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 3 B flow. The neoclassical radial electric field Er shows no bifurcation in contrast with earlier orbit loss models, but the shear in Er 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 3 B flow shear suppression. The parameter scaling of threshold temperature for strong turbulence shear suppression agrees with the H-mode threshold scaling in ASDEX Upgrade.