ExB circulation at the tokamak divertor X point

Detailed measurements in two dimensions by probes and Thomson scattering re veal unexpected local electric potential and electron pressure (p(e)) maxim a near the divertor X point in L-mode plasmas in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)]. The potential drives ExB circulation about the X point, thereby exchanging plasma between close d and open magnetic surfaces at rates that can be comparable to the total c ross-separatrix transport. The potential is consistent with the classical p arallel Ohm's law. A simple model is proposed to explain the pressure and p otential hills in low power, nearly detached plasmas. Recent two-dimensiona l edge transport modeling with plasma drifts also shows X-point pressure an d potential hills but by a different mechanism. These experimental and theo retical results demonstrate that low power tokamak plasmas can be far from poloidal uniformity in a boundary layer just inside the separatrix. Additio nal data, although preliminary and incomplete, suggest that ExB circulation across the separatrix might be a common feature of low confinement behavio r. (C) 2001 American Institute of Physics.