Plasma Thrusters

A wide range of new propulsion technologies are currently being investigated and electrodeless Radio-Frequency (RF) plasma discharges are particularly attractive because of the absence of directly-immersed electrodes. Such plasma discharges consist of an RF coil wrapped around the outside of a hollow dielectric tube and through which a feed gas is injected. The RF current in the coil produces a time-varying electric field that can sustain a high-density, partially-ionized, plasma. Such devices, also known as Inductively Coupled Plasmas (ICPs), can operate from pressures below 1 Pa to above 100 kPa. By applying a magnetic field, electromagnetic waves can be excited by the RF coil, and if the field is designed correctly, a magnetic nozzle can be created leading to plasma acceleration and thrust generation for space propulsion applications. Such systems, known as ambipolar thrusters operate at low pressure (typically less than 1 Pa) and are classified as electrostatic thrusters as ions are accelerated by ambipolar electric fields in the plasma. Although still a relatively immature technology, several companies have produced commercial products.

An alternative type of RF plasma thruster can instead be obtained by terminating the dielectric tube with a physical supersonic nozzle and operating at pressures almost 4-6 orders of magnitude higher (between about 10-100 kPa). Because of increased plasma-gas collisions at these higher pressures, intense gas heating (leading to temperatures above about 10,000 ^oC) occurs and thrust can be produced by accelerating this hot gas through a converging-diverging nozzle. Such systems, known as RF electrothermal plasma thrusters, offer several advantages over other propulsion technologies as no magnetic field or electron-emitting neutralizer is needed, there are no immersed electrodes (hence allowing reactive gases to be used), and only a single power supply is required. In contrast to RF ambipolar thrusters, the thrust-to-power ratio in RF electrothermal thrusters can be an order of magnitude higher, and thrust is nominally produced not by plasma ions, but predominately by hot neutral gas. RF electrothermal thrusters also generate a higher thrust-to-power ratio than gridded ion or Hall thrusters and so represent an attractive option for space missions requiring a combination of both reduced propellant consumption and faster orbital manoeuvres.

Together with collaborators at the Australian National University, UNSW Canberra Space is developing a new RF electrothermal plasma thruster technology that makes use of an innovative propellant injection configuration consisting of two counter-propagating vortices. Here, propellant is counter-intuitively injected with a tangential swirl from the downstream end of the thruster (just before the nozzle). Due to its initial angular momentum, the gas first spirals up along the inner surface of the thrusters towards the upstream end, before then reversing and spiralling back down towards the nozzle through the centre of the thruster. In this way, the gas flow field is segmented with a colder outer vortex flow helping to cool and reduce heat losses to the walls, and a hotter inner vortex flow that allows most of the input gas to pass through the hot central plasma region. Such bidirectional vortex flows offer a number of advantages including enhanced propellant mixing and heating, reduced heat losses to the walls, smaller thruster sizes, and the possibility of using alternative construction materials.

As with other propulsion technologies, propellant choice is an important consideration, and while noble gases such as helium and argon have previously been tested in RF electrothermal thrusters, such propellants are far from ideal. Water has emerged as a possible alternative propellant and is seen as an attractive green option. Water also has an important potential strategic advantage for future space missions as it is a substance thought to be found on the Moon, Mars, and certain types of asteroids. It may therefore be a viable propellant option to enable refuelling since it can be extracted and processed in-situ from various locations within the solar system. UNSW Canberra Space is exploring a range of possible propellant options, including water, which additionally has a number of attractive performance and engineering advantages.