Statistical characteristics of turbulent transport dominated by zonal flow dynamics

Matsumoto, Taro; Kishimoto, Yasuaki; Li, J.*

Characteristics of ETG-driven turbulent transport dominated by zonal flows and nonlinearly generated large scale structures with low toroidal/poloidal wave numbers are investigated by gyro-fluid simulations in slab geometry. Main results found in this research are as follows. (1) The zonal flows change the characteristics of turbulence from "homogeneous" structure to "inhomogeneous" one, in which micro-scale vortices and nonlinearly generated macro-scale vortices coexist at different radial zones, exhibiting a two-scale nature in turbulence. (2) The fractal dimension is simultaneously reduced at any radial region with an increase of the ratio of zonal flow energy to that of total fluctuations, accompanied by the disappearance of exponential PDF tail of the heat flux. (3) The reduction of heat flux in strong zonal flow regime results from two mechanisms in the relation between poloidal electric field and pressure perturbation, i.e. the reduction of coherence in the zone of micro-scale vortices and the phase synchronization in that of macro-scale ones.