Plasma potential measurements for a "volume"-type ECR ion source

A "volume"-type ECRIS, based on the flat central-field (flat-B) concept, has been developed at the Holifield Radioactive Ion Beam Facility, Oak Ridge National Laboratory. The superiority of the "volume-type" ECRIS concept over its conventional minimum-B source counterpart, in terms of higher charge-state and intensity within a particular charge-state, has clearly been demonstrated. As evidenced by comparing X-ray spectra for the two source geometries, improvements in charge-state and intensity within a particular charge-state are directly attributable to the increased physical "volume" of the ECR zone in the flat-B source that provides better coupling of RF power to the plasma, resulting in the acceleration of a much larger population of electrons to much higher energies. In this report, we provide plasma potential measurement data, made at various gas pressures, gas mixing ratios, and RF power, for the flat-B source. The plasma potential is found to increase with pressure and RF power, and to decrease with increase in gas mixing ratio whenever He gas is used as the mixing gas.