Global profile effects and structure formations in toroidal electron temperature gradient driven turbulence

Using a gyrokinetic toroidal particle code with global profile effects, the toroidal electron temperature gradient driven (ETG) turbulence in positive and reversed shear tokamaks is studied. In the simulation, initial saturation levels of the ETG mode are consistent with the mixing length theory, which shows a Bohm (gyro-Bohm) like ρ*-scaling for a ballooning type (slab like) ETG mode in a positive (reversed) shear configuration, where ρ* is the electron Larmor radius ρte divided by the minor radius a. In a realistic small ρ* positive shear configuration, the ETG mode has a higher saturation level than the large ρ* positive shear configuration and the reversed shear configuration. In the nonlinear turbulent state, the ETG turbulence in the positive and reversed shear configurations shows quite different structure formations. In the positive shear configuration, the ETG turbulence is dominated by streamers which have a ballooning type structure, and the electron temperature Te profile is quickly relaxed by enhanced heat transport in a turbulent time scale. In the reversed shear configuration, quasi-steady zonal flows are produced in the negative shear region, while the positive shear region is characterized by streamers. Accordingly, the electron thermal diffusivity χe has a gap structure across the qmin surface, and the Te gradient is sustained above the critical value for a long time. The results suggest a stiffness of the Te profile in positive shear tokamaks and a possibility of the Te transport barrier in reversed shear tokamaks.