Analysis of a Water Vapor Helicon Thruster Using Emission Spectroscopy

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A helicon thruster was developed to research the use of emission spectroscopy diagnostics. Helicon thrusters differ from conventional electric propulsion systems in that the conductive components that power the thruster do not contact the plasma propellant. Because of this design, propellants that would be corrosive in conventional thrusters can be used in the helicon thruster. Water vapor is a possible propellant option because it is cheap, easy to store, and available in space for refueling, making it an attractive choice for deep space missions. Conventional invasive plasma diagnostics do not work reliably with water vapor plasma because it is corrosive and contains multiple species of positively and negatively charged particles, which probes that measure only plasma current are unable to distinguish. These probes can also have measurement errors in the presence of radio frequency, electric, or magnetic fields, making analysis of thruster performance difficult.

This research explores the validity of a non-invasive diagnostic technique using emission spectroscopy with a helium seed gas to determine the plasma parameters for any propellant. A helium collisional radiative model was developed to estimate electron temperature and density from helium emission line ratios measured experimentally. Thruster tests conducted with pure helium were used to compare the collisional radiative model to existing models. Tests with pure argon and an argon/helium mix allow for a comparison of Langmuir probe measurements to the emission spectroscopy results and verification that the helium gas seeding does not significantly affect the thruster performance. The diagnostic technique is then tested on a water vapor/helium mix. The estimates for the electron density predicted those measured by the conventional probes well, but the diagnostic technique is less reliable for electron temperature at the experimental conditions.