Large E×B convection near the divertor X-point

Electric potential, electron temperature, and electron density were measured in two dimensions (R,Z) throughout the divertor and X-point in DIII-D tokamak plasmas. An electric potential hill (∼100 eV) and an associated electron pressure hill were discovered at the divertor X-point in L-mode plasmas. The potential hill extends previously reported divertor E×B circulation, convecting particles, energy and toroidal momentum into and out of closed magnetic surfaces, and contributes significantly to transport across the boundary. The potential is explained by classical parallel (to B) plasma physics, when the X-point Ti is clamped lower than upstream Ti. The low X-point Ti state might be self-sustaining at low heating power due to the same E×B circulation. We speculate that if the circulation transport is incompatible with H-mode, then the spontaneous L–H transition might require as a precondition that the X-point Ti become equalized on the near-separatrix magnetic surfaces.