Hall-Effect Thruster--Cathode Coupling, Part I: Efficiency Improvements from an Extended Outer Pole

This is the first part of a two-part paper in which the effect of cathode position and magnetic field configuration on Hall-effect thruster performance is explored. In this part, the effect of the cathode position on the efficiency of the thruster is explored. The study consists of two experiments performed on a Hall-effect thruster with its outer pole modified so as to change the external magnetic field topology with respect to the cathode position. In these experiments, the efficiency of the modified thruster is compared with the efficiency of the Hall-effect thruster in its original configuration. The importance of the magnetic field separatrix, a surface which divides the magnetic field lines into internal and external regions, is shown. The modifications provide a means to improve cathode coupling while keeping the cathode outside of the ion beam, which can erode the cathode components. The improvement in cathode coupling results in improved thruster performance, specifically improved cathode coupling voltages (the potential between the cathode and facility ground) of approximately 3% of the discharge voltage, increased beam divergence efficiencies on the order of 5 percentage points, and an improvement in total efficiency of up to 10 percentage points are noted. [PUBLISHER ABSTRACT]