AUTHOR=Charles Christine , Liang Wei , Raymond Luke , Rivas-Davila Juan , Boswell Roderick W. 

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

JOURNAL=Frontiers in Physics

VOLUME=Volume 5 - 2017

YEAR=2017

URL=https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2017.00036

DOI=10.3389/fphy.2017.00036

ISSN=2296-424X

ABSTRACT=A structurally supportive miniaturised low-weight ($\le$150~g) radiofrequency switch mode amplifier developed to power the small diameter {\it Pocket Rocket} electrothermal plasma micro-thruster called {\it MiniPR} is tested in vacuum conditions representative of space to demonstrate its suitability for use on nano-satellites such as `CubeSats'. Argon plasma characterisation is carried out by measuring the optical emission signal seen through the plenum window versus frequency (12.8-13.8~MHz) and the plenum cavity pressure increase (indicative of thrust generation from volumetric gas heating in the plasma cavity) versus power (1-15~Watts) with the amplifier operating at atmospheric pressure and a constant flow rate of 20~sccm. Vacuum testing is subsequently performed by measuring the operational frequency range of the amplifier as a function of gas flow rate. The switch mode amplifier design is finely tuned to the input impedance of the thruster ($\sim$16~pF) to provide a power efficiency of 88 $\%$ at the resonant frequency and a direct feed to a low-loss ($\sim 10 \%$) 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 6000~l.s$^{-1}$ and a vacuum pressure of the order of $\sim$ 2x10$^{-5}$~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.