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 nitrogendoped 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.

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Superconducting radio frequency cavities performance preservation is crucial, from vertical test to accelerator operation. Field emission 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 (hydrocarbon contaminants) and increasing the work function of Nb. The cleaning for Linac Coherent Light Source-II will follow the same plasma composition adopted at SNS, which allows in situ processing of cavities installed in cryomodules. A novel method for plasma ignition has been developed at the Fermi National Accelerator Laboratory: a plasma glow discharge is ignited using high order modes to overcome limitations imposed by the fundamental power coupler. The plasma can be easily ignited and tuned in each of the cavity cells using low radio frequency (RF) power, from 100 W to as low as 2 W depending on the gas and pressure. A method for RF plasma detection has been developed: the plasma location is identified within the cavity without the need of cameras. The presented method can be applied to other multicell cavity designs, even for accelerators where the coupling for the fundamental modes at room temperature is very weak.