Numerical modeling to predict ignition thresholds for plasma processing of superconducting radio-frequency cavities

Laboratories such as Oak Ridge, Fermilab, and Jefferson Laboratory have been developing plasma cleaning techniques for superconducting radio-frequency (SRF) cavities. These techniques show promise as an in-situ method of mitigating degradation in cavity performance via removal of surface hydrocarbon contamination. Plasma cleaning studies for SRF cavities in the driver linac at the Facility for Rare Isotope Beams (FRIB) began in 2020. Both neon-oxygen and argon-oxygen plasmas have been found to be effective in improving FRIB accelerating cavities. The ignition threshold RF electric field can be predicted for a diffusion-dominated plasma using the solution to the diffusion equation for a given cavity shape; a numerical solution is needed for realistic cavity shapes. Predictions for several cavity shapes will be presented and compared to available plasma ignition threshold measurements.