Energy and Particle Transport Across a Separatrix in Diverted Tokamaks.

摘要

Transport across a separatrix is shown to be significantly modified by poloidal gradients, which result from the flow of particles and heat along field lines toward the divertor region. The theory predicts an inward contribution to the particle flux, and a corresponding inward contribution to the convective energy flux, when the ion grad-B drift is toward the x-point, in a single-null configuration. This is due to a classical thermoelectric potential which exists near the separatrix when heat is conducted classically along magnetic field lines towards the x-point region. The magnetic surface-averaged classical (diamagnetic and E x B) radial particle flux is inward, when the x-point is correctly located. Estimates of the size of this particle flux indicate that it may be contributing significantly to the H-mode density rise just inside the separatrix in DIII-D. Estimates of the effect on the energy flux give values for the ratio of H-mode power thresholds, with two different grad-B drift directions, which agree approximately with the experimental results. 8 refs., 1 fig. (ERA citation 13:058103)