Abstract
This thesis covers the topic of Air-Breathing Electric Propulsion (ABEP): the concept
of collecting upper-atmospheric air as propellant for a spacecraft in very-low Earth orbit
(VLEO). The first part of the thesis describes a computational mission analysis, which
yields the thruster performance required for drag-compensation at varying altitudes.
The realistic orbit of an ABEP spacecraft is propagated with time and a thruster control
law is introduced to avoid a divergent altitude behaviour, based on the performance
data of a gridded-ion thruster tested with atmospheric propellants. The propagations
demonstrate a stable profile with an altitude range of around 10 km at 160 − 183 km.
The majority of the thesis focuses on the development of a novel Air-breathing Microwave
Plasma CAThode (AMPCAT). An initial magnetised design shows a behaviour
representative of electron cyclotron resonance (ECR) with xenon, whereby the extracted
current increase of 3.9 times corresponds to the growth of the ECR layer across the internal
cathode, but which is not observed with air. This motivates the development of
an unmagnetised AMPCAT, which includes the identification of a dual-mode current
emission, with transition to a higher current at a relative bias of around 70 V with air.
Diagnostics of the cathode plasma show that the transition is accompanied by a sharp
increase in the plasma potential, the internal electron density and the external electron
temperature, which suggests an ionisation mechanism based on secondary electron emission
(SEE).
For the final tests, the AMPCAT is coupled with a low-power cylindrical Hall thruster
running on xenon. A comparison of the AMPCAT operation with xenon to a conventional
hollow cathode shows an improved coupling of cathode electrons at low discharge
voltages. In general, the coupled testing validates the AMPCAT’s ability, when operating on
air, to support stable thruster operation at discharge currents of up to 1 A