Nb3Sn coating of SRF cavity by co-sputtering from a composite target

We deposited Nb3Sn film on the inner surface of a 2.6 GHz Nb superconducting radiofrequency (SRF) cavity by co-sputtering using a composite of Nb and Sn tube targets in a DC cylindrical magnetron sputtering system, followed by thermal annealing of the coated cavity. An aluminum mockup cavity, replicating a 2.6 GHz Nb SRF cavity geometry, was utilized to optimize deposition parameters, resulting in co-sputtered Nb-Sn films with Sn content of 32 - 42 at. % on the beam tubes and equator positions. Several annealing conditions were investigated to improve the surface homogeneity of the Nb3Sn film. The best co-sputtered Nb-Sn film was achieved after annealing at 600 C for 6 h, followed by annealing at 950 C for 1 h. The best process was applied to a Nb cavity, which was RF tested in a cryogenic dewar. RF testing of the Nb3Sn-coated cavity demonstrated a superconducting transition temperature (Tc) close to the highest reported, at 17.78 K. The Nb3Sn cavity underwent a light Sn recoating process, followed by additional RF testing, resulting in enhancement of the RF performance, primarily due to improved surface homogeneity of the Nb3Sn coating.