Keith R. Fuhrhop

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 /spl times/ 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 probes 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).

Current Collection to Electrodynamic-Tether Systems in Space

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..

Measurement of cross-section geometry effects on electron collection to long probes in mesosonic flowing plasmas

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 o wing plasma. A Hall thruster was used to simulate a o wing unmagnetized space plasma in a large 6m 9m vacuum chamber. Guarded tether samples were designed to mitigate end eects. Plasma parameters were determined based on the ion saturation and electron retardation regimes of a Langmuir probe’s current characteristics. Solid tape samples with eectiv e 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 o w leads to current enhancements over that predicted by the orbital-motion-limited theory; 2) the electron collection eciency 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 o w; 4) equivalent width slotted tapes are more ecien t electron collectors than solid tapes on a per area basis; 5) our data suggests the electron collection eciency of slotted tapes decreases with increasing line spacing until a possible minimum eciency 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 o w, but not in the case of the samples aligned with the o w, 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 o w.

A Comparison of Laboratory Experimental and Theoretical Results for Electrodynamic Tether Electron Collection Performance for Some Bare Tether Geometries

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 u 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.

Optimizing Electrodynamic Tether System Performance

A time-averaged electrodynamic tether (EDT) system simulation tool has been developed and used to conduct studies of tether performance under varying conditions. The studies included evaluating passive end-body electron collection and active ion emission approaches, a comparison of active electron emission technologies (hollow-cathode, electron field emission, hot cathode), adjustment of bare conductor versus insulated tether lengths, boosting and de-boosting conditions, and other various system element configurations. The study results indicate that in many cases bare tether anodes provide optimal electron collection. In addition, it was shown that while hollow cathodes may be the best active electron emission technique, field emitter arrays result in less than 1% difference in average system thrusting and use no consumables. This is based on the assumption that multi-amp field emitter arrays can be ultimately fabricated and qualified for space. Three case-studies were performed in order to better understand the trades for performance optimization. The cases were: (1) orbit maintenance of the International Space Station; (2) the use of an EDT system for reboost and deorbit of NASA’s GLAST spacecraft; and, (3) operation of the Momentum Exchange Electrodynamic Reboost (MXER) system. From evaluation of these cases, a recommended design “algorithm” is proposed. Case (1) is presented in this paper in its entirety, and cases (2) and (3) are briefly described.

Electron Emission for Electrodynamic Tether Systems in Space

This paper presents the theoretical analysis of three different types of electron emitters and three different system architectures. Thermionic cathodes, field emitter arrays, and hollow cathodes are evaluated for their potential use in various electrodynamic tether system applications. Basic grounded tip, basic grounded gate and series-bias system architectures are considered. It was found that the series-bias grounded gate configuration produced the overall best cases for the electron emitters when de-orbit time was a major factor because they yielded the highest deorbit forces. When power consumption was a major factor the basic grounded gate configurations was the best choice. As far as electron emitters, the spindt type field emitter array technology was always the best choice when comparing it against the thermionic cathode in all of the tether system setups when it involved low power. For many low power systems the field emitter technology is superior.

System analysis of the expected electrodynamic tether performance for the ProSEDS mission

This paper reviews some expected systems performance aspects of NASA’s ProSEDS (Propulsive Small Expendable Deployer System) electrodynamic (ED) tether mission after recently being required to lower its initial orbit from 360 km to 285 km. In addition, the ProSEDS tether, which has conductive and non-conductive sections, shortened its nonconductive section thereby reducing overall tether length from 15-km to 12-km long. The International Reference Ionosphere (IRI) model is not as accurate as previously predicted when the altitude is less than 300 km and it was found that a factor of 0.65 should be multiplied to the electron plasma density on the IRI 1990 model to compensate for this effect. The ED characteristics of ProSEDS are being theoretically predicted using software called TEMPEST developed at the University of Michigan. Using OML (Orbital Motion Limited) theory for tether collection TEMPEST demonstrated that ProSEDS will de-orbit in 90 hours instead of the originally predicted 160 hours. The induced EMF (electromotive force) range will remain approximately the same from 400 V to 1000 V as will the collected current range, which varies according to altitude. Also, it takes 50 hours for the atmospheric drag to become stronger than the electromagnetic drag from the tether at 285 km as opposed to the 130 hours at 360 km. Various errors in the potential measurement of ProSEDS stem from: (1) the electron sheath at the upper end of the tether, (2) a “phantom current” occurring throughout the tether, and (3) the measurement of the potential used for current collection on the nozzle of the Delta II module. The “phantom current” also causes an extra 2 km per day de-orbit. In addition, there is a 0.733-mA theoretical current that travels into the Delta II from the tether that must be emitted. This potential is plotted versus the atmospheric density to show how much current is collected. The total potential error in the ProSEDS system ranges from 4–71% without hollow cathode operating, depending on the density of the electron plasma.