High- and Low-Confinement Modes in Simple Magnetized Toroidal Plasmas

Three-field simulations of interchange turbulence are presented for a simple magnetized toroidal plasma with a vertical magnetic field. The simulations show the presence of two turbulent regimes characterized by low (L) and high (H) confinement properties. We evaluate analytically the properties of the L regime, obtaining expressions for the plasma gradients and for the density and heat fluxes that agree well with the simulations. By increasing the plasma source strength or reducing the vertical magnetic field, a transition to a H regime occurs, in which a strong velocity shear limits the perpendicular transport with respect to the L scaling and the plasma profiles steepen. The analytic estimate of the transition condition is in accord with the simulations.