Dual-mode operation of stationary plasma thrusters

The dual-mode operation of a stationary plasma thruster (SPT) has been investigated using the SPT-140 laboratory-model thruster. The SPT-140 is designed for operation at power levels between 4 and 6 kW. The first mode investigated is the standard one that the SPT-140 was optimized for and consists of mass-flow rates of 12-14 mg/s, discharge voltages between 300 and 400 V, thrust levels of 220-320 mN, and specific impulses ranging from 1900 to 2300 s. The second mode corresponds to high specific impulse (over 3000 s) at reduced thrust (less than 140 mN) operation over the same power range as the first mode. In spite of limitations in magnetic field optimization, anode thrust efficiencies over 55% have been obtained in the second mode. The physical features of high-specific-impulse SPT operation are discussed, including the limitations on the mass flow rates for efficient thruster operation and the possible impact of doubly charged xenon ions on Hall thrusters' characteristics. In addition, a thruster scaling analysis is presented that underlines the tendency for high-power thrusters to operate efficiently over a broader range of mass flow rates and discharge voltages than low-power SPTs.