Performance and Probe Measurements of a Radio-Frequency Plasma Thruster

The performance of a radio-frequency plasma thruster is evaluated using a displacement-type, inverted-pendulum thrust stand in the Large Vacuum Test Facility at the University of Michigan. A radio-frequency generator supplies up to 2000 W to the radio-frequency plasma thruster at a fixed frequency of 13.56 MHz. The matching network is placed inside the vacuum chamber at the thruster, and the radio-frequency power is measured at the matching network input port with a dual-directional coupler. A Faraday probe measures the current density in the far-field exhaust of the thruster, a retarding potential analyzer characterizes the ion voltage distribution, and a Langmuir probe measures the plasma potential. The radio-frequency plasma thruster is operated with argon propellant at mass flow rates of 2.4-7.6 m g/s, at corresponding corrected facility pressures of 1.5 x 10(-6)-4.6 x 10(-6) torr. The maximum thrust observed is 10.8 mN, and the maximum specific impulse is 303 s. Thrust efficiency is below 1% at all conditions. Faraday probe results show that the propellant utilization efficiency is below 15% and that the ionized exhaust has an effective divergence half-angle of 48 deg. Thrust is negatively correlated with the ion beam most probable voltage, showing that retarding potential analyzer results alone may not accurately characterize thruster performance. Charge-exchange collisions likely have a significant influence on radio-frequency plasma thruster operation.