Abstract
This thesis describes an experimental investigation into the performance of a radio frequency plasma thruster (RFPT). This type of thruster does not rely on the use of high voltage ion acceleration grids or beam neutralisers which are typically life limiting elements of an electric thruster. The RFPT excites an atomic or molecular gas into a plasma using an external RF antenna. This plasma is contained by a cylindrical dielectric source tube which is open at one end. An axial magnetic field is applied to the plasma using one or more solenoids or a permanent magnet source. The magnetic field is usually applied such that it is constant throughout the length of the source tube, after which the field diverges. The plasma couples with the imposed magnetic field, generating internal field structures, which can be used to regulate both the power coupling and the rate at which the plasma diffuses out of the source tube. In this study, the thruster performance is measured directly, thus providing missions designers with accurate performance data which can be used to assess applicability of the technology to future missions. A re-configurable lab prototype thruster was constructed and a pendulum type thrust balance was developed in order to make direct measurements of the thrust produced by the Surrey Space Centre (SSC) RFPT. The balance has been developed to allow mounting of the thruster together with the necessary RF and DC electrical feeds as well as the propellant feeds and incorporates sensors which allow measurements to be made in an RF plasma environment. A low magnetic field mode of RFPT operation was investigated in order to assess whether this mode of operation may be able to provide performance increases at reduced imposed magnetic fields (< 20 mT), which could enable the serious consideration of this technology for future flight opportunities. The lowered magnetic fields used here may reduce the risk of interference with spacecraft subsystems and perturbation to the spacecraft orbit, when compared to many of the proposed plasma thrusters which use fields in excess of 40 mT. Direct thrust measurements of an RFPT operating in a low field mode are presented as a function of the propellant flow, RF power and for two source tube lengths. The thrust is shown to peak at a field strength, B_0, which is demonstrated to vary with RF power and propellant flow. The peaks are also shown to correspond generally to peaks in the source plasma density, plasma potential and in some cases to exhaust ion beam current. Ion energy distribution function measurements show that strong beams are not present in thruster configurations which use a 170 mm length source tube but are shown to increase in strength when using a shorter 85 mm length tube. The low field mode is shown to generally provide performance increases over the non-magnetised case by a factor of 4. Low field mode thrust measurements using the shorter 85 mm source tube and matching double saddle antenna revealed enhanced peak performance gains over the 170 mm length case, resulting in an increase of the thrust efficiency by up to a factor of 15. The performance of the RFPT operating in this configuration is shown to produce enhanced or equivalent performance, when compared to similar state of the art configurations but at generally lower imposed magnetic fields. This particular optimisation may make possible a first flight opportunity for the RFPT as weak magnetic fields may be supplied easily by electromagnets. Electromagnets can be easily deactivated or field reversed to avoid interference with spacecraft subsystems and orbital parameters unlike permanent magnets which may be required to generate higher magnetic fields. Operation of the RFPT in the low field mode with an 85~mm source has also revealed new low field mode behaviour compared to the 170 mm case, producing sudden, large reductions in the performance of the thruster for imposed fields beyond B_0 which are shown to correspond to drops in source plasma density, plasma resistance and exhaust beam current.