Energetics of Propellant Options for High-Power Hall Thrusters

An emerging hurdle in high-power Hall thruster development has been the use of Xenon as the propellant of choice. While Xenon has several advantages as a propellant, namely low ionization energy, high atomic mass and easy storage and flow metering, there are several disadvantages that preclude the use of Xenon in very high power thrusters. The first disadvantage to Xenon as an EP propellant is its high cost. Currently Xenon can be purchased for approximately $5 per standard liter ($850/kg). Using current commercial prices, a 500-kW Hall thruster operating at 60% efficiency and 2,000-sec Specific Impulse (Isp) will consume $4,773 of Xenon per hour of operation. These costs can be extrapolated to $114,550 per test day, and $47.7M for a 10,000-hour mission. Longer-duration missions utilizing larger thrusters or many smaller thrusters can quickly become relatively expensive to supply with propellant. To defer this cost, more economical propellants need to be utilized. Studies have focused on Krypton as a more-economical alternative to Xenon, while maintaining the general design of a gaspropelled thruster. The second major disadvantage to Xenon is in ground testing. Thruster exhaust must be evacuated from a test facility in order to maintain a space-like vacuum. Typically this is accomplished with cryogenic vacuum pumps. For a 500-kW thruster operating at 2,000-sec Isp and 60% efficiency, 1.56g/sec of Xenon will enter the chamber. In order to maintain 5x10 Torr, the pumping capacity must be at least 40M liters per second. At a cost of roughly $1 per-liter per-second, this translates to roughly $40M in pumping equipment. The pumping requirements scale linearly with thruster power, so a 1MW thruster will require over $80M in pumping equipment. Additional costs include the large vacuum chamber, support infrastructure and recurring costs such as Liquid Nitrogen. Unlike propellant costs, facility costs cannot be reduced unless gaseous propellant usage is eliminated, as any gaseous propellant will require evacuation from the facility. The combination of the need for a more economical propellant, and also a propellant that does not require expensive pumping apparatus is apparent. Condensible propellants, defined as those species existing in either solid or liquid state at STP, offer significant advantages for facility cost. While Xenon, Krypton, and other gaseous propellants must be actively evacuated from the test chamber, condensible propellants will naturally condense on the chamber walls, requiring no pumping. Thus condensible propellants are ‘self-pumping,’ requiring only enough vacuum pump capacity to reach and maintain high vacuum with little to no gas load.