The baffle aperture region of an ion thruster

Abstract

During the period of this research project, electron bombardment ion thrusters have passed from an advanced state of development to highly successful flight demonstrations (e.g. Deep Space 1). Such advances made by NASA have been quickly followed in the commercial sector (e.g. NSSK of communications satellites). In the next few years, these early successes will be followed by many new electric propulsion missions, planned by the world's space agencies and commercial space organisations.

These early successes will spawn more ambitious and demanding missions, necessitating thrusters of different power demands, thrusts and sizes. Scaling of the UK series of electron bombardment ion thrusters has previously been carried out by semi-empirical scaling laws. These scaling laws have worked well within a certain range of thruster sizes with some iterative redesign necessary to produce acceptable efficiencies. However, when scaling beyond the ranges of the T5, T6 and UK25, the current scaling laws cannot be used due to the conflicting requirements of magnetic field strength in the discharge chamber and baffle aperture region. The baffle aperture region contains a plasma double layer that accelerates primary electrons into the discharge chamber and thus controls ionisation in the ion thruster.

Previously, the baffle aperture region was poorly understood. An extensive and unique experimental investigation of the plasma properties around this critical baffle aperture region has been carried out using Langmuir probes. The externally applied magnetic field has been mapped and the Langmuir probe results have been validated using emissive probes. Results are presented on both argon and xenon propellant and high-resolution maps of an extensive range of plasma properties are plotted over a continuous area from the coupling plasma, through the baffle aperture and into the discharge plasma. Maps were taken over a unique and wide range of thruster operating conditions in which all actively controllable thruster parameters were varied. Unique maps of mono-energetic primary electrons are presented.

Theoretical models have been used to create a novel and detailed understanding of the baffle aperture potential difference. Current flow through the baffle aperture is shown to be non-classical and can be modelled using the electron fluid equation with anomalous diffusion and mobility coefficients. The potential difference is classed as a magnetised or an unmagnetised double sheath, depending upon the operating conditions. Two types of primary electron spatial distributions were found and these strongly correlated to the type of double sheath. A new scaling law for the baffle aperture region is proposed and schematics presented on how such a law may obeyed, thus increasing the range of sizes for which the thruster scaling laws are valid.

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