Cryogenic secondary electron yield measurements on structural materials applied in particle accelerators

Multipacting phenomenon induced by large secondary electron yield (SEY) in cryogenic vacuum chambers and superconducting cavities is one of the significant issues in operating particle accelerators. Studying the characteristic and the factors influencing the secondary electron emission on cryogenic surfaces has become an inevitable task in the construction of accelerators. Here, custom cryogenic SEY measurement system with cold helium circulation facility has been established. Cryogenic SEY of structural materials (copper, aluminum, stainless steel, niobium and technical treated niobium) as a function of primary electron energy were evaluated. The experimental results show that SEY of materials at cryogenic temperature is lower than that at room temperature due to the adsorption of residual gases on the surfaces. Secondary electron emission can be effectively restrained by buffered chemical polishing treatment due to the removal of the contaminations and the build-up of graphitelike C–C bonds. After annealing treatments, the surface morphology of technical treated niobium has been modified, causing an increased roughness. Secondary electrons trapped by rough surface lower the SEY of technical treated niobium further. This paper is aimed to explore a rationale of cryogenic SEY characteristic of structural materials and evaluate the potential technical treatments for design and construction of Hefei advanced light facility (HALF).