Numerical simulation of beam extraction and optics of an NBI negative ion source

A negative ion source for the Neutral Beam Injection (NBI) system of China Fusion Engineering Test Reactor (CFETR) is under development in the Southwestern Institute of Physics. The aim is to extract an H– beam of 20 A for a pulse duration of 3600 s at an energy of 200 keV. In this work, the beam optics of the source is investigated with a single beamlet model in the COMSOL environment. The effects of varying the main electromagnetic parameters are studied. The boundary between plasma and beam, namely the meniscus, is calculated with a self-consistent method. The optimal condition at which the beam has a minimum divergence angle is found. Beam divergence is within limits. The magnetic field produced by permanent magnets in the extraction grid is verified to deflect co-extracted electrons effectively, while inducing a slight offset of the ion beam. As a conclusion, the system is capable to operate as required.