Role of zonal flow in turbulent transport scalings

Transport scalings with respect to collisionality (ν*) and device size (ρ*) are obtained from massively parallel gyrokinetic particle simulations of electrostatic toroidal ion-temperature-gradient (ITG) turbulence in the presence of zonal flows. Simulation results show that ion thermal transport from electrostatic ITG turbulence depends on ion-ion collisions due to the neoclassical damping of self-generated ExB zonal flows that regulate the turbulence. Fluctuations and heat transport level exhibit bursting behavior with a period corresponding to the collisional damping time of poloidal flows. Results from large-scale full torus simulations with device-size scans of realistic parameters show that Bohm-like transport can be driven by microscopic scale fluctuations in the ITG turbulence with isotropic spectra. These simulation results resolve some apparent physics contradictions between experimental observations and turbulent transport theories. (author)