The effect of an insulator coating on the plasma electrode of a duoPIGatron ion source on ion‐beam optics was examined theoretically and experimentally. The effect of a preacceleration potential, applied between the insulated electrode and the target cathode of the source plasma, on ion optics was also examined. A principal result is that this electrode arrangement with precel gives a much lower beam divergence than the same arrangement without precel. Detailed comparisons between the data and calculations are presented.