Effect of Coulomb collision on the negative ion extraction mechanism in negative ion sources

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.