Plasma cleaning of the SLAC Linac Coherent Light Source II high energy verification cryomodule cavities

Abstract

Plasma cleaning is a technique that can be applied in superconducting radio-frequency cavities in situ in cryomodules to decrease their level of field emission (FE). We developed the technique for Linac Coherent Light Source II (LCLS-II) cavities and we present in this paper the full development and application of plasma processing to the LCLS-II High Energy verification cryomodule (vCM). We validated our plasma processing procedure on the vCM, fully processing four out of eight cavities of this CM, demonstrating that cavity performance was preserved in terms of both accelerating field and quality factor. Applying plasma processing to this clean, record breaking cryomodule also showed that no contaminants were introduced in the string, maintaining the vCM FE-free up to the maximum field reached by each cavity. We also found that plasma processing eliminates multipacting (MP)-induced quenches that are frequently observed within the MP band field range. This suggests that plasma processing could be employed in situ in CMs to mitigate both FE and MP, significantly decreasing the testing time of cryomodules, the linac commissioning time and cost, and increasing the accelerator reliability.