Analysis of Wall Loss Currents in an ECR Magnetic Nozzle Thruster

The plasma loss currents through the walls of a low-power electron cyclotron resonance magnetic nozzle thruster are experimentally characterized. A test article is employed where the walls of the plasma chamber have been isolated from the surrounding thruster body and the device is tested in two configurations. The first has the two surfaces isolated, where the floating potential of each is characterized independently. The second configuration connects the thruster body and wall through a shunt resistor, enabling direct measurements of the current flow. It is found that nearly 15% of the input power is lost to driving these wall currents. As a result, the connected configuration yields thrust values approximately 20% lower than the isolated case. This difference in performance is primarily attributed to a decrease in ion current resulting from plasma losses to the wall. As a secondary method of eliminating loss current pathways, a dielectric plate is added to the exit plane of the thruster. This addition improves performance of the device compared to the two other configurations tested. Total efficiency is increased from 4% in the connected configuration to 10% when the dielectric plate is added. The plate also eliminates dependence on the electrical configuration of the internal thruster components and reduces the current measured between the body and wall by an order of magnitude. These results are discussed in the context of non-local loss currents that form at the boundaries between the plasma and thruster surfaces. Thin sheath theory is applied to quantify these losses and compare the theoretical values to those observed experimentally.