  Superconducting Radio Frequency (SRF) cavities performance preservation is
crucial, from vertical test to accelerator operation. Field Emission (FE) is
still one of the performance limiting factors to overcome and plasma cleaning
has been proven successful by the Spallation Neutron Source (SNS), in cleaning
field emitters and increasing the work function of Nb. A collaboration has been
established between Fermi National Accelerator Laboratory (FNAL), SLAC National
Accelerator Laboratory and Oak Ridge National Laboratory (ORNL) with the
purpose of applying plasma processing to the Linac Coherent Light Source-II
(LCLS-II) cavities, in order to minimize and overcome field emission without
affecting the high Q of nitrogen-doped cavities. The cleaning for LCLS-II will
follow the same plasma composition adopted at SNS, which allows in-situ
processing of cavities installed in cryomodules from hydrocarbon contaminants.
A novel method for plasma ignition has been developed at FNAL: a plasma glow
discharge is ignited using high order modes to overcome limitations imposed by
the fundamental power coupler, allowing in-situ cleaning for cavities in
cryomodule. The plasma can be easily ignited and tuned in each of the cavity
cells using low RF power. A method for plasma detection has been developed as
well, which allows the detection of the plasma location in the cavity without
the need of cameras at both cavity ends. The presented method can be applied to
other multi-cell cavity designs, even for accelerators where the coupling for
the fundamental modes at room temperature is very weak.
