The plasma-wall transition with collisions and an oblique magnetic field: Reversal of potential drops at grazing incidences

The plasma-wall transition is studied by using 1d3V particle-in-cell simulations in the case of a one dimensional plasma bounded by two absorbing walls separated by 200 Debye lengths (lambda(d)). A constant and oblique magnetic field is applied to the system, with an amplitude such that r < lambda(d) < R, where r and R are the electron and ion Larmor radii, respectively. Collisions with neutrals are taken into account and modelled by an energy conservative operator, which randomly reorients ion and electron velocities. The plasma-wall transition (PWT) is shown to depend on both the angle of incidence of the magnetic field with respect to the wall, theta, and on the ion mean-free-path to Larmor radius ratio, lambda(ci)/R. In the very low collisionality regime (lambda(ci) >> R) and for a large angle of incidence, the PWT consists of the classical tri-layer structure (Debye sheath/Chodura sheath/pre-sheath) from the wall towards the center of the plasma. The drops of potential within different regions are well consistent with already published models. However, when sin theta <= R= lambda(ci) or with the ordering lambda(ci) < R, collisions cannot be neglected, leading to the disappearance of the Chodura sheath. In this case, a collisional model yields analytic expressions for the potential drop in the quasi-neutral region and explains, in qualitative and quantitative agreement with the simulation results, its reversal below a critical angle derived in this paper, a regime possibly met in the scrape-off layers of tokamaks. It is further shown that the potential drop in the Debye sheath slightly varies with the collisionality for lambda(ci) >> R. However, it tends to decrease with lambda(ci) in the high collisionality regime, until the Debye sheath finally vanishes.