Minisymposium Presentation

The magnetised plasma sheath is a boundary layer, whose thickness is a few ion Larmor radii, appearing next to the solid targets of a fusion device. It is a multi-scale region, including the Debye length and electron gyroradius as additional smaller length scales. It is therefore rich in physics and complex, in particular in the fusion-relevant case of grazing magnetic field incidence at the target. From the point of view of the plasma, the main role of the sheath is to reflect electrons such that a (globally) ambipolar flow of ions and electrons to the solid target is achieved. Therefore, simplified conducting or logical sheath boundary conditions that exploit a constant parallel electron velocity reflection cutoff, related to an unresolved sheath potential drop, have been and are being used in gyrokinetic simulations. Yet, only actual sheath solutions can provide more accurate reflection conditions for electrons, as well as predict the correct plasma-surface interaction by computing the strongly distorted ion distribution function at the target. We present a scheme to iteratively obtain fast numerical solutions of the steady state magnetised sheath for grazing magnetic field incidence. A magnetic moment dependence in the electron reflection cutoff provides more accurate sheath boundary conditions.