Multistage Gun for Production of Low Energy Ion Beams

Abstract

An ion gun using separate acceleration and deceleration stages to produce ion beams in the 1–100 eV energy region has been constructed and tested. The gun is based on an existing electron gun design but uses a thermal emitter Li2O·Al2O3·nSiO2 (n = 2 and n = 4) as a source of Li+ ions. Current densities of 10−8−10−6 A/cm2 at beam energies of 2–100 eV were obtained and both the spatial and the energy distributions of the ions were investigated. Minimum beam radii (typically about 1 mm) occurred a few centimeters from the last aperture. Beam profiles were measured as a function of axial distance and well collimated beams up to 16 cm in length were obtained. The energy spread in the beam was found to be 0.22 eV (total width at half maximum intensity), in good agreement with the expected thermal energy spread from the source which operates at about 1200°C. Mass analysis shows that beam purities are in excess of 99% 7Li+ for isotopically enriched emitter material. Theoretical calculations of space charge effects on image distance and image size are presented as well as design parameters and graphs for the operation of the gun under various conditions.