Models, assumptions, and experimental tests of flows near boundaries in magnetized plasmas

We present the first measurements of ion flows in three dimensions (3Ds) using laser-induced fluorescence in the plasma boundary region. Measurements are performed upstream from a grounded stainless steel limiter plate at various angles (ψ=16° to 80°) to the background magnetic field in two argon helicon experiments (MARIA at the University of Wisconsin-Madison and HELIX at West Virginia University). The Chodura magnetic presheath model for collisionless plasmas [R. Chodura, Phys. Fluids 25, 1628 (1982)] is shown to be inaccurate for systems with sufficient ion-neutral collisions and ionization such as tokamak scrape off layers. A 3D ion fluid model that accounts for ionization and charge-exchange collisions is found to accurately describe the measured ion flows in regions where the ion flux tubes do not intersect the boundary. Ion acceleration in the E→×B→ direction is observed within a few ion Larmor radii of the grounded plate for ψ=80°. We argue that fully 3D ion and neutral acceleration in the plasma boundary are uniquely caused by the long-range presheath electric fields, and that models that omit presheath effects under-predict observed wall erosion in tokamak divertors and Hall thruster channel walls.