Towards understanding the cesium cycle of the persistent H^- beams at SNS

This paper describes the accomplishments of the SNS H^- ion source, which delivers routinely ∼50 mA at a 5.4% duty factor with ∼99% availability, enabling 1 MW beams for neutron production with ∼90 % availability. It discusses the need for increasing reliability and beam current. But mostly it focuses on its unexpected feature: H^- beams that are apparently persistent for up to 5 weeks without adding Cs after an initial dose of less than ∼5 mg. Thermal emission and sputtering are qualitatively evaluated, and appear consistent with a negligible Cs sputter rate after the initial dose disappears from the Cs plasma. It concludes with a list of future experiments that can shed more light on this apparently unique Cs cycle.