As part of an effort1 to better understand the performance of the National Electrostatics Corporation (NEC) MC-SNICS ion source for AMS applications we set up a test stand with two NEC profile monitors to measure the divergence of the extracted beam from a carbon sample as function of the beam current. The configuration of the 134-cathode ion source was changed from the manufacturer's design in one detail: the conical ionizer was replaced with the ANU2 spherical design (R=15 mm). Beam currents were varied by adjustment of cesium supply, power supply settings, and distance of the cathode from the Cs focus aperture. The profile monitors (NEC models BPM82 and BPM83) were mounted in a cryo-pumped straight beam pipe section at 0.85 m and 1.5 m distance from the cathode with an isolated beam dump at 2.0 m. All measurements were performed with 15 kV extraction, while cathode, Cs focus, and Einzel lens voltages were optimized. The acceleration tube was set at 0 or 20 kV, resulting in beam energies between 19 and 40 keV. Prior to this test we analyzed the composition of the extracted beam by applying a deflecting magnet field between the monitors. The dominant fractions of the beam were 12C- and 16O- with similar shapes and a much weaker heavier ion and neutral component. Calculations were performed to simulate the observed beam profiles. SIMION3 could not reproduce the results, even with its coulomb or beam repulsion features. To account for space charge effects near the cathode caused by Cs+ ions, code PBGUNS4 was used in sequential mode. This resulted in a larger divergence of the extracted negative ions, an effect that could only be achieved by reducing the Cs focus voltage in the SIMION calculations. Only in this way was it possible to create a beam waist far enough away from the extraction electrode to explain the measured widths on the profile monitors. The measured divergence half angles varied between less than 16 mrad for extracted currents of 40 ľA and 27 mrad for currents exceeding 100 ľA.
1. Collaboration of UC Irvine, U. Arizona, NEC, and WHOI on SNICS improvements. 2. D. C. Weisser et al., Pramana Journal of Physics, Vol 59, 6 (2002) 997. 3. Code SIMION 3D 7.0, D. A. Dahl, INEEL-95/0403, 2000. 4. Code PBGUNS 5.04, J. E. Boers, Garland, TX, 2001.
See more of Poster Session I
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)