A High-Intensity Source of Heavy Negative Ions

An early version of the developmental H- ion source for the Argonne Zero Gradient Synchrotron (ZGS) produced about 4 mA of negative ions in the mass range of 15-18 amu under conditions such that discharge cleaning of the source could not occur. The source was constructed almost entirely of carbonbearing, oxide-covered mild steel and stainless steel. The gas outflow was pulsed. At low arc and gas pulse rates, wall-plasma interactions produce volatile compounds such as CO, CH4, C2H6, H2O, etc. Ionization of these compounds and the fragments formed by their dissociation results in high levels of contaminants in the plasma and the positive ion beam emerging from the source; correspondingly high levels of contamination are obtained in the negative ion beam produced by charge exchange in H2 gas. The heavy ion contaminant fraction was reduced to about 2% of an 8-10 mA negative ion beam by running the source at 30 Hz with a continuous gas flow of ~ 0.25 Torr-l/s. It is possible to exploit the capability of the present H- source to produce large currents (~ 10 mA) of specific heavy negative ions.