To improve the H- ion beam optics, it is necessary to understand the energy relaxation process of surface produced H- ions in the extraction region of Cs seeded H- ion sources. Coulomb collisions of charged particles have been introduced to the 2D3V-PIC (two dimension in real space and three dimension in velocity space particle-in-cell) model for the H- extraction by using the binary collision model. Due to Coulomb collision, the lower energy part of the ion energy distribution function of H- ions has been greatly increased. The mean kinetic energy of the surface produced H- ions has been reduced to 0.65 eV from 1.5 eV. It has been suggested that the beam optics of the extracted H- ion beam is strongly affected by the energy relaxation process due to Coulomb collision.
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