Experimental mercury bombardment thruster at submillipound thrust.

An ion thruster system, including a new type of mercury feed system and a shielded neutralizer, has been designed and tested at a thrust level of 0.5 mlb. The use of a radial flow propellant distributor and an oxide-coated brush cathode resulted in discharge power losses onefourth of those previously encountered at this thrust level. Different lengths and diameters of the discharge chamber were tested to establish a compromise between discharge power losses and propellant utilization. The final design discharge chamber was 5 cm in diameter and 7.5 cm long. The complete flight-type thruster system used a permanent magnet field and weighed (without propellant) 1.36 kg. The power-to-thrust ratio, including power to heat the feed system and the iieutralizer, was 222 w/mlb at a thrust of 0.65 mlb and a specific impulse of 3050 sec. The oxide-coated brush cathode was endurance-tested 1553 hr in a thruster, and an identical cathode was heat-cycled in a separate test for 418,000 cycles before failure.