A Kaufman‐type broad beam ion source, used for sputtering and etching purposes, has been operated with Ar, Kr, O2 and N2 gas inputs over a wide range of beam energies (200–1200 eV) and gas flow rates (1–10 sccm). The maximum ion beam current density for each gas saturates at about 2.5 mA/cm2 as gas flow is increased. The discharge threshold voltage necessary to produce a beam and the beam efficiency (beam current/molecular current), however, varied considerably. Kr had the lowest threshold and highest efficiency, Ar next, then N2 and O2. The ion beam current varied only weakly with beam energy for low gas flow rates, but showed a factor of two increase when the gas flow was higher.