The H- ion production and transport processes are numerically simulated to clarify the origin of H- beam nonuniformity. The three-dimensional transport code using the Monte Carlo method has been applied to H0 atoms and H- ions in the large "JAERI 10A negative ion source" under the Cs-seeded condition, in which negative ions are dominantly produced by the surface production process. The results show that a large fraction of hydrogen atoms is produced in the region with high electron temperature. This leads to a spatial nonuniformity of H0 atom flux to the plasma grid and the resultant H- ion surface production. In addition, most surface-produced H- ions are extracted even through the high Te region without destruction. These results indicate a correlation between the production process of the H- ion and the spatial nonuniformity of the H- ion beam.
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