Magnetic Presheath Boundary Conditions for NIMROD VDE Computations*

In general, VDE computations are expected to be sensitive to their boundary conditions. Axisymmetric resistive-MHD computations with the NIMROD code have shown that the computed halo current distribution depends on the temperature boundary condition and on the particle density boundary condition [Bunkers and Sovinec, BAPS 62, No. 12 (2017)]. Actual plasma contact with material surfaces leads to sheath effects that are impractical to resolve in global simulations. A practical alternative is a set of boundary conditions that represents conditions at the magnetic presheath entrance, which has been developed for plasma turbulence simulations [Loizu, et al., Phys. Plasmas 19, 122307]. Implementing these presheath-entrance conditions in NIMROD requires adapting the model for the non-reduced system of equations. The axisymmetric VDE calculations presented here compare the influence of various boundary conditions. Application of the lowest-order magnetic presheath effects produces significantly different behavior relative to the standard, mathematically simpler boundary conditions. In particular, the flow smoothly connects to the wall when the magnetic presheath boundary conditions are applied.