Turbulent transport of poloidal momentum in toroidal plasmas

In this article, we discuss the turbulent transport of the momentum (component perpendicular to main magnetic field) in toroidal plasmas. The fluctuations in the quasi-linear response tend to induce the shear of mean poloidal flow, as in the case of driving the zonal flows. The case in which the shear of mean poloidal flow and the mesoscale zonal flows coexist is studied. Zonal flows, which have a more rapid growth rate than the shear of mean flow, reaches the nonlinear saturation state, and modify the interaction between microscopic fluctuations and the shear of mean flow. Taking into account the screening effect of zonal flows, the shear of mean poloidal flow is subject to diffusive damping. Responses to the external force are also studied. A new bifurcation (from the state where mesoscale zonal flows are dominant to the one where zonal flows are suppressed) is found.