Progress on Nb₃Sn Coating Technique for Superconducting Radiofrequency Cavities

The past five decades have seen many successes in superconducting radio frequency（SRF） tecknique, and Nb SRF cavities have been widely used in frontier particle accelerators. However, in the twenty-first century, the development of niobium cavities has decelerated due to the limitation of intrinsic material properties, making it difficult to meet higher requirements for future accelerators. Nb₃Sn, with higher transition temperature, higher superheating field and lower surface resistance than Nb, is not only able to improve cavity performance, but also save operating costs by raising the operating temperature to 4.2 K. During the past few decades, several coating techniques of Nb₃Sn have been development to produce Nb₃Sn coated thin film superconducting radiofrequency cavities, including vapor diffusion, sputtering, bronze processing and chemical vapor deposition. Herein, this review article summarizes these coating techniques from the aspects of fabrication, results, properties and prospects. The Nb₃Sn cavity representing the-state-of-art performance was a single cell 1.3 GHz elliptical cavity fabricated with vapor diffusion technique on Nb substrate by FermiLab. It showed accelerating gradient E_acc reaches ～22.5 MV/m at 4.4 K and a quality factor Q₀ of ～2×10¹⁰ at 10 MV/m and 4.4 K. By the way, the-state-of-art performace of Nb₃Sn cavity in China showed an E_acc of ～16.1 MV/m at 4.2 K and a Q₀ of ～6×10⁹ at 10 MV/m and 4.2 K. The acceleration gradient of the Nb₃Sn cavities can theoretically reach 90 MV/m, thus there is still great improvement potential in terms of performance.