  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.
