Microdroplet emission and instabilities in liquid-metal ion sources

Abstract

When the liquid metal ion sources (LMIS) operate in the high-current mode, an instability arises which is accompanied by the comparatively low-frequency ion current oscillations and microdroplet emission. Two distinct groups of microdroplets are observed, the large ones (with radius higher than 10 3 Å). and the smaller ones (10–100 Å). The rate of formation of small-scale droplets proves to be two or three orders higher than that of the large droplets. In this work it is shown that the appearance of the above groups of microdroplets is due to two kinds of instability developed in an LMIS. The smaller droplets are due to the Rayleigh instability on the surface of thin cylindrical jets ejected from the tip of the Taylor cone at higher currents. This instability results in jet fragmentation and formation of small droplets (with sizes of the order of the jet radius) till the jet completely disappears. Following this the jet ejection reappears and the jet fragmentation process is repeated. Thus, the Rayleigh instability is accompanied by the comparatively low-frequency ion current oscillations ( f = 1–10 MHz) which are due to the process of fragmentation and recovery of the jets. This process leads to the liquid-metal pressure modulation and causes the comparatively large-scale capillary wave extension on the Taylor cone surface as a result of the Faraday parametrical effect. It is this effect that leads to formation of the large droplets generated on the Taylor cone surface. We have made measurements of the angular distributions of the large and small droplets, as well as their intensities in the instability mode in tin LMIS. The measurements were carried out with transmission electron microscopy (TEM). The higher harmonic dependence of low frequencies on the current value have been defined. Also, the comparison between the computed and the experimental data have been carried out.