Beam extraction from a large vacuum-immersed negative-ion source

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Abstract
A large vacuum‐immersed negative‐ion source has been investigated for the development of the neutral beam injection system in a large helical device. The structure of the vacuum‐immersed source is compact and light because it is free from atmospheric pressure. Therefore, this is very advantageous for a large‐scaled ion source. Moreover, since the grids of the accelerator are supported by post insulators, enhanced pumping of gaps between grids is expected, leading to a reduction of stripping loss of negative ions. The gas pressure distribution between grids was calculated and the stripping loss of H− ions was estimated and the effectiveness of the vacuum‐immersed source structure was confirmed. The gas temperature effect of the gas pressure distribution is also considered by using a Monte Carlo code.

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