LEVIS ion source and beam characterization on PBFA-II

We report on the continuing development of the LEVIS (Laser Evaporation Ion Source) lithium active ion source for the 15-cm radial focussing ion diode on PBFA-II. We found previously that DC-heating of the anode surface to 150°C maximum for 5 hours resulted in a pure lithium beam. This paper discusses the characterization of LEVIS source uniformity by Faraday cup arrays and multiple lines of sight for visible light spectroscopy. These diagnostics give some evidence of nonuniformity in both A-K gap electric fields and ion current density. Despite this, however, the measured focal spot size appears smaller than with a passive LiF source operated in the same magnetic field topology. Experiments using a curved anode for vertical beam focussing show reduced ion beam turn-on delay by 5 ns by altering the magnetic field topology as well as anode curvature. Another 3–5 ns reduction was achieved by switching from a passive LiF to the active LEVIS source.