High-Energy Acceleration of an Intense Negative Ion Beam

概要

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A high-current, more than 10A, H^- ion beam has been accelerated to more than 100 keV with the two-stage acceleration for the first time. A large negative hydrogen ion source with an external magnetic filter, which has been developed for the LHD-NBI system, produces more than 10A of the H^- ions from the grid area of 25 cm × 50 cm with the arc efficiency of 0.1 A/kW by seeding a small amount of cesium. A 13.6A of H^- ion beam has been accelerated to 125 keV at the operational gas pressure of 3.4 mTorr. The maximum H^- ion current which can be accelerated is determined by the perveance of the accelerator, and is dependent on the 3/2-power of the total beam energy. The optimum beam acceleration with a small divergent angle is achieved with nearly the same electric fields in the first and the second acceleration gaps on condition that the ratio of the first acceleration to the extraction electric fields is adjusted for an aspect ratio of the extraction gap. The ratio of the acceleration drain current to the H^- ion current is more than 1.7. That is mainly due to the secondary electron generated by the incident H^- ions on the extraction grid and the electron suppression grid. The neutralization efficiency was measured and agrees with the theoretical calculation result. These results satisfy the specification of the negative-ion-based LHD-NBI system.
社団法人プラズマ・核融合学会の論文
1995-07-25