A liquid caesium field ion source for space propulsion

Liquid metal ion sources usually employ a single point geometry for the anode either in the form of a wetted needle or a capillary. The realisation of high ion currents whilst, at the same time, maintaining a low neutral mass emission rate has necessitated a different approach. A Cs field ion source is reported which is novel in that emissions issue from a multitude of regularly spaced liquid cusps. These sites are anchored at the mouth of a micrometre-sized slit which forms a channel of high flow impedance through which liquid caesium is constrained to flow. The properties of this emitter have been investigated by electrical and mass-loss measurements, the latter having been performed on a sensitive dual-axis vacuum microbalance permitting independent real time measurements of the total mass emission rate and of the thrust. The results indicate a decrease in the neutral mass fraction of the beam with increasing flow impedance inside the emitter, brought about by decreasing the width of the slit. More suprising is the observation that the neutral mass fraction decreases with increasing ion current; a result in direct contrast with observations reported for needle sources. Some properties of the emitter may be understood from a consideration of the hydrodynamic stability where the applied electric field acts on the liquid surface at the mouth of the emitter slit. Following this, a simple viscous flow model of the emitter is given which agrees well with experiments.--AA