Plasma-Wall Interactions in the Cesiated SNS H− Ion Source

High-current H− ion beams are needed to drive high-power accelerators as well as to heat future fusion reactors with neutral beams. The most productive H− sources enhance the production of H− ions with caesium. Cs lowers the work functions of the metal walls, which increases the probability of ions to capture a second electron when bouncing back from the metal plasma walls. However, caesium also causes voltage break downs that can be severe and cause significant downtime. SNS has developed a frugal caesium management, which uses a single injection of ∼4 mg of caesium to produce ∼50 mA of H− beam without decay for up to 6 weeks. This paper presents calculations and experimental data, which suggest the persistence of the caesium enhanced H− beams are due to 1) thermally limiting the Cs emission, and 2) conditioning for high purity hydrogen plasma, which eliminates the sputtering of Cs by non-hydrogen ions, and 3) hydrogen ions being too light to sputter Cs.