Global Gyrokinetic Simulations of Toroidal Electron Temperature Gradient Driven Mode in Reversed Shear Tokamaks

Abstract: Using a global gyrokinetic toroidal particle code, the toroidal electron temperature gradient driven (ETG) turbulence is studied in positive and reversed shear tokamaks. In the positive shear configuration, the ETG mode shows a ballooning structure, and its envelope width ∆r/ρte, which is limited by a global ω*te-shearing effect, is proportional to ρ*-1/2, where ω*te is the electron diamagnetic frequency and ρ * is the electron Larmor radius ρte divided by the minor radius a. In the reversed shear configuration, the mode width of a slab like ETG mode, which is determined by a global magnetic field structure around the qmin surface, does not depend on ρ*. According to the mixing length theory, a ballooning mode gives a Bohm like ρ*-scaling, while a slab like mode shows a gyro-Bohm like ρ*-scaling. In realistic small ρ * tokamaks, the saturation level of the ETG mode in the positive shear configuration is order of magnitude higher than that in the reversd shear configuration. In the nolinear turbulent state, the ETG turbulence in the positive and reversed shear configurations show 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 to the maginally stable state in a turbulent time scale. In the reversed shear configuration, quasi-steady zonal flows are produced in the regative 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 gradinet is sustained above the marginal value for a long time in the quasi-steady phase. 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. 1