Reverse trajectory analysis of the hydrogen negative ion beam in a prototype accelerator for ITER

Reverse calculation of negative ion trajectory based on the measured beam emittance has been performed for the first time in multi-stage accelerator in order to reconstruct negative ion profile near meniscus, which has been a long-term issue for negative ion sources. According to reverse calculation, negative ions are mainly extracted from the periphery of extraction area, and the particles extracted from the aperture edge is lost on acceleration grids. By taking into account the lost component with reconstructed negative ion profile, the negative ion trajectory became consistent with observed beam traces on the acceleration grids. This result can be applied directly to the design of ITER accelerator as well as the other Cs-seeded negative ion sources.