Vacuum pressure considerations on the performance and lifetime of negative ion sources

Modern high power proton accelerators and fusion facilities usually exploit negative ion injection. The ISIS pulsed spallation neutron and muon facility uses a high power Penning-type H− ion source. Over the years, several variations of the source have been tested with a view to higher duty-cycle operation in future facilities. It has been observed that the different source configurations vary in their operational stability and wear patterns, despite using the same power supplies, gas feed-rates, magnetic fields, temperature settings etc. It is proposed that the vacuum chamber setup contributes to the observed performance variations. Pressure profiles output from vacuum simulations were used with collision cross sections to estimate stripping losses along the beam’s flight path. It was found that up to 20% difference in H− stripping can occur depending on the vacuum environment. Therefore caution must be used when comparing perveance scans of transported beam current from different ion sources and different experimental setups. In addition to stripping losses of negative ions, the vacuum environment affects the flux of positive ions backstreaming into the ion source, increasing erosion. This paper discusses the practical considerations needed when designing the vacuum system to improve source performance and lifetime, with results applicable to any type of ion source.