Theoretical and Experimental Performance of an Electron Cyclotron Resonance Thruster Operating on Water Vapor Propellant

Abstract

A coaxial electron cyclotron resonance (ECR) thruster is analyzed experimentally and theoretically for operation on water vapor. A one dimensional model using chemical kinetics theory and empirical electron impact collision cross sections is developed to determine plasma composition along the length of the thruster. A peak in thruster efficiency occurs in a region dominated by molecular ions at electron temperatures greater than 15~eV. An ECR thruster is designed, fabricated, and tested to verify model predictions. Water vapor performance is compared to argon, krypton, and xenon to understand how mass utilization efficiency and frozen flow losses vary across propellant type. Thrust performance is measured using an inverted pendulum thrust stand; plasma characteristics are measured using a retarding potential analyzer, ExB probe, Langmuir probe, and emissive probe; and species concentrations in a water vapor plasma are measured using optical emission spectroscopy. Frozen flow losses increase as electron temperature decreases and are found to be a factor of two higher for water propellant compared with the atomic species at comparable electron temperatures. It is found that higher propellant atomic mass and lower ionization energies generally exhibit higher mass utilization, due in part to lower neutral density in the thruster. Multiply charged ions did not constitute a significant fraction of the plume for the atomic species. Mass utilization efficiency and frozen flow losses describe the trends in the thrust efficiency but not the overall magnitude. Microwave coupling and diffusion to the walls are also identified as dominant loss processes. Incorporating wall and coupling losses to the control volume model enables good agreement between the experimental and predicted values for thruster performance and species concentrations.