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I enjoyed working with all of them. I was impressed with their skills and professionalism. I would like to thank them for all the assistance they provided me over the course of my research. Although they had doubts that I would ever collect enough data, they never stopped supporting me. The technicians at GRC were always willing to listen my crazy ideas and figure out how to make them a reality. I would like to thank these technicians for their high quality workmanship. Thanks especially to Pat Barber, Jim Coy, Wayne Gerber, Chris Kandrach, Mike Pastel, Rich Polak, Dennis Rogers, and Bob Roman. They will receive a copy of my diploma as proof that I actually finished my degree. Materials folks at GRC, Dave Hull, Joy Buehler, and Tiffany Biles provided assistance in analyzing my molybdenum samples. I want to thank them for helping me with sample preparation and showing me how to use the microscope equipment. I would like to thank ESML at PNNL for the use the RBS facility. I would like to personally thank V. Shutthandan and Yanwen Zhang for their willingness to schedule time for me in the laboratory. They did a professional job in the RBS analysis. Thanks, too, to the other students I worked with at GRC. Alex Kieckhafer, Sonca Nguyen, and Tyler Hickman made time in the lab go quickly. Ken Street at GRC helped me use the microbalance. I appreciated his time and willingness to support my research. I would like to thank Sharon Miller for serving as my grant monitor. Her effort to take care of the administrative aspects of the grant was appreciated. I want to thank Chris Hall for serving on my advisory committee. I enjoyed having him as a professor at Virginia Tech. I greatly appreciated the support of my family and close friends throughout my graduate school career. They never wavered in cheering me on to the finish line.

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Public reporting burden 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)10-08-2011 REPORT TYPE Conference Paper DATES COVERED (From -To) TITLE AND SUBTITLE 5a. CONTRACT NUMBER A Performance Comparison of Xenon and Krypton Propellant on an SPT-100 Hall SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)Air Force Research Laboratory (AFMC) AFRL/RZS SPONSOR/MONITOR'S Pollux Drive NUMBER(S)Edwards AFB CA 93524-7048 AFRL-RZ-ED-TP-2011-342 DISTRIBUTION / AVAILABILITY STATEMENTApproved for public release; distribution unlimited (PA #11558). SUPPLEMENTARY NOTESFor presentation at the 32 nd International Electric Propulsion Conference, Wiesbaden, Germany, 11-15 Sep 2011. ABSTRACTThe use of krypton as an alternative to xenon for Hall thruster propellant is an interesting option for satellite system designers due to its lower cost. However, this cost-savings comes at the expense of thrust efficiency. Reduction in efficiency can be caused by energy losses from Joule heating, radiation, and the ionization process as well as degradation of plume quality from an increase in velocity distribution spread (most often from an increase in multiply charged ion populations) and geometric beam divergence.1 In order to quantify this performance reduction for the case of the flight model SPT-100 HET (1.35 kW), an ongoing series of experimental measurements is being conducted to measure how various thruster efficiency terms change with propellant and operating condition. This study will combine thrust measurements with plume data from electrostatic probes. This paper presents the results of performance measurements made using an inverted pendulum thrust stand. Krypton operating conditions were tested over a large range of operating powers from 800 W to 3.9 kW. Analysis of how performance is impacted by propellant and operating condition is presented. A simple mission analysis was done based on these measurements to evaluate the practicality of krypton propellant for an SPT-100 subsystem using krypton propellant for north-south station keeping (NSSK) for a typical communications spacecraft in geosynchronous orbit.. SUBJECT TERMS

The plume of a Busek BHT-200 xenon Hall thruster has been characterized through measurements from various plasma electrostatic probes. Ion current flux, plasma potential, plasma density, and electron temperatures were measured in the near-field of the plume to 60 cm downstream of the exit plane. These experimentally derived measurements were compared to simulations of the thruster/vacuum chamber environment using the plasma plume code DRACO. The goals of this study were to gain understanding of the effect of the vacuum facility on the thruster plume and to determine the fidelity of the DRACO numerical simulation.

Increased background pressure in vacuum chamber test facilities as compared to on-orbit operation has been shown to influence the operation of electric propulsion devices such as Hall thrusters. This study aims to elucidate the impact of pressure on the ionization and acceleration mechanisms in a stationary plasma thruster, model SPT-100 Hall thruster, using time-averaged and time-resolved laser-induced fluorescence velocimetry. The results are compared for the thruster operating at an applied 300 V (∼4.25 A), with vacuum facility background pressures ranging from 1.7 × 10 −5 to 8.0 × 10 −5 torr. Time-averaged measurements reveal that, in general, an upstream shift in the position of the ionization and acceleration regions occurs as the facility pressure is increased above the nominal 1.7 × 10 −5 torr. Time-resolved measurements, implemented using a sample-hold scheme with 1 μs resolution, emphasize that similar acceleration profiles are present within the Hall thruster discharge channel regardless of background pressure. Measurements taken at 3.5 × 10 −5 torr, where the facility background neutral density is similar to the neutral density emitted from the thruster, unexpectedly show increased ion acceleration over the next highest pressure condition at 5.0 × 10 −5 torr. These results indicate a not-yet well defined balance of the impacts of neutral ingestion, classical and turbulent electron transport on thruster operation, and that the ratio of the background to thruster neutral density is a more relevant benchmark than background pressure alone when evaluating Hall thruster operation.