This paper presents the analysis of new measurements of electron current collection to porous tape probes in a high-speed flowing plasma, and a comparison to similar measurements with round cylinder, solid and slotted tape samples previously reported §. In these experiments, a Hall thruster was used to create a high-speed (~8 km/s) flowing unmagnetized plasma in a large 6-m 9-m vacuum chamber. Experimental results of solid tape samples with widths spanning from 7.2 to 20.4 Debye lengths and slotted tapes with center-to-center line spacings spanning from 2.1 to 6.0 Debye lengths (gap widths from 1.3 to 3.6), were compared to measurements of holed tapes with hole diameters ranging from 1.4 to 9.4 Debye lengths. Several conclusions can be drawn from the analysis of the results in the regime tested: 1) Beyond a threshold bias potential probably close to the beam energy, holed tapes collect more current when oriented transverse (perpendicular) to the flow, just like solid and slotted tapes; 2) Holed tapes are more efficient electron collectors than both solid and slotted tapes in terms of collected electron current per unit area when oriented perpendicular to plasma flow. However, when oriented parallel to plasma flow, slotted tapes are more efficient than holed or solid tapes; and 3) When the tapes were oriented parallel to the flow, the electron current collected on holed tapes decreases with increasing hole size until a minimum is attained, beyond which it starts increasing again. The opposite effect occurred when the holed probes were oriented transverse to the flow, and a maximum efficiency was observed. We conclude that the holed tethers, which have better structural stability, also have the greatest mass equivalent electron current collection compared to that of solid and slotted tethers.
This paper presents the analysis and comparison of measured electron current collection to cylindrical, 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 6m × 9m vacuum chamber. Guarded tether samples were designed to mitigate end effects. Plasma parameters were determined based on the ion saturation and electron retardation regimes of a Langmuir probe's current characteristics. Solid tape samples with effective widths spanning from 4.9 to 41.9 Debye lengths, and slotted tapes with 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 current enhancements over that predicted by the orbital-motion-limited theory; 2) the electron collection efficiency of solid tapes (on a per area basis) 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) equivalent width slotted tapes are more efficient electron collectors than solid tapes on a per area basis; 5) our data suggests the electron collection efficiency of slotted tapes decreases with increasing line spacing until a possible minimum efficiency 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 the elongation of the sheath associated with plasma flow.
Three important electrodynamic-tether system configurations have been investigated: an insulated tether with an end body collector, bare tether, and bare tether with end body collector. This paper discusses the current collection capabilities of these configurations and their respective advantages and disadvantages. University of Michigan's TEMPEST computer model was used to conduct the analyses of the three configurations. Analysis has determined that all three configurations allow orbit raising from 400 km to 700 km in around 18.5 days under similar ionospheric and system conditions. In addition, the best tether geometry to use for any of these configurations would be a slotted tether oriented perpendicular to the plasma flow with the individual wires as far apart as possible and as narrow as possible. This would minimize atmospheric drag, increase collision survivability, and keep the electron collection level close to the orbital-motion limit, while increasing the redundancy of the tether in case of micrometer collision..
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