The fundamental properties of steep neoclassical plasma pedestals in a quiescent tokamak plasma have been investigated with a new guiding center particle code XGC: an X-point included Guiding Center code. It is shown that the width of the steepest neoclassical pedestals is similar to an experimentally observed edge pedestal width, and that a steep pedestal must be accompanied by a self-consistent negative radial electric field well. It is also shown that a steep neoclassical pedestal can form naturally at a quiescent diverted edge as the particle source from the neutral penetration (and heat flux from the core plasma) is balanced by the sharply increasing convective ion loss toward the separatrix. The steep neoclassical pedestal and the strong radial electric field well are suppressed by an anomalous diffusion coefficient of a strength appropriate to an L-mode state; nonetheless, the E×B shearing rate increases rapidly with pedestal temperature. Additionally, the present study shows that a steep pedestal at the diverted edge acts as a cocurrent parallel momentum source.

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A new nonambipolar neoclassical transport (X-transport) mechanism has been identified which can be an irreducible baseline source of a strong radial electric field and edge pedestal formation immediately inside the separatrix in a diverted tokamak. Due to the vanishingly small poloidal magnetic field in the vicinity of a divertor X-point, there exists an ion velocity space hole at the thermal energy level. This becomes a source of a nonambipolar, collisional, convective radial transport of plasma ions, as the ions scatter into and out of the loss hole in the vicinity of an X-point. The widths of the Er and edge pedestal layers are somewhat smaller than the ion poloidal gyroradius, measured at the midplane. A simple estimate shows that the X-transport rate can be significant enough to influence a high mode transition and edge pedestal formation.