Electric Propulsion (EP) systems are an emerging solution for propulsion in modern spacecraft, as they have been proven to produce a higher specific impulse in comparison to traditional chemical systems, resulting in significantly lower propellant mass requirements - leading to lower launch mass, reduced cost, and extended mission lifetime. The term "electric propulsion" encompasses a wide range of systems including gridded ion engines and Hall effect thrusters. In general, EP systems make use of electric and magnetic fields to ionise a propellant gas and accelerate the resulting ions away from the spacecraft at high velocity, imparting an impulse and producing thrust. These systems are potentially suitable for North-South station keeping of telecommunication geostationary satellites, attitude control, and orbit transfer in low and medium Earth orbit satellites and geostationary "all-electric" platforms, as a means of re-orbiting end-of-life spacecraft to graveyard orbits, for de-orbiting manoeuvres, and also for large delta-V interplanetary missions. Some examples of applications of EP systems in modern spacecraft: ESA's GOCE satellite, which mapped the Earth's gravitational field, employed an ion thruster to compensate for atmospheric drag caused by its low orbit altitude of ~250 km; NASA's Dawn probe's ion engine has, so far, achieved a record breaking delta-V of 10.8 km/s during its mission to Vesta and Ceres in the asteroid belt; and the ESA/JAXA BepiColombo satellite, that is planned to launch in October 2018, will use a solar-powered ion engine (along with a number of gravity assists) for its 7.2 year cruise to Mercury.

Although EP systems are a proven solution for many space applications, there are two main challenges that must be addressed: scaling-down mature EP technologies results in much lower efficiency and reduced specific impulse; and the main candidate for EP propellant gas currently in use is Xenon, which is very rare and expensive to produce. Xenon is widely used as a propellant for EP as it is an inert gas with low ionisation energy per unit mass, and high density at moderate pressure. Alternatives to Xenon are being investigated, with the aim of not only reducing cost, improving availability, and increasing the small-scale model efficiency, but also broadening the applications and capabilities of EP systems by enabling in-situ propellant harvesting, or air-breathing electric propulsion, to further contribute to reductions in launch mass and cost,.

This PhD project will focus on the experimental investigation of low-power electric propulsion with unconventional propellants. By combining methods from physics and engineering, this project will result in an understanding of the physics involved in running various plasma sources with molecular propellants, such as air and water vapour, by employing an array of combined diagnostic systems, such as electrostatic and spectroscopic probes. These diagnostics will reveal the composition of the molecular propellants after plasma ignition and molecular dissociation, leading to a greater understanding of extraction efficiency and propellant utilisation. Additionally, the propulsive performance of the chosen unconventional propellants will be experimentally determined using a number of available Surrey Space Centre EP systems (microwave electron cyclotron resonance thruster and Halo thruster) as a means of measuring their performance capabilities against known metrics with noble gases, such as Xenon and Argon. If an affordable, more readily available propellant with similar or greater qualities to Xenon is found, this will provide a novel solution that will greatly improve the prospects of electric propulsion in the future.