Direct measurements of ion dynamics in collisional magnetic presheaths

Abstract

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.

Three-Dimensional Multispecies Distribution Functions in a Plasma Boundary With an Oblique Magnetic Field

“…The Chodura model is intended for plasma boundaries in which λ i /ρ i 1, which is not the case in the HELIX experiments, and therefore is expected to be a poor description of the boundary structure. That said, these measurements corroborate the observations of Siddiqui et al [22], which span λ i /ρ i = 0. x + u 2 y ] 1/2 .…”

“…Since that time, magnetic presheath research has enjoyed broad theoretical progress [11][12][13][14][15][16][17]. Only a few experiments exist in literature to provide data to validate these models [18][19][20][21][22][23]. Of these experiments, the early focus was on mapping the plasma potential structure of the boundary region with electrostatic probes.…”

“…Of these experiments, the early focus was on mapping the plasma potential structure of the boundary region with electrostatic probes. Siddiqui et al were the first to report measurements of argon ion distribution functions and 2D flow fields in a boundary with ψ ≤ 60 • [22], which they compared to a collisional fluid model. Preliminary results related to the experiments reported in this dissertation were described two years later in Ref.…”

“…Measurements of the background metastable neutral population were unavailable to the authors of Refs. [22] and [23]. Through elastic scattering collisions, ionization, and charge exchange, ion and neutral populations modify each other's distribution functions.…”

“…4.20.Observed λ mps ranges from 6.1 mm (≈ ρ i ) to 13.2 mm (2.1ρ i ), with an average distance of 9.3 mm (1.5ρ i ). Siddiqui et al found that at ψ = 60 • , the Chodura criterion is predicted to be met at λ mps /ρ i = 6.3, but is observed to occur at λ mps /ρ i = 0.6 ± 0.01[22]. It is worth noting that Chodura, in his initial 1D1V model, excluded flows u y and u z , which simulations suggest couple with u z (see, for example, Ref [11]…”