Krypton, cadmium, iodine, cesium, mercury, and bismuth are compared with xenon in several areas of performance, including thrust, specific impulse, probability of ionization, maximum theoretical efficiency, and sputter yield. The lighter propellants such as krypton and cadmium are favorable for high-I sp , low-thrust applications, whereas heavier propellants such as mercury and bismuth are preferable for low-I sp , high-thrust missions. Calculations of the ionizing collision rate show that cesium had the highest ionization probability (and lowest ionization energy), krypton had the lowest probability (and highest ionization energy), with the other propellant falling between cesium and krypton. Sputter erosion calculations show that for a carbon surface, heavier atoms will sputter less at low ion energies (less than 2000 eV) than light atoms, and will sputter much less on a kilograms-per-kilogram basis.

Krypton, cadmium, iodine, cesium, mercury, and bismuth are compared with xenon in several areas of performance, including thrust, specific impulse, probability of ionization, maximum theoretical efficiency, and sputter yield. The lighter propellants such as krypton and cadmium are favorable for high-I sp , low-thrust applications, whereas heavier propellants such as mercury and bismuth are preferable for low-I sp , high-thrust missions. Calculations of the ionizing collision rate show that cesium had the highest ionization probability (and lowest ionization energy), krypton had the lowest probability (and highest ionization energy), with the other propellant falling between cesium and krypton. Sputter erosion calculations show that for a carbon surface, heavier atoms will sputter less at low ion energies (less than 2000 eV) than light atoms, and will sputter much less on a kilograms-per-kilogram basis.

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The 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 the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S NUMBER(S) DISTRIBUTION/AVAILABILITY STATEMENTAppi'oved for publ.Je reteoato, AFRL-SR-AR-TR-07-0207,distrlbution unliglitod SUPPLEMENTARY NOTES 14, ABSTRACTThis report summarizes major findings from a three-year effort to develop and characterize a 2-kW bismuth Hall thruster. The device utilizes a set of segmented anodes, wherein discharge current can be shifted to control the temperature of a bismuth evaporator. Thruster performance on Xe is reported to establish a baseline attributable to segmented anode geometry separate from propellant species. Results are presented on bismuth, confirming the ability to maintain a self-sustaining metal-vapor plasma discharge using only waste heat from the thruster. Thermal failure of stainless-steel porous propellant diffusers is documented along with a fabrication strategy to construct diffusers using porous molybdenum. Results are presented for current and voltage behavior of a LaB6 cathode operating on bismuth vapors. SUBJECT TERMS SECURITY