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

In the design of the neutral-beam injectors for large tokamaks such as JT-60 planned in Japan Atomic Energy Research Institute, 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 of extracting a proton beam with energy 75 keV and current 15 A of about 0.6° divergence from the 12-cm-diam 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.