Abstract
The optimum shape of the extraction aperture in the positive electrode is investigated experimentally by using a duopigatron ion source with single hole extraction electrodes. We find that a slightly chamfered aperture gives the best beam divergence without decreasing the optimum perveance. This result is applied to the ion sources of the JFT-2 neutral beam injector and the injection power to the torus can be improved appreciably. The computational result about the optimum emitting surface by the ion beam simulation code indicates that a slight chamfering is preferable for beam optics.