Managing Sn-supply to tune surface characteristics of vapor-diffusion coating of Nb3Sn

Nb 3 Sn promises better RF performance ( Q and E acc ) than niobium at any given temperature because of superior superconducting properties. Nb 3 Sn-coated SRF cavities are now produced routinely by growing a few microns thick Nb 3 Sn films inside Nb cavities via the tin vapor diffusion technique. Sn evaporation and consumption during the growth process notably affect the quality of the coating. Aiming at favorable surface characteristics that could en-hance the RF performance, many coatings were produced by varying Sn sources and temperature profiles. Coupon samples were examined using different material character-ization techniques, and a selected few sets of coating parameters were used to coat 1.3 GHz single-cell cavities for RF testing. The Sn supply's careful tuning is essential to manage the microstructure, roughness, and overall surface characteristics of the coating. We summarize the material analysis of witness samples and discuss the performance of several Nb 3 Sn-coated single-cell cavities linked to Sn-source characteristics and observed Sn consumption during the film growth process.