Nb₃Sn-coating experiments on samples and single-cell cavities were conducted at Peking University (PKU) to understand the Nb₃Sn growth process using the vapor diffusion method. The evaporation of tin and tin chloride used in the vapor diffusion process was simulated and experimentally analyzed. The results show that the nucleation process is generally uniform, and the atomic ratios of Nb/O and Sn/O were found to be 1:2 within the 10 nm surface of the nucleated samples. Three tin sources were distributed along the cavity axis to obtain a uniform grain size on the cavity surface, and a surface tin content of 25~26% was achieved. The tin segregation effect was found in long-time coatings or coatings with insufficient tin, resulting in a low Sn% and bad cavity performance. By overcoming the tin segregation problem, a Nb₃Sn cavity with a 750 nm grain size was produced by 1200 °C coating for 80 min and 1150 °C annealing for 60 min. The acceleration gradient reached 17.3 MV/m without quenching and an obvious Q-slope at 4.2 K. The relationship between coating recipes and vertical test results is discussed and conclusive advice is provided in this paper.

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.

Resonant rings which can amplify RF power through the coupling of waves are used for high power breakdown tests, unidirectional filters, or pulse-shaping techniques. Usually, the RF output terminal of a resonant ring is connected to a matched load. For the resonant ring at Peking University, the matched load has been replaced by a waveguide shorting plate to obtain higher conditioning power for the 1.3 GHz capacitive type power couplers. The power gain is increased significantly with this short termination with the same input RF power. Working mechanism analysis, experiments, and results of this modified resonant ring will be presented.

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.

A miniaturized 150 kV DC photocathode gun is developed at Peking University to generate electron beam which can be manipulated in temporal and spatial distribution as requirements freely. To measure the bunch length which is an important temporal parameter of the low energy electron beam from the DC photocathode gun, a 1.3 GHz medium-β disk-loaded deflecting cavity is adopted. In this paper we present the design of the deflecting cavity which involves the microwave design including the geometry optimization and the separation of the orthogonal dipole modes as well as the power coupling, the mechanical design including the determination of the cavity wall thickness and the tuning as well as brazing structure, and the thermodynamic analysis. Particle tracking simulation shows that the best resolution of 190 fs can be achieved for the 150 keV electron beam by using the deflecting cavity. Its fabrication is completed and the RF measurements are carried out with a vector network analyzer. It is shown the measured values of the RF physical parameters are in good agreement with the simulation design ones.