Plasma ignition and detection for in-situ cleaning of 1.3 GHz 9-cell cavities

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.