Minimum emission current of liquid metal ion sources

Abstract

The inability of liquid metal ion sources (LMIS) to operate at low dc emission currents limits their performance. We briefly describe a model that explains why LMIS have a minimum dc emission current (Imin) and also predicts Imin as a function of the temperature and the properties of the liquid metal. The model predicts that Imin=217+0.744T(nA) for gallium LMIS, where T is the temperature (K). Measurements of Imin for gallium LMIS between 30 and 890 °C are in reasonable agreement with the model. A better fit to this data, however, is given by Imin=1187 exp(−0.026/kT)(nA) where k is Boltzmann’s constant (eV/K). Below 30 °C, Imin drops precipitously—values as low as 380 nA have been measured at temperatures as low as 25.8 °C. This drop is attributed to a supercooling effect that is not accounted for in the model. Imin is also calculated for 17 pure-elemental LMIS at their melting points, and found to vary from 10 nA for mercury to 1.0 μA for aluminum. Imin is measured to be much lower for bismuth LMIS than for gallium LMIS, as predicted by the model, although difficulties with the bismuth LMIS have allowed only an upper limit of Imin⩽77 nA to be measured. The model also suggests possibilities for improving ion sources by reducing or eliminating Imin.