Turbulence and sheared flows in fusion plasmas*

The performance of magnetic-confinement-fusion devices is often limited by turbulent fluctuations that lead to enhanced transport and energy losses. Both experimental and numerical investigations have shown that these fluctuations, and thus the transport properties of the plasma, can be altered significantly by the presence of sheared flows. We derive a novel phenomenological model for the scaling of turbulent transport coefficients with the imposed flow shear and present numerical evidence to support our theory. In the near-marginal regime, where the flow shear is large but not large enough to completely suppress the turbulence, we uncover a plethora of interesting nonlinear effects, such as bistability and the formation of long-lived coherent structures. We describe the physics of these coherent structures in a simple fluid model of ion-scale turbulence and discuss the relationship between the imposed (equilibrium) sheared flows and the intrinsically generated zonal flows. We show that interpreting near-marginal turbulence as an ensemble of coherent structures allows for an accurate prediction of important properties, such as the turbulent viscosity. Finally, we demonstrate the relevance of these findings to the Dimits transition between low and high turbulent transport.