Trends in Hall Thruster Efficiency Modes with Current Density

Increasing the current density of Hall thrusters while maintaining efficient operation may enable their use for more rapid transits in deep space. Historically, however, Hall thruster performance has suffered with increased current density. To improve Hall thruster designs for these conditions, we must first understand what efficiency modes drive these performance losses. To investigate this, we used an efficiency model that breaks the total efficiency into five individual modes. We parametrically increased the current density of a Hall thruster and employed a far field probe suite to measure the individual efficiency modes at each condition. We gathered data for the thruster operating on xenon, krypton, argon, and nitrogen. We found that there is a tradeoff between increasing mass utilization and decreasing beam utilization. Mass utilization efficiency is a metric for how well the thruster ionizes the propellant, and beam utilization is a proxy for how well the electrons are confined in the channel. We compared these results to a theoretical model for how these two efficiency modes should scale for a given propellant. Our model correctly predicted that the mass utilization should peak at a higher current density for more mobile propellants with smaller ionization cross sections. Comparison of the experimental beam utilization with our model showed that the electron cross field transport was likely governed by a combination of classical and anomalous collisions. Ultimately, this work will help inform new Hall thruster designs and optimize performance at high current densities.