Development of a Focus Electrode for an Indium FEEP Thruster

The geometry of the electrode array in a field emission electric propulsion (FEEP) thruster is extraordinarily important, not only for thrust-relevant parameters, but also to ensure long-term reliability. Ion beams require good focusing because stray ions and charged droplets cause damage to the device in multiple ways. The aim of this paper is to investigate the correlation of initial spray angle at the Taylor-cone to the emitter current for an indium Liquid Metal Ion Source (LMIS) and to design an improved focusing electrode which guides the ion beam in such a manner that even very divergent beams caused by large emitter currents are focused and expelled without contact to the extractor, the focus electrodes or the plume shield. To that goal, the electrode array of an Indium-based thruster being developed by the Austrian Research Center Seibersdorf (ARC-sr) is simulated in an ion-trajectory program and various geometries tested for their focusing qualities. Finally, a focus electrode is designed which shapes the ion beam in such a manner that the expelled ion beam conforms to the specifications imposed by the requirements for the LISA mission.