Electron-Bombardment Thrusters Using Liquid-Mercury Cathodes

Kaufman-type mercury electron-bombardment ion thrusters equipped with LM (LiquidMetal) cathodes have been built in 10-, 15-, and 20-cm diam. One such 20-cm thruster has successfully completed a 4000-hr life test, permitting the system life to be extrapolated beyond 104 hr. At a specific impulse of 6400 sec, the propellant efficiency was 81%, and the power loss in the discharge was reduced from 10% to 7% during the test. In shorter runs, both mass utilization and electrical efficiency were further improved. The propellant feed system for this thruster is unique in that no vapor phase is required because liquid mercury is fed directly to the LM cathode. A system has been developed for monitoring and controlling small (< 1 cm3/hr) liquid flow rates in a zero-gravity environment. Closed-loop operation has been achieved in which the beam current was used to stabilize and control the propellant feed rate, thus holding the beam current at a set value.