Neoclassical radial current balance in tokamaks and transition to the H mode

Monte Carlo ion simulation based on neoclassical radial current balance in a divertor tokamak gives a stationary sheared (E) over right arrow X (B) over right arrow flow. The neoclassical radial electric field E-r shows no bifurcation in contrast with earlier orbit loss models, but the shear in E-r reaches values at which a transition to enhanced confinement has been observed. Also, MHD turbulence analysis shows that a smooth transition can occur through the neoclassical (E) over right arrow x (B) over right arrow flow shear suppression. The parameter scaling of threshold temperature for strong turbulence shear suppression agrees with the H-mode threshold scaling in ASDEX Upgrade.