Abstract: In a negative hydrogen (H.sup.-) ion cyclotron, a system and method for improving the efficiency of the cyclotron by minimizing loss, i.e., neutralization, of H.sup.- ions within the acceleration region of the cyclotron caused by gas stripping. The system includes a cyclotron volume, an ion source within the ion source volume is maintained at a negative potential and located proximate the cyclotron center on the plane of acceleration. The vacuum system includes a main vacuum pump for evacuating the cyclotron volume and an ion source pump for separately evacuating the ion source volume to remove hydrogen (H.sub.2) gas molecules which could cause gas stripping if injected into the cyclotron volume. In the preferred embodiment, the system further has a pumping volume, communicating between the ion source volume and the cyclotron volume, and a separate pumping volume vacuum passageway whereby the ion source volume is evacuted in two stages.