Ion-Source Modeling and Improved Performance of the CAMS High-Intensity Cs-Sputter Ion Source

The interior of the high-intensity Cs-sputter source used in routine operations at the Center for Accelerator Mass Spectrometry (CAMS) has been computer modeled using the program NEDLab, with the aim of improving negative ion output. Space charge effects on ion trajectories within the source were modeled through a successive iteration process involving the calculation of ion trajectories through Poisson-equation-determined electric fields, followed by calculation of modified electric fields incorporating the charge distribution from the previously calculated ion trajectories. The program has several additional features that are useful in ion source modeling: 1) averaging of space charge distributions over successive iterations to suppress instabilities, 2) Child’s Law modeling of space charge limited ion emission from surfaces, and 3) emission of particular ion groups with a thermal energy distribution and at randomized angles. The results of the modeling effort indicated that significant modification of the interior geometry of the source would double Cs ion production from our spherical ionizer and produce a significant increase in negative ion output from the source. The results of the implementation of the new geometry were found to be consistent with the model results.