1. For future high powered orbit transfer and planetary missions, analyses have indicated system benefits through the use of fewer larger thrusters and increased power per thrster.' Preliminaryevaluationsof 50cmdiameter inert gas ionthruster performancehave beenpresentedinReferences 25-27. The thrust andpower perthrusterhavebeen increased over the state-of-art 30 cm diameter ion thruster, and the thrust-topowerratiosincreasedwellbeyondthose of earlier 50 and 150 cm diameter mercuryion thruster^.^^^*^ Unfortunately, the ion extraction capabilities of the 50 cm diameter dishedelectrodes havenot scaled with area as expected and, therefore,thethrusterhasnotrealizedits fullpotentialtowardreducingthe specific mass of high power ion propulsion systems. But, the ion extraction capability of ion thrusters operating at high values of specific impulse for planetary missions may not be as importantas it isfor near-Earthmissions requiringlowervaluesofspecificimpulse.
2. The ion extraction capability or perveance of nine 30 and 50 cm diameter dished molybdenum, twoelectrodeionacceleratingsystemswere evaluated with beam currents to 4 and 7.5 amperes, respectively, and minimum total accelerating voltages up to about 1800 volts. Reductions in perveance, with decreasing accelerator-hole-toscreen hole diameter ratios, anticipated from published theoretical and experimental results were observed in this effort. In addition, the perveance values were observedto increasewith a decreasing ratio of discharge voltage to totalacceleratingvoltage,asexperienced by others using small area electrodes. This effect is believed to be the primary reason that the impingement limited beam currents from large area ion acceleratingsystems increasewith total acceleratingvoltagefasterthanthethreehalves power predicted by Child's law whichgovernsspacechargelimitedflow.