Brian E. Gilchrist

The current-voltage characteristics of a large probe in low Earth orbit: TSS-1R results

Measurements of the current collected by the Tethered Satellite System (TSS) satellite as a function of voltage and ambient plasma parameters are presented. The satellite current is found to vary approximately with the square root of the potential from below 10 to nearly 1200 V. The collected current exceeded premission expectations, based on the Parker and Murphy [1967] collection model, by factors of two to three. Possible reasons for discrepancies between the measurements and model are briefly discussed.

Vehicle charging effects during electron beam emission from the CHARGE-2 experiment

The CHARGE-2 sounding rocket payload was designed to measure the transient and steady-state electrical charging of a space vehicle at low Earth orbit altitudes during the emission of a low-power electron beam from the vehicle. In addition to the electron gun, the payload contained several diagnostics to monitor plasma and waves resulting from the beam/space/vehicle interaction. The payload was configured as a mother-daughter arrangement with the two parts connected by an insulated conducting wire tether that extended to a maximum length of 426 m during the flight. In the paper we discuss the configuration of the payload, a description of the science instruments, and the flight operation plan. We then present some initial results from the flight, addressing the prime objective of the mission to study vehicle charging. The results show that the 1-keV electron beam was generated at beam currents of 1-48 mA emerging from the exit aperture of the electron gun. The beam emission always resulted in positive charging of the vehicle, which occurred as a two-step process; a transient charging took place about 10 its after beam turn-on, followed by a slower rise to a steady-state potential in a period of the order of milliseconds. The maximum steady-state potential we measured was 560 V for a 17-mA beam emission current. The highest transient potential we measured was 290 V for a 31-mA beam current. Studies of the distribution of currents and voltages in the complete tethered system show that the return currents are supplied from the space charge limited sheath currents above 240 km. Below 240 km the return current to the mother appears to be enhanced by the effects of beam plasma interactions. Detectable effects of the electron beam in vehicle potential and photon emission from the beam are seen for beam currents as low as 1 mA.

The Shuttle Electrodynamic Tether System (SETS) on TSS-1

SummaryThe Shuttle Electrodynamic Tether System (SETS) experiment formed part of the scientific experiments comprising the first flight of the NASA/ASI Tethered-Satellite System flown at an altitude of 300 km and an orbital inclination of 28.5 degrees in July–August 1992. The SETS experiment was designed to study electrodynamic behavior of the Orbiter-Tether-Satellite system as well as provide background measurements of the ionospheric environment near the Orbiter. The SETS experiment was able to operate continuously during the mission providing a large data set. Details of the SETS objectives, its instrumentation, and initial results from the mission highlighting voltage, current, and charging measurements are presented here.

Self-Consistent 2-D Kinetic Simulations of High-Voltage Plasma Sheaths Surrounding Ion-Attracting Conductive Cylinders in Flowing Plasmas

Using the self-consistent steady-state 2-D Kinetic Plasma Solver (KIPS-2D), thorough characterizations are performed of high-voltage cylindrical sheaths surrounding ion-attracting conductive cylinders immersed in stationary as well as flowing collisionless plasmas. Analytical fits are obtained that allow for the accurate prediction of stationary sheath sizes for round-cylinder radii anywhere from one thousandth of a Debye length to five Debye lengths and for any bias potential beyond a small lower bound. Plasma flow is shown to progressively compress the sheath on its ram and lateral sides, down to a limit that closely matches the stationary frozen-ion sheath radius. Conversely, plasma flow is shown to cause a significant wake-side elongation of the sheath. The quasi-elliptical sheath-edge contours observed under flowing conditions can be characterized by their along-flow and across-flow dimensions. By normalizing these dimensions against stationary-sheath diameters, contour plots of the corresponding flow-effect correction factors can be obtained that account for plasma-flow velocity effects in a wide range of speed regimes and bias potentials. In this paper, Mach numbers up to ten and bias potentials from -10Te to -500Te (where Te is the electron temperature in units of volts) are simulated and corresponding correction factors are computed, although KiPS is capable of simulating even higher speeds and bias potentials. These correction factors appear to stabilize at high voltages, suggesting that their values at the highest simulated potential bias possibly can be used with reasonable accuracy to predict performance at even higher (but nonrelativistic) bias-potential values using analytical equations derived from stationary simulations. For example, at a Mach number of 1.1, the along-flow and across-flow sheath dimensions at high voltages are expected to be around 115% and 85% of the stationary-sheath diameter, respectively. Flow-effect correction factors for current collection are also obtained for the ram-side, wake-side, and total collected current. For the same plasma-velocity example, at high voltages, total current collection is minimized to about half of the stationary value, which would translate into a 50% reduction in power to collect the current. This example is of significance for Earth-radiation-belt remediation-system concepts using high-voltage tethers

