Speaker
Description
In magnetic confinement fusion devices, the divertor is often negatively charged because electrons are more mobile and are preferentially captured by the divertor. This negative charge is shielded by a positively charged Debye sheath. In magnetised plasmas, another layer of width of the order of the ion gyroradius, known as the magnetic presheath, forms on top of the Debye sheath. In this work, we consider a simplified 1D problem in space, focusing on a distribution function that only has gradients perpendicular to the wall. We show that for a sufficiently collisional plasma, there exists another layer with a width that scales with the ion mean free path, the collisional layer. This layer connects the plasma far away from the wall, where fluid equations are used to model the plasma, to the collisionless magnetic presheath. The collisional layer is described by the steady state electrostatic ion drift kinetic equation in one spatial dimension, together with quasineutrality and adiabatic electrons. We show that the kinetic Chodura condition must be satisfied at the magnetic presheath entrance and that the potential at the magnetic presheath entrance diverges. Finally, we introduce a semi-Lagrangian finite element code CLOVER (Collisional Layer Solver) developed to solve our system of equations numerically. This code provides the distribution function at the magnetic presheath entrance and the potential drop across the collisional layer.