Negative Halogen Ion Sources

This paper reviews recent progress in developing high current density ion sources for positive and negative halogen ion beams. These sources have produced CI^- current densities almost equal to their positive chlorine current densities, and also close to the current densities of Ar⁺ beams extracted under similar conditions. The emittance of the CI^- and positive chlorine beams was at least as low as that of an Ar⁺ beam extracted from the same source. The coextracted electron ratios (e/Cl^-) as low as six-seven were much lower than would be expected from the mass ratio dependence of mobility, and appear to be due to the existence of a novel plasma near the extraction plane composed primarily of positive ions and negative ions with few electrons. The results of research across the past eight decades into discharges with electronegative gases and vapors lend insight into the processes occurring in recent experiments, which employed innovations developed in the magnetic fusion energy program to produce high current D beams.