Numerical simulation for design of a two-stage acceleration system in a high power ion source

In design of the neutral beam injectors for JT-60, the optimum configuration of extraction grids and the operating conditions were studied numerically for a two-stage acceleration in a high power ion source. Results of the calculations show the feasibility to extract a proton beam with energy 75 keV and current 15 A of about 0.6 degree divergence from the 12 cm diameter extraction grids of transparency 40%. Beam divergence in the two-stage configuration depends largely on the field intensity ratio; the optimum ratio that gives a minimum beam divergence is about 0.3 - 0.5. Possibility of beam focusing by aperture displacement is also treated with a simple model of thin-lens approximation. (auth.)