Investigation of a miniature differential ion thruster

Abstract

Complex space missions involving formation flying or drag compensation are driving the need for spacecraft propulsion systems capable of providing low but also highly accurate thrust levels. Currently, no single propulsion device exists that is able to provide both precision and coarse thrust capability over the micro-Newton to milli-Newton thrust range required by these missions. A need for a precision, low thrust, miniature electric propulsion device with a wide throttling range therefore exists. The concept of a differential ion thruster was initially proposed by the Ion Propulsion Group of QinetiQ to address this requirement. It was proposed that an unprecedented throttling range and thrust resolution could be achieved through differential control of opposing ion beams, by which very small net offsets in thrust could be achieved. Single ion beam operation, as for conventional gridded ion thrusters, would permit higher thrust levels to be achieved with high specific impulse. The extraction and independent control of two ion beams from a single gridded ion thruster has never previously been reported. Prototype and breadboard models of the proposed Miniaturised Differential Gridded Ion Thruster (MiDGIT) were designed and manufactured in collaboration with QinetiQ to provide a proof-of-concept and to demonstrate preliminary performance. Test campaigns were conducted at the QinetiQ Large European Electric Propulsion Facilities and within the EP1 vacuum chamber at the University of Southampton. The work reported in this thesis contributes to the first detailed characterisation of a twin-ended radio frequency gridded ion thruster utilising a common plasma discharge. Two control methods were identified which permitted independent control of the ion beams extracted from either end of the thruster. These were: variation of the accelerator grid potential in order to induce changes in the plasma sheath geometry upstream of each screen grid leading to variations in the extracted ion currents, and variation of the RF power delivered to each end of the thruster to generate a higher plasma density on one end of the discharge and ultimately a net thrust out of that end of the thruster. The performance of the MiDGIT thruster has been evaluated with regards to both coarse thrust and fine thrust control requirements. Though the MiDGIT thruster has demonstrated a wide thrust range surpassing competing single-ended miniature ion thrusters, the extraction of two ion beams to achieve very low thrust levels leads to low specific impulse and high specific power for the MiDGIT thruster compared to any other single-ended ion thruster that can achieve the same thrust levels. Recommendations to improve efficiency are made and suggestions for future work and further development of the MiDGIT thruster are given.

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