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.

Ion velocities and temperatures are measured in the presheath of a grounded plate downstream from an argon helicon plasma source using laser-induced fluorescence (Prf≈450→750 W, Te=2.5→5 eV, Ti=0.1→0.6 eV, n0≈1×1012cm−3, pn=1→6.5 mTorr, λ=0.3→2 cm, ρi≈ 0.5 cm). The plate is held 16°→60° relative to the 1 kG background axial magnetic field. The velocity profiles are compared to a 1D fluid model similar to those presented by Riemann [Phys. Plasmas 1, 552 (1994)] and Ahedo [Phys. Plasmas 4, 4419 (1997)] for the 1 mTorr dataset and are shown to agree well. The model is sensitive to parameters such as collision and ionization frequencies and simplified models, such one presented by Chodura [Phys. Fluids 25, 1628 (1982)], are shown to be inaccurate. E→×B→ flows as large as 40% of cs at the sheath edge are inferred. Definitions for the term “magnetic presheath” and implications for ion flow to tokamak divertors and Hall thruster walls are discussed.

The location and existence of a double layer observed in the core of an argon helicon discharge (3 to 4 mTorr, 500 W rf power) with uniform magnetic fields (900 G), previously reported by Siddiqui (2014 Phys. Plasmas 21 020707), is shown to be modulated by the distance of the downstream conducting boundary from the helicon antenna and the length of its presheath. A region of locally hot electrons (T e ≈ 10 eV) is observed in all cases. When the downstream grounded boundary is 33 cm away from the antenna no double layer is observed and the electrons cool due to heat conduction, requiring the electron density to increase concurrently in order to maintain axial electron pressure balance. When the boundary is held 27 cm or closer to the antenna, the boundary's presheath, which requires the density to decrease monotonically, does not allow enough room for the electrons to cool via conduction. The double layer only exists in these cases, and is shown to trap the hot electrons upstream from the boundary. As such, the interaction of the boundary's presheath with the upstream rf electron heating is shown to determine the overall discharge equilibrium in the system.