Dynamics of large-scale structure and electron transport in tokamak microturbulence simulations

An important issue whether the zonal flow or streamer is preferentially formed in plasma turbulence with electron gyro-radius scale is studied. It is found that the generation of different large-scale structures is determined by the spectral anisotropy of turbulent fluctuation. 3D gyrofluid simulations show that the magnetic shear governs the pattern selection in slab electron temperature gradient (ETG) driven turbulence. Weak shear favors the enhancement of zonal flows so that the electron transport is strongly suppressed. Contrarily, radially elongated streamers are formed in stronger shear ETG turbulence. In a toroidal plasma, streamers are excited in linearly stable region along the field (i.e., good curvature region) after the initial saturation of ETG modes, and higher electron transport is dominated by the fluctuations with peaked poloidal spectrum. Further, we show that the enhanced zonal flows in weak shear ETG turbulence may be damped by a Kelvin-Helmholtz (KH) instability. Also, results on the complicated mutual interaction among ETG turbulence, zonal flows and KH mode in finite-Beta toroidal ETG turbulence will be addressed. (author)