Erosion and Performance Scaling of Electrodeless Plasma Thrusters with a Magnetic Nozzle

Abstract

The scaling of erosion and performance characteristics of electrodeless plasma thrusters with magnetic nozzles is investigated analytically and numerically. A quasi 1-D, two species model was used in conjunction with sheath and sputtering models to develop a numerical scheme for erosion rates and thruster performance. Analytical scaling laws were derived for thrust efficiency and erosion rates and analyzed through the numerical model. Wall impulse, which is derived as the total impulse obtained per unit thickness of wall material eroded, was analyzed as an objective metric for lifetime characteristics. Scaling laws strongly depended on the dominant diffusion mechanism, and the inclusion of anomalous transport introduced unique trade-offs between thrust efficiency and wall impulse. Generalized asymptotic scaling laws for thrust efficiency and wall impulse were calculated for argon and xenon. Comparison with wall impulse of existing Hall thrusters suggest that the erosion rates in electrodeless plasma thrusters can be comparable to those of Hall thrusters and lifetime limitations cannot be ignored. In addition to the investigation of scaling laws, an EXB probe, or Wein filter, was designed and made as a diagnostic method for future research. The probe was tested on the SPACE Lab Alternative Propellant ECR eXperimental (APEX) thruster along with a Langmuir probe. The probe was able to collect velocity data as expected and was also able to distinguish doubly charged ions at high input powers. Direct correlation between the numerical model and measured data was not possible likely due to differences in geometry and energy injection methods.