Vacuum Testing of a Miniaturized Switch Mode Amplifier Powering an Electrothermal Plasma Micro-Thruster

A structurally supportive miniaturized low-weight (≤150 g) radiofrequency switch mode amplifier developed to power the small diameter Pocket Rocket electrothermal plasma micro-thruster called MiniPR is tested in vacuum conditions representative of space to demonstrate its suitability for use on nano-satellites such as "CubeSats." Argon plasma characterization is carried out by measuring the optical emission signal seen through the plenum window vs. frequency (12.8-13.8 MHz) and the plenum cavity pressure increase (indicative of thrust generation from volumetric gas heating in the plasma cavity) vs. power (1-15 Watts) with the amplifier operating at atmospheric pressure and a constant flow rate of 20 sccm. Vacuum testing is subsequently performed by measuring the operational frequency range of the amplifier as a function of gas flow rate. The switch mode amplifier design is finely tuned to the input impedance of the thruster (~16 pF) to provide a power efficiency of 88% at the resonant frequency and a direct feed to a low-loss (~10 %) impedance matching network. This system provides successful plasma coupling at 1.54 Watts for all investigated flow rates (10-130 sccm) for cryogenic pumping speeds of the order of 6,000 l.s^-1 and a vacuum pressure of the order of ~2 × 10^-5 Torr during operation. Interestingly, the frequency bandwidth for which a plasma can be coupled increases from 0.04 to 0.4 MHz when the gas flow rate is increased, probably as a result of changes in the plasma impedance.