Nb₃Sn is emerging as one of the focal points in superconducting radio frequency (SRF) research, owing to its excellent superconducting properties. These properties hold significant possibilities for cost reduction and the miniaturization of accelerators. In this paper, we report the recent efforts of the Shanghai Advanced Research Institute (SARI) in fabricating high-performance Nb₃Sn superconducting cavities using the vapor diffusion method. This includes the construction of a Nb₃Sn coating system with dual evaporators and the test results of 1.3 GHz single-cell coated cavities. The coated samples were characterized, and the growth state of the Nb₃Sn films was analyzed. The first coated superconducting cavity was tested at both 4.4 K and 2 K, with different cooldown rates passing through the Nb₃Sn critical temperatures. The causes of Sn droplet spot defect formation on the surface of the first cavity were analyzed, and such defects were eliminated in the coating of the second cavity by controlling the evaporation rate. This study provides a reference for the preparation of high-performance Nb₃Sn-coated cavities using the vapor diffusion method, including the setup of the coating system, the comprehension of the vapor diffusion process, and the test conditions.

Electropolishing (EP) has become a standard procedure for treating the inner surfaces of superconducting radio-frequency (SRF) cavities composed of pure niobium. In this study, a new EP facility was employed for the surface treatment of both 1.3 GHz and 3.9 GHz single-cell cavities at the Wuxi Platform. The stable “cold EP” mode was successfully implemented on this newly designed EP facility. By integrating the cold EP process with a two-step baking approach, a maximum accelerating gradient exceeding 40 MV/m was achieved in 1.3 GHz single-cell cavities. Additionally, an update to this EP facility involved the design of a special cathode system for small-aperture structures, facilitating the cold EP process for 3.9 GHz single-cell cavities. Ultimately, a maximum accelerating gradient exceeding 25 MV/m was attained in the 3.9 GHz single-cell cavities after undergoing the cold EP treatment. The design and commissioning of the EP device, as well as the electropolishing and vertical test results of the single-cell cavities, will be detailed herein. These methods and experiences are also transferable to multi-cell cavities and elliptical cavities of other frequencies.

Buffered chemical polishing (BCP) is an important and widely used polishing technique for superconducting radio-frequency (SRF) cavities made of niobium. A common problem encountered during BCP is the formation of bubbles and W-shaped pits on the cavity surface, which may severely limit the RF performance. We report a method to address the problem of W-shaped pits through optimizing the BCP acid ratio. We systematically investigate the effect of the BCP acid ratio through sample and cavity BCP experiments and determine an optimal ratio for the three acids. The new BCP recipe with the optimal acid ratio is verified through the development of niobium cavities with several different shapes, which are shown to be free of pits and demonstrate excellent RF performance; notably, several 3.9 GHz nine-cell cavities present unprecedented accelerating gradients. Furthermore, the findings suggest a simple pit-free BCP recipe that does not require ${\mathrm{H}}_3{\mathrm{PO}}_4$, using only HF and ${\mathrm{HNO}}_3$. The method proposed in this study is also appropriate for suppressing pit formation with other acid mixtures or when polishing other metal objects.

The Shanghai high repetition rate XFEL and extreme light facility linear accelerator employs two 3.9 GHz third harmonic cryomodules. The 3.9 GHz fundamental power coupler is one of the key components of the cryomodule and is designed to handle 2 kW of continuous-wave rf power, meeting operational requirements with a beam current of 0.3 mA and cavity gradient of 1 3.1 MV / m . A key innovation of the 3.9 GHz coupler is the incorporation of an adjustable antenna insertion depth, which enables online external quality factor ( Q ext ) tuning and eliminates the need for a three-stub tuner, reducing system complexity and cost. Two 3.9 GHz coupler prototypes were fabricated and underwent comprehensive rf testing. The couplers achieved excellent performance with an insertion loss of − 0.1 6 dB and return loss of − 4 3.2 2 dB at 3.9 GHz after structural optimization. High-power rf conditioning demonstrated stable operation at 2 kW in traveling wave mode and 1 kW in standing wave mode. Sixteen production 3.9 GHz couplers were manufactured and integrated into two third harmonic cryomodules, which achieved maximum total cavity voltages of 63.6 and 59.8 MV, respectively, with average usable accelerating gradients of 2 3.0 and 2 1.6 MV / m . The cryomodules have been installed in the tunnel and initial beam commissioning has been completed successfully.