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Kari Fluegel June 25, 1993

Release: 93 -049



A new technique for dissipating electrical charges from spacecraft will be put to the test when NASA's Plasma Motor Generator takes to orbit on an Air Force Delta II rocket this week.

The Plasma Motor Generator is the second in a series of three Delta-launched tether application payloads that are part of the Flight Demonstration Program sponsored by the Office of Space System Development. It will be launched as a secondary experiment that will be tested after the successful deployment of the Delta's primary payload, an Air Force Navstar Global Positioning Satellite.

The Delta II is set to be launched from Cape Canaveral's Launch Complex 17 at 9:04 a.m. June 26.

The PMG, a low-cost experiment developed at the Johnson Space Center, will assess the effectiveness of using Hollow Cathode Assemblies to deploy an ionized gas and to "ground" electrical currents by discharging the energy to space.

A spacecraft can build up an electrical charge while moving in orbit, either due to natural phenomena or due to operation of high power electrical devices on the spacecraft. Typically, this type of electrical build up is not seen with the shuttle in low Earth orbit. It is, however, seen among satellites in geosynchronous orbit that are exposed to magnetic storms from solar events. It also can occur with large spacecraft using high power electrical loads like a space station or high voltage experimental systems. In all such cases, the electrical charge buildup is due to an imbalance between electron or ion currents reaching the spacecraft from its surrounding space environment and the total current leaving the spacecraft due to operation of electrical devices. To eliminate this charge build up, the spacecraft needs to complete an additional circuit path between itself and the surrounding environment (the ionosphere), thereby "grounding" itself to the ionosphere.

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One experimental system capable of producing spacecraft charging is the electrodynamic tether. The basic principle of operation involves collection of current by a spacecraft at one end to be driven by substantial voltages to another spacecraft at the opposite end of the tether wire. This requires the operation of spacecraft grounding techniques at each end and provides a good method for testing the performance of such grounding devices. The PMG experiment uses a 500 meter wire as an electrodynamic tether to test the hollow cathode "plasma contactors" as grounding device and to study electrodynamic tether behavior.

In order to understand the operation of an electrodynamic tether system, the Earth can be thought of as a giant magnet, a principle used for direction-finding with a magnetic compass. If a copper wire is placed in orbit around the Earth, it will move through the Earth's magnetic field and a voltage will be induced in the wire. The wire will function like the armature in an electrical generator or motor, and the Earth itself will act as the magnetic pole-piece and frame against which the resulting forces on the "armature" wire react. If an electric current is allowed to flow through the tether wire by grounding it to space at both ends, the wire conductor will experience a mechanical force. This force can be either thrust or drag, depending on the direction of the current flow.

The Plasma Motor Generator is designed to complete an electrical circuit with a hollow cathode plasma generator to make a good electrical contact with the ionosphere. The PMG also seeks to demonstrate the theoretical prediction that electricity can be generated, and that thrust and drag can be induced.

The PMG's mission begins shortly after the Delta's third stage separates to take the GPS to its geosynchronous orbit leaving the second stage behind in a 100 by 480 nautical mile orbit. At that point, the power and telemetry units of the Delta's second stage will be controlled by the PMG team for its experiment. A spring will eject the Far End Package of the PMG on an electro-dynamic tether wire which will spool out trailing the Far End Package to a distance of about 500 meters. A Near End Package will stay fixed to the rocket body and will use the stage's remaining power for the experiment.

The deployment device is similar to the Small Expendable Deployer System and was developed by Johnson Space Center and Tether Applications Inc. The deployer, which weighs only 25 pounds, uses a "spinning reel" concept and cannot retrieve the attached package. The separation of the PMG from the rocket body will take about four minutes at a rate of two meters per second.

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Prior to the release of the Far End Package, plasma contactors will be energized releasing the ionized xenon gas from 1/4 inch diameter cathode tubes on each of the packages. This gas will then create a plasma ball or a pocket of ionized gas that will serve as the "ground" for electrical charges carried through the tether. The Far End Package will collect an electrical charge as it moves through space. The charge will then move through the tether to the Near End Package which will then allow it to dissipate into space. A series of tests will be conducted to better understand the performance of the Hollow Cathode Tubes. During "motor" operations, the direction of current flow will be reversed to collect a charge at the Near End Package and release it at the Far End Package.

Voltage and current generated by movement of the conducting tether through the Earth's magnetic field also will be measured. In another portion of the experiment, current will be forced through the tether to measure the effects of thrust and drag, and to use those effects to change altitude. The experiments will continue for only a few hours until the rocket stage's battery power is exhausted.

A combination of onboard instrumentation as well as ground-based sensors will be used to evaluate the performance of the flight demonstration. Observations from approximately 10 ground-based radars, magnetometers and optical sensors will provide independent evaluations of the overall PMG performance from both the dynamics and plasma interaction standpoints.

The Principal Investigator for the PMG is Dr. James McCoy, and Project Manager is Christine O'Neill, both of JSC. Suzanne Sawyer is Project Engineer and John Stanley is the project manager for ground-based measurements. The co-investigator team includes Dr. M. Grossi of the Smithsonian Astrophysical Observatory, Dr. M. Dobrowolny of IFSI of Italy; Dr. R. Chris Olsen of the University of Alabama, and Dr. R. Jerry Jost of System Planning Corporation.

NASA's Lewis Research Center in Cleveland provided for the Plasma Diagnostics Package and the Marshall Space Flight Center in Huntsville, Ala., provided the SEDS electronic box. The Goddard Space Flight Center integrated the PMG into the Delta II.

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