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An experimental investigation was conducted to develop a 30-cm-diam, mercury bombardment thruster for potential space applications. Thruster operating efficiencies were improved by variation of the discharge-chamber geometry and magnetic field shape. A single, glass-coated system was used to achieve efficient thruster operation at lower specific impulse than previous bombardment thrusters. Propellant utilization efficiencies greater than 90% and discharge losses less than 200 ev/ion were attained. Estimated over-all thruster efficiency was about 58% at a specific impulse of 2200 sec. The investigation included a control system that afforded enough flexibility to throttle the ion beam current over a range of 2 to 1. This was accomplished without compromising expected lifetime of thruster components. NomenclatureA 0 -annular open area between distribution pole piece and baffle, cm 2 d = diameter, cm / sp = specific impulse, sec J = neutral propellant flow, equivalent amp L/D = thruster-length/anode-diameter ratio I = length, cm AV = potential voltage, v 7] = efficiency e = energy loss per beam ion, ev/ion Subscripts B = baffle / = discharge chamber OK,OM,OT = cathode, main, and total (flow), respectively P = distributor pole piece p = power (efficiency) SC = screen collar T = total U -propellant utilization

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