Microdroplet generation 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 ion current oscillations and microdroplets emission [Mair and Engel (1979), Sudraud et al. (1982, 1985)]. Two groups of microdroplets are observed differing in size, i.e., large ones (with radius of 2×102 to 2×103 Å) and small ones (with a radius of 10–100 Å). The intensity of small-scale droplet formation proves to be two or three orders of magnitude higher than that of the large droplets [Thompson (1982)]. In this work it is shown that the appearance of the above groups of microdroplets is due to two instabilities developed in LMIS. The small 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. The recovery of the jets occurs within the time interval necessary for the most charged droplets to move towards the extractor as the screening effect of the droplets fades out and the electric field distribution on the cone surface is recovered. When the above time interval elapses the jet ejection reappears from the tip of the Taylor cone, and the jet fragmentation process is repeated. Thus, the Rayleigh instability is accompanied by the comparatively low-frequency ion current oscillations which are due to the process of fragmentation and recovery of the jets (f≂107 Hz). This process leads to liquid metal pressure modulation and causes the comparatively large-scale capillary wave extension on the Taylor cone surface (wavelength ≂104 Å) 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. Emission of the small droplets occurs only in the cone axis direction, and that of the large droplets in a wider angular range. We have made measurements of the angular distributions of the large and small droplets, as well as of their intensities in the instability mode (I=120 μA, f≂10 MHz) in tin LMIS. The intensity of small droplet generation (with a radius of 15–100 Å) is approximately 1010–1011 s−1 and that of large droplet generation is two to three orders of magnitude less. The computed intensity values approached the ones observed. The small droplets are emitted mainly along the LMIS axis but as to the large ones, a wider spatial spread is observed. The measurements were carried out with transmission electron microscopy (TEM).