Understanding how the liquid-metal ion source works

Abstract

There is some interesting physics in how the liquid-metal ion source (LMIS) works. This short review describes our current understanding, covering the following topics. Distinction between the real Taylor-Gilbert cone and Taylor's mathematical cone. Thermodynamic origin of the pressure-difference formula. Shape of an operating LMIS: the cusp-on-a-cone as expected shape and as an electrically driven vena contracta. Onset behaviour and Taylor's slender-body formula. Ion energy distributions, and interpretation in terms of surface processes and space-charge. Swanson's proof that the emission mechanism is field evaporation. Predicting the low-current Ga LMIS apex radius as 1.5 nm. The low-current Ga LMIS as a device driven by a negative pressure of order 40 atm. Re-examination of Mair's formula for LMIS current/voltage characteristics, and the role of space-charge. Cusp length as a function of emission current: theory and modelling. Emitter stability and control mechanisms: space-charge stabilisation, inertial/geometrical stabilisation, pressure-change destabilisation. Secondary emission phenomena: electrons, neutrals and photons. The high-temperature anomaly in the ion energy distribution, and a possible explanation. Instability and the formation of microdroplets and nanodroplets. Numerical modelling of basic LMIS behaviour: current/voltage characteristics for sources with and without viscous drag; modelling of cusp length and limiting half-angle; modelling of time-dependent effects. Current research tasks. Copyright © 1996 Elsevier Science Ltd.