Edge Pedestal and Er-Layer Formation by X-Transport in a Tokamak Plasma

X-transport can be a baseline source of a strong edge Er and pedestal formation immediately inside the last closed flux surface in a diverted tokamak. It is a strong unconventional collisional transport localized to the X-region, caused by a lack of poloidal magnetic field. Some collisionless ions near the X-point cannot be confined (see Fig. 1), forming a loss hole in velocity space. Ions scatter into and out of the loss hole by Coulomb collisions to contribute to the convective collisional X-transport. It is intrinsically non-ambipolar and stronger than the usual neoclassical tokamak transport. A Monte Carlo guiding center code has been developed to study the X-transport physics, including the pedestal (see Fig. 2) and Er layer formation, and flow development in the plasma edge. Neutral particle effect has recently been added to the code and found to have a significant influence. Numerical study finds that the ion temperature pedestal is broader than the density pedestal, and the ion temperature pedestal is anisotropic between parallel and perpendicular directions. The X-transport shed light on many unexplained phenomena observed in the H-mode and edge pedestal experiments, including H-mode threshold power scaling, role of Grad-B drift direction, single null versus double null, limiter versus divertor, and an edge flow generation, etc.