Effect of mid-electrode potential on multi-charged ion beam extracted from electron cyclotron resonance ion source

We are constructing a tandem type electron cyclotron resonance ion source (ECRIS) and a beam line for extracting, analyzing ion beams, and their applications. The ion beam is extracted from the second stage by an accel-decel extraction system with single-holes and the ion beam current on each electrode is measured. The total ion beam current is measured by a faraday cup installed at downstream from the extraction electrodes. The ion beam current of each charge state is measured by the faraday cup beyond the sector magnet. The most of the total ion beam is consisted of mainly Ar^{+ ~ 4+} and we can measure up to Ar⁹⁺. We measure the total ion beam current and the currents of each charge state as a function of the mid-electrode potential. It is found that the ion current of each charge state depends on the same manner to the mid-electrode potential as similar to the total ion beam currents. The results obtained experimentally against the mid-electrode potential show qualitatively good agreement with a simple theoretical consideration including sheath potential effects for formation of ion beams. The beam loss is estimated by comparing the total ion beam currents with the sum of the currents after mass/charge analysis. The effect of mid-electrode potential is very useful for decreasing the beam loss and then optimizing beam transport for enhancing ion beam current extracted from tandem type ECRIS with many ion sources.