Numerical simulation on neutral beam generation mechanism by collision of positive and negative chlorine ions with graphite surface

We investigated ion neutralization by collision with graphite by numerical simulation based on time-dependent density functional theory. It is known that the neutral beam source developed by Samukawa (2001 Japan. J. Appl. Phys. Part 2 40 L779), where neutral particles are generated by the collision of ions from plasma with a graphite electrode with numerous high-aspect-ratio apertures, can achieve very high neutralization efficiency of over 90% when negative ions (Cl^-) are used compared with about 60% when positive ions (Cl ₂⁺ are used. To understand the neutralization theoretically, we developed a numerical simulator and calculated the dynamic process of electron transfer between an ion and graphite during the whole collision process. Multiple collisions were considered in the calculation. We found that Cl^- had higher neutralization efficiency (more than 90%) than Cl₂⁺ (about 34%), which is in excellent agreement with the experimental result, so our simulator could successfully simulate the neutralization process. The difference in neutralization efficiency between Cl₂⁺ and Cl^- may be due to the relationship between the ion and graphite orbital energy levels.