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Ion Source

Since the applications proposed for the HIBAL facility included only standard ion beam analysis techniques such as RBS, PIXE and PESA, the external ion source for this facility is the NEC “Alphatross” source. There are two accessible bottles of hydrogen and helium gas (shown in the figure) that feed into an RF source chamber, which creates a plasma (a typical hydrogen plasma is shown in the right panel of the figure). The entire source is isolated from ground and is typically run with 10-20kV of bias potential to force the positive ions out of the RF source. Adjacent to the RF source is the rubidium charge-exchange oven, a vertical steering element and an E X B velocity selector assembly. A 500 L/sec turbomolecular pump is inverted above the source and a plungable faraday cup completes the ion source. The ion source sits at a ~2 degree offset from the central axis of the accelerator tank to clean up the beam from the source. After the ion source there is a focusing einzel lens before the accelerator tank. The ion source area is shown in an orange framework in the CAD drawing shown in the upper right corner of Figure 5. The manual controls of the ion source are shown in the right panel of Figure 6, and access is possible to these controls behind the accelerator control consoles shown in the lower right panel of figure 5. The maximum output of the ion source we have obtained in the first 14 months of operation (as measured on the low-energy cup) has been approximately 5 microamps of protons and approximately 7 microamps of alphas. The ion source operation has generally been extremely stable to date. After an initial warm-up period of about 30 – 60 minutes, the ion source usually remains at the same output for over 8 hours at a time, with no (or very minimal) operator intervention. On average the ion source output seems to vary less than a few percent over 8 hours when the rubidium oven is full and well-behaved. We have had to replace the 10g of rubidium in the ion source twice in the first 16 months of operation. This ~8-month cycling time is slightly more frequent than the infinite lifetime quoted in the operations manual, even with very careful attention paid to the shutdown procedure. The rubidium appears to cake out on the baffles of the charge exchange oven and it does get into the RF ion source over time. We have not determined the optimum running conditions to prevent this from happening entirely yet.