  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.
