This is the first part of a two part paper in which we explore the effect of cathode position and magnetic field configuration on Hall-effect thruster (HET) performance. In this part, the effect of the cathode position on the efficiency of the thruster is explored. The study consists of two experiments in which we compare the efficiency of an HET in such a way as to change the external magnetic field topology with respect to the cathode position to its original configuration. We show the

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... nce of the magnetic field separatrix, a surface which divides the magnetic field lines into "internal" and "external" regions. The modifications provide a means to improve cathode coupling while keeping the cathode outside of the damaging ion beam. The improvement in cathode coupling results in improved thruster performance. Specifically, better cathode coupling voltages, lower beam divergences, and an improvement in efficiency of up to 10 percentage points are noted.