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.
Highlights
Doleans et al report in [14] that evidence of volatile hydrocarbon has been found through residual gas analysis on thermally cycled Spallation Neutron Source (SNS) [17]) cryomodules; they explain that these signals must originate from the released gases that were previously condensed on the cavity walls at cryogenic temperature or from species produced during accelerator operation by the interaction of electrons with the cavity surface contaminants
Plasma processing can be used on superconducting radio frequency (SRF) cavities in situ in the cryomodules to remove the hydrocarbon contamination and restore the niobium work function obtaining a decrease in Field emission (FE) and a corresponding increase in the accelerating gradient
In this paper we present the results of plasma processing applied to multiple 1.3 GHz cavities
Summary
A collaboration among Fermi National Accelerator Laboratory (FNAL), SLAC National Accelerator Laboratory and Oak Ridge National Laboratory (ORNL) is working to develop plasma processing for LCLS-II [1,2]. Doleans et al report in [14] that evidence of volatile hydrocarbon has been found through residual gas analysis on thermally cycled Spallation Neutron Source (SNS) [17]) cryomodules; they explain that these signals must originate from the released gases that were previously condensed on the cavity walls at cryogenic temperature or from species produced during accelerator operation by the interaction of electrons with the cavity surface contaminants. Plasma processing can be used on superconducting radio frequency (SRF) cavities in situ in the cryomodules to remove the hydrocarbon contamination and restore the niobium work function obtaining a decrease in FE and a corresponding increase in the accelerating gradient This technique was first applied to SRF cavities at ORNL, where Doleans et al developed plasma cleaning for SNS high beta 805 MHz cavities [18]. Given the results of these tests, it was decided to investigate the efficacy of plasma processing on cavities artificially contaminated with carbon-related (C-related) sources or through vacuum failure simulations
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