Numerical analysis of negative ion temperature in a negative ion source

To understand the origin of the two energy groups of volume produced H- ions observed in the multicusp, hybrid-type H- ion source (Camembert III), numerical analysis by the three-dimensional Monte Carlo simulation code has been done. Realistic multicusp geometry and the electric field near the filament are taken into account in the model. The energy relaxation process by Coulomb collision is also included in the model by the Monte Carlo method. Under the typical low-pressure condition of Camembert III, high-energy H- ions are clearly observed in the central region. The origin of these high-energy H- ions is possibly explained by the combined effects of (1) acceleration by electric field near the filament and (2) relatively large energy-loss time of these high-energy H- ions. However, the low energy group is not clearly observed in the simulation. The low energy H- ions with original energy (∼0.05 eV) have enough time to relax their energy towards the relatively high temperature of the background H+ ions, because their energy relaxation time is much smaller than the confinement time