Study on Nb₃Sn Coating Technology of 1.3 GHz Superconducting Cavity by Electrochemical Bronze Method

Superconducting RF cavities have been widely used in modern accelerators because of their advanced characters, such as high quality factor, large beam aperture etc. After years of development, the performance of bulk Nb cavity is close to the material's theoretical limit. Using Nb₃Sn thin film cavities to replace current bulk Nb cavities could be a viable way to break such limit. Because of its high superconducting transition temperature and superheating magnetic field, Nb₃Sn can elevate both the working temperature and the acceleration gradient of the SRF cavities. Currently, there are several preparation methods of Nb₃Sn thin films under development. Among them, Nb₃Sn thin films with decent performance were successfully prepared on the inner surface of Nb cavity by tin vapor diffusion method. However, because the reaction temperature is above 1 100 °C, tin vapor diffusion method cannot get rid of bulk niobium substrate, so it inevitably has defects in mechanical stability, thermal conductivity and other aspects, which is difficult to meet the application of high reliability accelerator in the future. Bronze method have been successfully used for Nb₃Sn cable preparation. The maximum heat treatment temperature of this method is under 700 °C, which can potentially be applied to copper-based Nb₃Sn thin film cavity preparation. In addition, electrochemical coating method is also considered because of its advantages, such as low cost, easy control of reaction process, normal temperature and pressure. In this work, we combined the advantages of above two methods. Specifically, on 1.3 GHz Nb based cavity, bronze precursor films were prepared by electrochemical plating firstly, and then were heated to synthesize Nb₃Sn thin film by annealing. The vertical test results show that the intrinsic $Q_{0}^{}$ of the film cavity at 4.2 K is about $6 \times 10^{8}$ and still has a lot of room for improvement.