Examining the ionization reactions in water-vapor Hall thruster plasma
Examining the ionization reactions in water-vapor Hall thruster plasma lead image
Hall-effect thrusters use electric fields to accelerate a propellant, generating thrust for spacecraft. Typical Hall thrusters use noble gasses like argon (Ar) and xenon (Xe) as propellants due to their high mass and low reactivity. However, these gases are rare on Earth, making them expensive to use in space. In contrast, water could also be used in Hall thrusters, leading to cheaper propellants and the possibility to one day harvest more fuel in space.
Shirasu et al. used a fast-moving Langmuir probe to measure the electron energy distribution functions (EEDFs) in the discharge plasma of a water-vapor Hall thruster. Their goal was to better understand the discharge processes to improve the thruster design.
In a water-vapor Hall thruster, the ionization process produces not only ionized water but also fragments like ionized hydrogen (H+) and hydroxide (OH−). These dissociated ions, and their effects on the discharge plasma, are not well understood.
The Langmuir probe used by the researchers provided a detailed look at these ion interactions. The team identified the relative concentration of ionic species present in the plasma and estimated their impact on thruster efficiency.
“The observed EEDFs exhibited quite kinetic characteristics, which means that the electron in the discharge plasma is not necessarily thermalized or isotropic,” said author Kento Shirasu. “This emphasizes the necessity of experimental investigations in addition to fluid-type simulations.”
The authors are planning to supplement their experimental data with additional studies.
“We have to validate the chemical species information estimated in this study,” said Shirasu. “We are planning to try direct measurements of ion species using probe or optical techniques.”
Source: “Experimental investigation of electron-impact reactions in the plasma discharge of a water-vapor Hall thruster,” by K. Shirasu, H. Koizumi, H. Sekine, K. Komurasaki, Journal of Applied Physics (2024). The article can be accessed at https://doi.org/10.1063/5.0230606 .
