A Continuous-Flow Accelerator Mass Spectrometry (CFAMS) system is under construction at the National Ocean Sciences AMS Facility, Woods Hole Oceanographic Institution. This system is based on a NEC Model 1.5SDH-1 0.5 MV Pelletron accelerator and will be used to analyze carbon isotopes in the +1 charge state. A key component of the system is a combination of a microwave plasma ion source and a magnesium vapor charge exchange canal, capable of accepting continuously flowing CO2 gas at about 0.2 ml/min and producing up to 80 mA of C- ions for injection into the accelerator. The C- beam from the charge exchange canal is characterized by a large emittance (~80 p mm mrad at 30 keV) and large energy spread (~300 eV) from the charge exchange process. For high efficiency transmission of this beam through the entire system, we designed an achromatic low-energy injection system consisting of an Einzel lens, a 45° electrostatic spherical analyzer (ESA), and a 90° double-focusing magnet. We also customized the accelerator and the high-energy achromat (90° analyzing magnet and 90° ESA) for larger acceptance. The 45° ESA is rotatable to allow a second ion source, which will be a Cs sputter type ion source. Infolytica MagNet and ElecNet were used for 3-D electromagnetic field modeling of each optical element. Ray tracing calculations were done with both Infolytica Trajectory Evaluator and RAYTRACE. The results of these calculations will be presented with special emphasis on the ESA design.
Keywords: Ion optics, Computer modeling, ESA, Continuous-flow AMS
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