A class of miniaturized pulsed plasma thrusters (PPT), known as MicroPPTs,' is currently in development at the Air Force Research Laboratory. The MicroPPTs use a surface discharge across solidpropellant to provide precise impulse bits in the 10 range. In the near term, these thrusters can provide propulsive attitude control on spacecraft using the dry mass of conventional torque rods and reaction wheels.' Eventually these thrusters are designed for primary and attitude control propulsion on 25-kg class spacecraft. Efforts to characterize MicroPPT performance and the thruster plume are underway. To this end, a modified torsional thrust stand has been developed for the purpose of accurately measuring the low-level thrust generated by the MicroPPT. A Hemott Cell interferometer is introduced to establish the electron and neutral densities in the thruster plume. Comparison of the measured electron density with modeling predictions shows close agreement. Additionally, a Pockels cell has been developed to provide a zero-impedance MicroPPT breakdown voltage measurement, and an intensified CCD array has been used to characterize the divergence of both the thruster plume and the particulate emission. A synopsis is also presented of the status of the thruster development, including lifetime, thermal, and environmental testing.

Public reporting burden for this collection of information is estimated to average 1 hour per response, including t gathering and maintaining the data needed, and completing and reviewing the collection of information. Send c) collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, 2 Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwoi.

Publicreporting burSen for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. REPORT DATE (DD-MM-YYYY)2. REPORT TYPE The Aerospace Corporation, El Segundo, CA AbstractThe United States Air Force Research Laboratory's Electric Propulsion Space Experiment (ESEX) was launched and operated in early 1999 in order to demonstrate the compatibility and readiness of a 30-kW class ammonia arcjet for satellite propulsion applications. As part of this flight, an array of on-board contamination sensors was used to assess the effect of the arcjet and other environments on the spacecraft. The sensors consisted of microbalances to measure material deposition, radiometers to assess material degradation due to thermal radiation, and solar cell segments to investigate solar array degradation. Over eight firings of the ESEX arcjet (and 33 min. 26 sec operating time), the following preliminary results are reported. The microbalances show no measurable deposition from the arcjet, in agreement with predictions. The radiometer near the thruster, viewing the arcjet plume and body, experiences a change in the thermal properties of its coating from the firings. Radiometers with no view of the arcjet, or a view of only the plume, show no change. During firings, the solar cell segments, near the thruster, show decreasing open-circuit voltage; probably attributable to an additional electrical load provided by the plume plasma. The solar cells also exhibit a 3% decrease in non-firing, solar-illuminated short-circuit current over the eight arcjet firings, attributable to decreased solar transmission of the cover glass. However, no effects associated with the arcjet are observed on the spacecraft solar arrays. These data are in good agreement with model predictions, where available. In general, contamination effects are observed only on sensors near the thruster exhaust nozzle, a location unlikely to be used in an operational high-power electric propulsion system. No contamination effects are observed in the backplane of the thruster. For future programs, while engineering measures may be needed for spacecraft equipment in the immediate vicinity of the thruster body, the arcjet environment is generally benign.

No abstract