System size and shear flow effects on ion temperature gradient turbulence

We explore the role of system size ($\rho^*$ scaling) and shear flow levels on ion temperature gradient (ITG) turbulence in gyrokinetic simulations. Firstly, the scaling of heat flux level versus system size in plasmas with zero initial flow shear is reexamined. Averages over an ensemble of simulations are made to quantify errors in the mean flux estimate. Our scaling results are compared with others in light of this error. Secondly, the effect of poloidal shear flow on ITG turbulence is examined. Turbulence levels are suppressed when the flow shearing rate is similar to the ITG growth rate. Even at flow levels many times smaller than the supression threshold, nonlinear turbulence is only seen in the simulation if the initial perturbation is large: the system manifests subcritical turbulence. Inward or outward propagating avalanches and inward and outward shifted diffusivity profiles are seen according to the sign of the flow shear. In cases with strong flow shear almost all the flux is associated with avalanches, which propagate across the entire region with strong pressure gradient. The non-local and non-diffusive physics are explored by performing a second scan in system size.