Field emission mitigation studies in LCLS-II cavities via in situ plasma processing

TL;DR

This study investigates in situ plasma processing to mitigate field emission in superconducting RF cavities, demonstrating its effectiveness in cleaning surfaces, preserving quality factors, and improving performance in contaminated or emitting cavities.

Contribution

The paper introduces plasma processing as a novel in situ method for reducing field emission in LCLS-II cavities, maintaining surface quality and enhancing cavity performance.

Findings

Plasma cleaning preserves high quality factors and quench fields.

It effectively removes hydrocarbon contamination and reduces field emission.

Performance improvements were observed after plasma processing in contaminated cavities.

Abstract

Field emission is one of the factors that can limit the performance of superconducting radio frequency cavities. In order to reduce possible field emission in LCLS-II (Linac Coherent Light Source II), we are developing plasma processing for 1.3 GHz 9-cell cavities. Plasma processing can be applied in situ in the cryomodule to mitigate field emission related to hydrocarbon contamination present on the cavity surface. In this paper, plasma cleaning was applied to single cell and 9-cell cavities, both clean and contaminated; the cavities were cold tested before and after plasma processing in order to compare their performance. It was proved that plasma cleaning does not negatively affect the nitrogen doping surface treatment; on the contrary, it preserves the high quality factor and quench field. Plasma processing was also applied to cavities with natural field emission or artificially contaminated. It was found that this technique successfully removes carbon-based contamination from the cavity iris and that it is able to remove field emission in a naturally field emitting cavity. Vacuum failure experiments were simulated on four cavities, and in some cases plasma processing was able to achieve an increase in performance.