On the identification and mitigation of life-limiting mechanisms of ionic liquid ion sources envisaged for propulsion of microspacecraft

Abstract

Life-limiting processes affecting ionic liquid ion sources (ILIS) are investigated in this research, motivated by the development of ILIS for propulsion of microspacecraft and other industrial applications. Micropropulsion in particular has received considerable interest in recent years due to the increasing utility of and demand for small satellites and the services they provide. A passively-fed, porous ILIS system which includes the ion thruster head, propellant supply system, and power electronics has been proposed by researchers in the Space Propulsion Laboratory at the Massachusetts Institute of Technology to address the gap in micropropulsion and provides the primary impetus for the work presented in this thesis. Spacecraft mission durations can reach up to 15 years with ongoing propulsion requirements and in ground-based applications, minimal servicing to replace the ion source is desirable. Thus, any embodiment of ILIS would benefit from operational lifetimes in excess of 1000 h. To date, successful long-duration operation of porous ILIS beyond tens of hours has not been achieved, precluded primarily by two critical challenges: electrochemistry and electrical discharges. Electrochemistry-chemical reactions between the source emitter and the ionic liquid-has been shown to be capable of fully deteriorating the source in fewer than 100 h. Electrical discharges, on the other hand, can cause source failure within a matter of seconds and are also challenging to predict and avoid. ILIS as a subcomponent of a larger spacecraft system exposed to the environment of space must also overcome challenges such as radiation and threats of micrometeorite impact. This research aims to investigate, quantify, and mitigate the primary life-limiting mechanisms of ILIS to support their successful long-life operation. A description of the onset of electrochemistry is applied to ILIS, and it is shown that existing methods for preventing electrochemistry would present a challenge to the drive electronics, requiring alternation of the polarity applied to the devices at kilohertz frequency. The relevance of an alternative contact method is revealedthe distal contact, which is shown to be effective at avoiding electrochemistry at the critical emission site. By contrast, a metal emitter with the traditional, direct electrical contact was electrochemically etched as it operated for a similar time and conditions, showing severe degradation. The distal technique has been implemented in the current version of the MIT propulsion system and more in-depth studies are ongoing to aid in distal electrode material selection and design. The source of another severe life-limiting mechanism was unknown prior to this work, but was known to be capable of causing device failure within seconds through electrical shorting, material ablation, polycondensation of the ionic liquid, and so on. Experiments performed as part of this effort revealed that electrical discharges were transpiring under certain conditions and inducing these source failures. An analytical framework for predicting the steady-state discharge is outlined for ILIS, though the process is likely complicated by the electrohydrodynamic interactions during porous ILIS operations. Given this difficulty, an experimental study was undertaken to preliminarily explore the operational and design conditions that support discharges so that they may be avoided in practical implementations and to guide future modeling efforts. Gas contamination and flooding of the substrate or unrestricted flow from the porous substrate are identified as the likely factors leading to this failure mechanism. Qualitative descriptions of those processes are provided along with suggestions for techniques for preventing them. The work concludes with a summary of other life-limiting mechanisms such as grid erosion and micrometeorite impact. Suggestions for future work related to electrochemistry include optimizing the distal electrode material properties, and understanding gas evolution there. For electrical discharges, an experimental study that resolves the challenges of the setup presented could provide more detail to the actual processes by helping to identify the ionized species. It is the hope of the author that the work discussed here and the work to come on mitigating these processes will contribute to the successful embodiment of ILIS as an appealing option for micropropulsion and for ground-based applications.