Producing persistent, high-current, high-duty-factor H− beams for routine 1 MW operation of Spallation Neutron Source (invited)a

Since 2009, the Spallation Neutron Source (SNS) has been producing neutrons with ion beam powers near 1 MW, which requires the extraction of ∼50 mA H− ions from the ion source with a ∼5% duty factor. The 50 mA are achieved after an initial dose of ∼3 mg of Cs and heating the Cs collar to ∼170 °C. The 50 mA normally persist for the entire 4-week source service cycles. Fundamental processes are reviewed to elucidate the persistence of the SNS H− beams without a steady feed of Cs and why the Cs collar temperature may have to be kept near 170 °C.