Testing a frequency-tuned radiofrequency plasma thruster having tiny matching circuit immersed in vacuum

A vacuum-immersed radiofrequency plasma thruster containing a permanent-magnet-induced cusp and a magnetic nozzle is designed and powered by an automatically frequency-tuned radiofrequency generator for the frequency of 40 ± 3 MHz and the maximum rf power of about 120 W, where tiny ceramic capacitors are installed near the thruster head for the impedance matching. Operational ranges of the rf power and the gas flow rate are preliminarily characterized for argon, krypton, and xenon, toward further thruster assessment. A gas flow rate giving a maximum ion saturation current of a Langmuir probe located 30 cm downstream of the thruster is found to be changed by the propellant species, which can be qualitatively explained by the measured voltage standing wave ratio and the cross-field plasma loss from the magnetic nozzle. It is confirmed that a steady-state operation causes no thermal issue of the tiny matching circuit.