Gallium Droplet Emission as Observed with an Ion Streak Camera.

Droplets emitted from a gallium liquid metal ion source have been observed using an ion streak camera. The camera records the angle and time of arrival of individual atomic ions at a chevron multichannel plate detector (MCP) with a minimum time resolution of ~30 nsec. The ion hits are recorded with a charge coupled device (CCD) camera observing light from the MCP. Individual droplets are recorded as shadows in the image produced by the broad gallium atomic ion beam. Droplet shadows have large angular widths at times immediately after droplet creation and shrink rapidly in angle with increasing time as the droplets move away from the emitting tip. Associated with each droplet streak is a momentary decrease in atomic ion emission at all angles, as might be anticipated from the perturbation produced by the emission of a droplet. The shape of a droplet streak has been modelled by geometrical shadowing and by coulomb repulsion shadowing between the droplet and the atomic ions. Through nonlinear fitting methods, both the mass and charge of the droplet may be deduced from this model. Results show that the charge on most large droplets is substantially below the Rayleigh limiting charge. The momentary decreases in atomic ion emission at all angles are roughly periodic under some emission conditions.