Overview of future NASA tether applications

Abstract The groundwork has been laid for tether applications in space. NASA has developed tether technology for space applications since the 1960s. Important recent milestones include retrieval of a tether in space (TSS-1, 1992), successful deployment of a 20-km-long tether in space (SEDS-1, 1993), closed loop control of tether deployment (SEDS-2, 1994), and operation of an electrodynamic tether with tether current driven in both directions—power and thrust modes (PMG, 1993). Various types of tethers and systems can be used for space transportation. Short electrodynamic tethers can use solar power to “push” against a planetary magnetic field to achieve propulsion without the expenditure of propellant. The planned Propulsive Small Expendable Deployer System (ProSEDS) experiment will demonstrate electrodynamic tether thrust during its flight in early 2000. Utilizing completely different physical principles, long nonconducting tethers can exchange momentum between two masses in orbit to place one body into a higher orbit or a transfer orbit for lunar and planetary missions. Recently completed system studies of this concept indicate that it would be a relatively low-cost, in-space asset with long-term, multimission capability. Tethers can also be used to support space science by providing a mechanism for precision formation flying and for reaching regions of the upper atmosphere that were previously inaccessible.

Enhanced electrodynamic tether currents due to electron emission from a neutral gas discharge: Results from the TSS‐1R Mission

During the reflight of the first electrodynamic Tethered Satellite System (TSS-1R) mission, the unplanned separation of the tether at the Orbiter end resulted in the highest tether current during the mission. In the moments just prior to the tether separation with 19.7 km of tether deployed and a generated electromotive force (EMF) of 3482 V, currents reaching approximately 0.97 A were shunted through the tether to the Orbiter electrical ground, which was in contact with the ionosphere primarily through its main engine surfaces. This current level was nearly twice as large as observed during any nominal operating period. As the failure point of the tether entered into the ambient plasma, the current increased to 1.1 A and maintained this level even after the break for approximately 75 s. The principal surprise in these results was that the broken end of the tether, with only a few short strands of copper wire, could support higher currents than the much larger Orbiter conducting surface areas. Analysis of possible current enhancement mechanisms revealed that only a gas-enhanced electrical discharge, providing an electron emission source, was plausible. Ground plasma chamber tests confirmed this analysis. The TSS-1R results thus represent the highest electron current emission from a neutral plasma source yet demonstrated in a space plasma. This is of interest for current collection processes in general and plasma contactor development in particular.

