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.
Highlights
The global space industry and related research and development is being disrupted by many factors including the emergence of nano-satellites, such as the small standardized “CubeSats”
The present study is a proof of concept of switch mode amplifier technology to power the electrothermal capacitively coupled radiofrequency Pocket Rocket thruster
Future work will aim at complete integration within a 1 “CubeSat” Unit and prediction of thrust output for various thruster geometries using computer fluid dynamics modeling
Summary
The global space industry and related research and development is being disrupted by many factors including the emergence of nano-satellites, such as the small standardized “CubeSats” (based on the 10 cm square unit or “1U”). Miniaturized Amplifier Powering a Plasma Micro-Thruster to provide propulsion for nano-satellites with some of the technology already under testing in orbit (resisto-jets, ion spray thruster, low power arcjet, hollow cathode thruster, pulsed plasma thruster [3,4,5,6,7]). A good example is the development by a university group of the micro cathodic arc jet with a recent satellite detumbling demonstration in orbit [3, 7]. The result of having on-board propulsion rather than only attitude determination control systems (reaction wheels, magnetorquers) would be satellites that survive longer, have a wider envelope of performance, and which can produce research data and commercial returns of greater quality and quantity, especially if formation flying and orbit maneuvers could be routinely achieved
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