Monte Carlo simulation of negative ion transport in the negative ion source (Camembert III)

Abstract

Transport process of negative hydrogen ions (H−) in a large hybrid multicusp H− source, “Camembert III,” has been analyzed by a three-dimensional Monte Carlo simulation code. The realistic geometry and multicusp magnetic-field configuration are taken into account. Various important destruction processes of H− and Coulomb collision with background plasma are also included in the model. Both the volume- and surface-produced H− ion trajectories are followed. For volume-produced H− ions, most of the H− ions can reach the wall in the low-pressure case (1 mTorr), while in the high-pressure case (3 mTorr) most of the H− ions are destructed by volume loss reactions before reaching the wall. This shows that the wall loss is significant at low pressure as in the experiments. For surface-produced H− ions, the influence of its birthplace on the H− current is studied. Negative ions created on the sidewall hardly can reach the center of the source due to trapping by the multicusp magnetic field. As a result, H− ions created on the sidewall do not have a significant effect on the H− current.