A novel Eclair-shaped filter magnetic field with no primary electron leakage has been proposed to reduce the co-extracted electrons and improve the spatial uniformity of negative ion production. A primary electron analysis was conducted to clarify its spatial distribution and the issue of current magnetic field configuration because the experimental data imply that the negative ion population is low on the edge segments due to the many primary electrons around there. A simulation study revealed that the low capability of electron confinement at the KAMABOKO source’s edge region due to the localized magnetically weak points induces the primary electron leakage, which causes high co-extracted electron current and negative ion non-uniformity. Calculation results showed a new design index that contributes to the complete suppression of primary electron leakage to the extraction region with a permanent magnet connection angle of 108 degrees or more. The Eclair-shaped ion source has been designed based upon the obtained index about the connection angle to realize the ideal magnetic configuration without primary electron leakage to the extraction region.

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