The central topic of my PhD research, the Pocket Rocket is a radio-frequency capacitively coupled electrothermal microthruster. Designed for use on CubeSats the Pocket Rocket operates on very low power, typically < 50W, and produces thrusts in the range of 1-10 mN. My work involved the numerical modelling and experimental validation of the device for operation in different propellants, and with multi-harmonic tailored voltage waveforms.

The Pocket Rocket source facilitates the coupling of electrical power into neutral propellant through ion-neutral charge exchange collisions, where the ions primarily serve as an intermediary. Ions within the thruster source are radially accelerated through the sheath potential, enhanced by means of a dc self-bias voltage that forms as a negative surface charge on the insulating alumina dielectric walls, located between the plasma and the powered electrode. This negative dc self-bias voltage forms due to the physically asymmetric geometry of the Pocket Rocket, and enforces equal positive and negative charge loss to both the smaller powered electrode and larger grounded electrode areas. The requirement for equal net charge loss through the smaller powered electrode area maintains a significant positive ion flux through the powered electrode sheath. Ions accelerated through the sheath potential collide with neutral propellant traversing the axial pressure gradient, resulting in neutral heating via ion-neutral charge exchange, generating thrust.

The application of multiple superimposed voltage harmonics to the Pocket Rocket facilitates enhanced control of plasma parameters and discharge conditions. Such control allows for the concurrent optimisation of ionisation pathways (electron dominated processes) and neutral gas heating pathways (ion dominated processes) through the so-called Electrical Asymmetry Effect (EAE). Through careful tailoring of the phase offsets and amplitudes of the voltage harmonics our work was able to numerically predict, and experimentally validate, discrete control over the ion and electron species within the plasma – and hence demonstrate variable impulse operation within the Pocket Rocket thruster.

Ongoing work in this area involves enhancing control of the spatial power deposition through the application of magnetic fields. Through varying the magnetic topology, the charged fluxes to surfaces may also be altered via the so-called Magnetic Asymmetry Effect (MAE). Careful combination of the aformentioned EAE and the MAE lead to the generalised ElectroMagnetic Asymmetry Effect (EMAE), the non-linear behaviour of which – and subsequent effect on discharge performance – is still poorly understood.

Published outputs from this research area include:

1) S. J. Doyle, et. al., “Decoupling ion energy and flux in intermediate pressure plasmas utilizing tailored voltage waveforms”, Plasma Sources Sci. Technol. Special Edition on Tailored Voltage Waveforms, vol. 29, p. 124002, (2020), doi:10.1088/1361-6595/abc82f

2) S. J. Doyle, et. al., “Inducing localised beam-like ion energy distribution functions in intermediate-pressure plasmas”, Physics of Plasmas 26, 073519 (2019), doi:10.1063/1.5111401

3) S. J. Doyle, et. al., “Control of electron, ion and neutral heating in a radio-frequency electrothermal microthruster via dual-frequency voltage waveforms”, Plasma Sources Sci. Technol., vol. 28, p. 035019, (2019), doi:10.1088/1361-6595/ab0984

4) S. J. Doyle, et. al., “Spatio-temporal plasma heating mechanisms in a radio-frequency electrothermal microthruster”, Plasma Sources Sci. Technol., vol. 27, p. 085011, (2018), doi:10.1088/1361-6595/aad79a