This 10 cm diameter mercury electron bombardment ion engine was used on the first successful flight test of ion propulsion. The engine provided flow control via a boiler and a porous stainless steel plug. A hot tantalum wire was used as the discharge cathode. Beam and accelerator power supply voltages were 2500 V and 2000 V, respectively. The engine had a 1.4 kW power level with 28 mN of thrust at a specific impulse of about 4900 s. Ion beam neutralization was provided by a heated tantalum filament