Plasma Cleaning of LCLS-II-HE verification cryomodule cavities

TL;DR

This paper demonstrates plasma cleaning as an effective in situ technique for superconducting cavities in cryomodules, reducing field emission and multipacting, thus improving performance and reducing testing and commissioning time.

Contribution

The study develops and validates plasma processing for LCLS-II-HE cryomodules, showing it preserves cavity performance and mitigates field emission and multipacting.

Findings

Cavities maintained high accelerating fields and quality factors after plasma processing.

Plasma cleaning eliminated multipacting-induced quenches.

No contaminants were introduced during plasma processing.

Abstract

Plasma cleaning is a technique that can be applied in superconducting radio-frequency (SRF) cavities in situ in cryomodules in order to decrease their level of field emission. We developed the technique for the 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 (HE) verification cryomodule (vCM). We validated our plasma processing procedure on the vCM, fully processing four out of eight cavities of this CM, demonstrating that cavities performance were 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 field emission-free up to the maximum field reached by each cavity. We also found that plasma processing eliminates multipacting (MP) induced quenches that are typically observed frequently within the MP band field range. This suggests that plasma processing could be employed in situ in CMs to mitigate both field emission and multipacting, significantly decreasing the testing time of cryomodules, the linac commissioning time and cost and increasing the accelerator reliability.