Characteristics of ETG-driven turbulence dominated by zonal flows (ZFs) and nonlinearly generated large scale structures with low toroidal/poloidal wave numbers are investigated by gyrofluid simulations in slab geometry. Main results found in this research are as follows. (i) The electron transport analysis in various parameter regions shows that the electron heat flux was not necessarily enhanced as the electron temperature gradient increases. (ii) The ZFs change the characteristics of turbulence from a 'homogeneous' structure to an 'inhomogeneous' one, in which micro-scale vortices and nonlinearly generated macro-scale vortices coexist in different radial zones, exhibiting a two-scale nature in turbulence. (iii) The fractal dimension is simultaneously reduced in any radial region with an increase in the ratio of ZF energy to that of total fluctuations. Namely, macro-scale vortices, which are the origin of low fractal dimension and play a role in saturating ZFs, survive in the system without causing large thermal transport by adjusting the phase relation.
Observation and characterization of radially sheared zonal flows in DIII-D