Physical mechanism of enhanced stability from negative shear in tokamaks: Implications for edge transport and the L-H transition

Abstract

The enhancement of stability to ballooning modes from negative shear in tokamaks is shown to be a simple consequence of the orientation of the convective cell with respect to the toroidally outward effective gravity, g↘. For modest positive shear, convective cells remain oriented along g↘ as they map along field lines. In contrast, for negative shear or very positive shear convective cells twist strongly away from g↘ and are less strongly driven. The twist of convection cells is controlled by the profile of the vertical magnetic field along the outer midplane, Bz. Twist is a minimum in regions where Bz is independent of the major radius. Transport should be highest in such locations. Resistive ballooning modes in the tokamak edge are strongly stabilized by modest values of negative shear. Tokamak discharges with finite values of βp develop regions of local negative shear on the outside midplane of the plasma torus. This local negative shear should self-stabilize resistive ballooning modes at finite values of the poloidal beta. This effect may impact the transition to high confinement operation (H-mode).