Field emission mitigation studies in the SLAC Linac Coherent Light Source II superconducting rf cavities via in situ plasma processing

Field emission is one of the main factors that can limit the performance of superconducting radio frequency cavities. To reduce possible field emission in the Linac Coherent Light Source II (LCLS-II), we are developing plasma processing for 1.3 GHz nine-cell cavities. The ultimate goal of plasma processing will be to apply the technique in situ in the cryomodules in order to mitigate hydrocarbon-related field emission without disassembling them. Herein is presented the first systematic study of plasma processing applied to LCLS-II superconducting radio frequency cavities. Having developed a new method of plasma ignition for LCLS-II cavities, we applied plasma processing to 1.3 GHz cavities starting with a clean nitrogen doped cavity and proceeding with studying natural field emission and artificially contaminated cavities. All the cavities were cold tested before and after plasma cleaning in order to compare their performances. It was proved that this technique successfully removes carbon-based contamination from the cavity iris and that it is able to eliminate field emission in a naturally field emitting cavity. The effect of plasma processing on cavities exposed to vacuum failures was also investigated, showing positive results in some cases. This work shows how successful plasma processing is in removing hydrocarbon related contamination from the cavity surface without affecting the high Q-factors and quench fields characteristic of nitrogen doped cavities.