The plasma-wall transition layers in the presence of collisions with a magnetic field parallel to the wall

International audienceThe plasma-wall transition is studied by mean of a particle-in-cell (PIC) simulations in the configuration of a parallel to the wall magnetic field (B), with collisions between charged particles vs. neutral atoms taken into account. The investigated system consists in a plasma bounded by two absorbing walls separated by 200 electron Debye lengths (λ d). The strength of the magnetic field is chosen such as the ratio λ d /r l , with r l the electron Larmor radius, is smaller or larger than the unity. Collisions are modelled with a simple operator that reorients randomly ion or electron velocity, keeping constant the total kinetic energy of both the neutral atom (target) and the incident charged particle. The PIC simulations show that the plasma-wall transition consists in a quasi-neutral region (pre-sheath), from the center of the plasma towards the walls, where the electric potential or electric field profiles are well described by an ambipolar diffusion model, and in a second region at the vicinity of the walls, called the sheath, where the quasi-neutrality breaks down. In this peculiar geometry of B and for a certain range of the mean-free-path, the sheath is found to be composed by two charged layers, a first, positive, close to the walls, and a second one, negative, towards the plasma and before the neutral pre-sheath. Depending on the amplitude of B, the spatial variation of the electric potential can be non-monotonic and presents a maximum within the sheath region. More generally, the sheath extent as well as the potential drop within the sheath and the pre-sheath are studied with respect to B, the mean-free-path and the ion and electron temperature