CRAFT (Comprehensive Research Facility for Fusion Technology) is a large scientific device that is preferentially deployed for the construction of major national science and technology infrastructures. A negative ion–based neutral beam injection system with a beam energy of 400 keV, a beam power of 2 MW, and a beam duration of 100 s, it was designed to deliver an energetic neutral beam for fusion research. Among the crucial components of this system, the high-power negative ion source stands out, and the voltage holding capability of its accelerator with double-stage is a commonly encountered issue.

To address this concern, a comprehensive investigation has been conducted that focused on the gaps between the acceleration grids and grid supports in terms of voltage holding capability utilizing empirical formulas. The results of this investigation revealed that an acceleration gap of 81 mm and a grid support gap of 65 mm can be achieved through the implementation of empirical formulas, which aligns with the requirement of 200 kV for each stage. In addition, the preliminary experimental results showed that the voltage holding capability of the adjacent grids can reach up to 200 kV when the gap between the adjacent grids was designed to be 90 mm. These findings provide a foundation for the subsequent design of a high-power ion source characterized by both high energy and a large area.