Experimental Investigation of Electron Collection to Solid and Slotted Tape Probes in a High-Speed Flowing Plasma

This paper presents the analysis and comparison of measurements of electron current collection to round cylinder, solid tape, and slotted tape electrodynamic-tether samples in a mesosonic flowing plasma. A Hall thruster was used to simulate a flowing unmagnetized space plasma in a large 6-m 9-m vacuum chamber. Guarded tether samples were employed to mitigate end effects. Plasma parameters were determined based on the ion saturation and electron retardation regimes of a cylindrical Langmuir probe\´s current characteristics. Solid tape samples with widths spanning from 4.9 to 41.9 Debye lengths, and slotted tapes with center-to-center line spacings spanning from 1.4 to 13.2 Debye lengths were tested. Several conclusions can be drawn from the analysis of the results: 1) the plasma flow leads to significant current enhancements over that predicted by the orbital-motion-limited theory; 2) the electron current collected per unit area on solid tapes decreases as the width of the tape is increased; 3) beyond a threshold bias close to the beam energy, solid and slotted tapes both collect more current when oriented transverse to the flow; 4) slotted tapes are more efficient electron collectors per unit area than solid tapes; and 5) our data suggests that the electron current collected on slotted tapes decreases with increasing line spacing until a possible minimum is attained, beyond which it is expected to start increasing again. The minimum was attained in the case of the samples oriented transverse to the flow, but not in the case of the samples aligned with the flow, for which the critical spacing is likely higher (due to an increased sheath interaction radius of each line caused by flow-induced sheath elongation).