Direct measurements of ion dynamics in collisional magnetic presheaths

Ion velocities and temperatures are measured in the presheath of a grounded plate downstream from an argon helicon plasma source using laser-induced fluorescence ($P_{\mathit{r}\mathit{f}}\approx 450\to 750$ W, $T_e=2.5\to 5$ eV, $T_i=0.1\to 0.6$ eV, $n_0\approx 1\times {10}^{12}{\text{cm}}^{-3},\hspace{0.17em}{p}_n=1\to 6.5$ mTorr, $\lambda =0.3\to 2$ cm, ${\rho }_i\approx$ 0.5 cm). The plate is held $16°\to 60°$ 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. $\overrightarrow{E}\times \overrightarrow{B}$ flows as large as 40% of c_s 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.