascades downward a stronger Navier-Stokes type nonlinearity couples the intermediate-scale ETG and ZF modes, thus allowing for relevant ZF generation. We provide gyrokinetic-ion ETG simulation results from GENE, a 5-d gyrokinetic continuum code, with both single-mode ETG and full ETG spectra results compared to the aforementioned theory. We will specifically look at the strength of ZF generation via ETG modes at intermediate and short-wavelength scales, as well as the role of magnetic shear and collisionality in affecting the strength of the ZF generation.

[1] Haotian Chen et al 2021 Nucl. Fusion 61 066017