Propulsive Small Expendable Deployer System (ProSEDS) space experiment

ProSEDS is a secondary (i.e. piggyback) payload on a Delta-11 GPS 8 mission scheduled for launch in August 2000. It will test the feasibility of generating generate electrodynamic thrust without propellant using a 5 kilometer conducting wire (tether). The ProSEDS obtains thrust as the tether cuts across the magnetic field, a voltage is induced across the wire. Electrons are attracted to the positively based far end of the wire. Electrons flow downward through the conductive tether. Earths magnetic field exerts a drag force on the current in the tether segments, that is mechanically transferred via the wire to the stage. The primary objective for the ProSEDs mission is to demonstrate that a significant, measurable electrodynamic thrust through a tether in space. The primary mission will last one day, as the primary battery assures at least three orbits of data will be collected, the remaining power will be provided by the secondary battery, which uses tether generated power to recharge. The extended mission begins using the power provided through the tether, and wil terminate when a system ceases to function; (i.e., either degradation of the tether,through Atomic Oxygen contact, a micrometeoroid or other debris impact, or another malfunction.) The technology has many potential applications. Amongst the applications, which are reviewed in detail, are: (1) satellite deorbit, (2) reboost of the International Space Station, (3) propellantless reusable Orbit Transfer Vehicles, (4) Propulsion and power generation for future Jovian missions.

Nanoparticle Electric Propulsion for Space Exploration

A new electrostatic thruster technology is under development at the University of Michigan using nanoparticles as propellant with micro‐ and nano‐electromechanical systems. Termed the nanoparticle field extraction thruster (nanoFET), this highly integrated propulsion concept is a high efficiency, variable specific impulse engine type that can be readily scalable for a large range of future space science and exploration missions.

Analysis of chamber simulations of long collecting probes in high-speed dense plasmas

Chamber tests of simulated electrodynamic tethers (EDTs) of different geometries operating in a dense high-speed plasma are described. The geometries tested and described here are cylindrical and flat-ribbon tape. By moving the probe samples relative to the plasma source it was possible to vary the density and therefore the effective width over a range of approximately 1 to 2 Debye lengths (/spl lambda//sub D/) for the cylinder sample and 6 to 19 /spl lambda//sub D/ for the tape samples. Several important conclusions can be drawn from the tests. 1) The current-voltage characteristics of the cylinder behave as predicted by orbital-motion-limited (OML) current collection theory in the saturation region. 2) The tape tether had comparable current levels to a theoretical equal area OML cylinder up to an effective width of at least /spl sim/11/spl lambda//sub D/ and possibly wider. 3) Orienting the tape samples parallel or perpendicular to the plasma flow yielded different current responses (perpendicular is larger) above a bias potential that is near the estimated energy of the incoming beam ions. The observed difference was generally more pronounced at larger effective widths (higher densities). 4) It was also necessary to be above this bias potential to have a V/sup 0.5/ current-voltage character appropriate for an ideal cylinder in the OML regime. It is concluded that wide ribbon-like tape tethers can be effective current collectors but that velocity effects will be a factor to consider, especially as relative width of the tape tether (with respect to /spl lambda//sub D/) grows.

Nonintrusive electron number density measurements in the plume of a 1 kW arcjet using a modern microwave interferometer

Reported is the use of a microwave interferometric technique for making nonintrusive measurements of plasma electron number density in the plume of a space electric propulsion thruster. The technique is capable of providing good sensitivity and accuracy as well as resolution using modern microwave network analyzer technology. Density profiles were obtained throughout the plume of a 1 kW hydrogen arcjet based on accurate microwave differential phase measurements. Spatially resolved integrated phase shifts for a 17.5 GHz signal radiated through the plume at various radial positions were Abel inverted to calculate radial electron density profiles. All measurements were taken in the University of Michigans Large Chamber Plasma Facility, a 6 m by 9 m vacuum chamber, at pressures of 2/spl middot/10/sup -4/ Torr or less. The interferometer measured electron densities as low as 1/spl middot/10/sup 15/ m/sup -3/ with a predicted capability to measure peak densities as high as 3/spl middot/10/sup 18/ m/sup -3/. The accuracy of this technique is estimated to be on the order of /spl plusmn/10%. Comparison with Langmuir probe electron number density measurements demonstrate relative agreement between the two methods. Further, a previously reported tendency for the Langmuir probe to underpredict electron number density was consistent with our measurements. It is postulated that this underprediction may be due to small Langmuir probe perturbations in the local plasma of the far-field plume and errors in estimation of the probe collection area. >