Q degradations in superconducting niobium cavities

In the past year, several laboratories around the world have observed degradations of the Q value of superconducting niobium cavities made from high-thermal-conductivity niobium under certain cooldown conditions. Especially under slow cooldown or warmup to temperatures <200 K of the larger systems, severe degradations have been reported. A systematic study of the influence of the cooldown speed, warmup conditions, multiple cooldowns, and chemical surface treatment on cavity performance of cavities manufactured from niobium of different purity has been conducted. Possible cures such as anodic oxidation are being explored and results of these investigations are reported. Those experiments provide some evidence that anodic oxide layers on niobium surfaces of high-purity niobium are capable of significantly reducing Q degradations due to precipitation of the niobium hydride phase, and thicker Nb/sub 2/O/sub 5/ layers seem to be more effective. This observation, coupled with fact that hydrogen preferentially is located in a thin surface layer, suggests that diffusing oxygen into the surface by moderate 'in situ' heating of an anodized cavity might further reduce the danger of Q degradations.<>