Anomalous Ionization Regime in a Forced Electron Beam Induced Arc Discharge Ion Source for Singly Charged Radioactive Ion Beam Production

At TRIUMF's ISAC facility, 1+ Radioactive Ion Beams (RIBs) of noble gases, halogens, and molecules are created using a Forced Electron Beam Induced Arc Discharge (FEBIAD) ion source. Reported ionization efficiencies for FEBIADs range from 10% to 25% for Ar-40(+), while TRIUMF-FEBIAD ionization efficiency seems to be < 1% with a 90% emittance <15 mu m. As RIB ion sources aim for a high ionization efficiency, an experimental and numerical campaign was conducted to investigate the comparably low efficiencies observed. The experimental results for Ar-40(+) indicate that up to a 10% ionization efficiency is possible by operating the source at different parameters. The measurements agree with theoretical estimations; however, certain combinations of parameters produce an anomalously high electron current which enhances the ionization efficiency threefold. Present investigations aim to characterize and model the anomalous electron current because, if proven reliable, the argon ionization efficiency could reach 30% with no significant impact on beam emittance.