Spectroscopic Investigation of Water-Propellant Hall-Effect Thrusters: Toward Understanding Physicochemical Processes and Propulsion Characteristics

In this presentation, we investigate the characteristics of plasmas generated in Hall-effect thrusters using water as an alternative propellant and how such plasmas influence propulsion performance, primarily through spectroscopic diagnostics. Water, being a liquid at standard conditions, is easy to store and handle, and offers high safety and accessibility. These features make it a promising alternative to conventional propellants, especially for lowering the barrier to space access. Composed of light elements abundant in space, water also holds potential for in-situ resource utilization on the Moon, Mars, and beyond. Its applicability to life support, energy storage, and chemical propulsion further underscores its value as a key resource for future space missions. Nevertheless, practical applications of water as a propellant remain extremely limited, and studies on its use in electric propulsion are scarce. In particular, plasma propulsion, which involves complex physicochemical processes, still faces numerous scientific and engineering challenges, with a fundamental understanding and technology development largely unexplored. This study is part of an international collaborative project between Japan and France. The target thruster is a water-propellant Hall-effect thruster developed at the University of Tokyo. Electron energy distribution functions (EEDFs) are measured using a Langmuir probe in Japan. At the same time, measurements of excited hydrogen atoms via laser-induced fluorescence (LIF) and vibrationally excited species via emission spectroscopy are conducted at CNRS in France. The obtained EEDF data reveal the variety of reactions occurring in water plasmas and provide quantitative insight into electron-driven processes. These results, combined with neutral particle simulations, enable us to predict the transport behavior of radicals, excited species, and ions within the thruster. By validating these predictions against spectroscopic measurements, we aim to clarify the physicochemical processes that govern the operation of water-based Hall-effect thrusters.