Influence of condensed gases on field emission and the performance of superconducting RF cavities

In a program to study the field emission (FE) and to improve the performance of one-cell 1500-MHz superconducting Nb microwave particle accelerator cavities, the authors recently achieved peak surface fields as high as 51 MV/m through the use of 1200°C UHV annealing, methanol rinsing, and high-power He processing. Performance is limited by excess FE from localized points on the cavity walls. Cycling of these cavities to room temperature and admission of He processing gas frequently produce large changes in Q correlating with the appearance or disappearance of the dominant field emitter, suggesting that condensed residual and impurity gases play a significant role in enhancing FE. By intentionally condensing O₂ into a cold cavity, the authors have produced similar effects, increasing the dissipated power and reducing Q, each by an order of magnitude at the same field level. Preliminary tests have also been carried out with H₂ and water vapor. These results suggest that improvements in the outgassing and vacuum environment of these cavities may be important