Neoclassical transport effects and the H-mode transition

Ion neoclassical transport near the separatrix of a divertor tokamak is investigated. In the high-collisionality regime, large ion temperature gradients may develop with single-null operation if the ion grad-B drift is toward the x-point, and if the power is comparable to a 'critical' value. This may explain the observed dependence, of the power threshold for the H-mode in ASDEX, on the number and location of the x-points. In the low-collisionality regime, achieved experimentally with higher heating power, the ion temperature gradient is large and does not depend on x-point number or location.