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The hydrodynamic evolution of the surface of a liquid metal in the presence of an electric field is investigated using both analytical and numerical techniques. It is established that a free liquid surface with axial symmetry evolves with time into a cone-like shape, with the cone angle equal to Taylor's static value of 98.6°. The mechanism behind such interesting flow behaviour is that the system of electrohydrodynamic (EHD) equations has a self-similar asymptotic solution that generalizes Taylor's static result. The asymptotic solutions are found and the time-dependent behaviours of basic physical quantities (electric field strength, fluid velocity and surface curvature) near the singularity are established. The results and the analytical and numerical techniques used are thought to be useful in the development of time-dependent models of operating liquid metal ion sources and EHD sprayers.

The exact solution of Laplacian potential between a rod emitter and a plane extractor is obtained using the method of images. From the solution obtained in the polar coordinates whose origin is an emitter apex, the electric field strength and the angular distribution around the liquid metal surface are shown. The significance of this equation in predicting and verifying the equilibrium configuration and the characteristics of a field emission liquid metal ion source (LMIS) are discussed.

We introduce a thruster concept where the same electric field is responsible for both ionization of the neutrals and acceleration of the ions, by letting the propellant gas escape into a high-field region through a thin, hollow needle at high electric potential. Ionization occurs via the corona mechanism. The configuration is very similar to the FEEP, the difference being in the ionization mechanism and the use of gaseous propellant. Although tests showed that such a thruster only ionizes a small fraction of the neutral gas (

Ion currents of up to 30 mu A are readily obtained from liquid metal ion sources. The high electric field required to create the liquid cone from which emission occurs has given rise to the use of the term liquid metal field ion emission to describe the mechanism. Two candidate processes-field ionisation and field evaporation-are considered in the light of the available experimental data concerning the size of the cone apex, operating voltages etc. It is concluded that field ionisation or any other process requiring ionisation of thermally evaporated neutral atoms is unlikely to be the dominant process because of the absence of a heating mechanism capable of raising a sufficiently large area of the cone surface to the temperatures required. Field evaporation is, however, seen to be a possible mechanism provided an emitting feature of a few tens of angstroms is present. Alternatively, emission may be due to a moving asperity on the apex of a blunter emitting cone.

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. Here the authors report a Cs field ion source which is novel in that emissions issue from a multitude 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. 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.