A liquid helium-free cryostat for radio frequency (RF) test of the superconducting cavity is designed and constructed. Gifford-Mcmahon (G-M) cryocoolers are used to provide cooling capacity, and the heat leakage at 4 K is less than 0.02 W. Vertical and horizontal tests of two Nb₃Sn cavities are carried out in the cryostat with different surface treatments outside the cavities. Both of the cavities achieve stable continuous wave (CW) operation. A novel treatment, which cold-sprayed a 3.5 mm thick Cu layer onto the outside of the cavity, enables the maintenance of an average temperature of 5.5 K in the cavity at a RF loss of 10 W, implying that the thermal stability and uniformity of the cavity has been significantly improved. Through the synergistic control of four metal film resistors, a cooling rate of 0.06 K/min near 18 K is realized, and the cavity temperature gradient is reduced to 0.17 K/m, which effectively improves the RF performance of the cavity. The maximum $E_{acc}$ of the cavity reaches 3.42 MV/m, and the $Q_0$ is 1.1 × 10⁹. An electromagnetic–thermal coupling simulation model for the superconducting cavity is established and is in good agreement with the experimental results. The simulation results show that the cavity with a Cu-spraying treatment and the thermal links of 5N Al can satisfy the $E_{acc}$ of 10 MV/m under conduction cooling.

Nb₃Sn has a superconducting transition temperature of 18.1 K and a superheating magnetic field of 420 mT, making it one of the most promising materials for superconducting radiofrequency (SRF) cavities. The surface roughness reduction and mechanical stability of Nb₃Sn films are two important issues to improve the cavity RF performance and reliability in the application of conduction-cooling accelerators. This paper presents the studies on the surface roughness of Nb₃Sn films prepared by the tin vapor diffusion method and proves the advantages of buffered electropolishing (BEP) as a pre-polishing method. The smallest mean roughness of 26 nm, with a grain size of 760 nm, was achieved by fast BEP treatment on the niobium substrate. Nb₃Sn films on flat and curved substrates with the same coating process on Nb₃Sn cavities at Peking University (PKU) were tested under different tensile and compressive stress levels. The results showed that Nb₃Sn films had severe crack risks while loading stresses, and a safe strain range of (−2.3%, 0.9%) is suggested. To study the tuning problems for Nb₃Sn cavities, 150 kHz tuning was performed on the previously obtained high-performance cavity.