Gas discharge ion source. II. Duopigatron

Ion source performance characteristics consisting of total ion current, ion energy distribution, mass distribution, and ion current density distribution have been measured for several models of a duopigatron. Variations on the duopigatron design involved plasma expansion cup material and dimensions, secondary cathode material, and interelectrode spacings. Of the designs tested, the one with a copper and molybdenum secondary cathode, and a mild steel plasma expansion cup proved to give the best results. The ion current density distribution was peaked at the center of the plasma expansion cup and fell off to 80% of the peak value at the cup wall for a cup 15.2 mm deep. A total ion current of 180 mA consisting of 60%-70% atomic ions was produced with an arc current of 20 A and source pressure of 9.3 Pa. More shallow cups produced a larger beam current and a more sharply peaked ion current density distribution. Typical ion energy distributions were bellshaped curves with a peak 10-20 V below anode potential and with ion energies extending 30-40 V on either side of the peak.