Speaker
Description
KEK and CERN are conducting collaboration research to fabricate superconducting RF cavities by hydroforming. To date, several 1.3 GHz single‑cell cavities have been produced and their performance measured. It is well known that hydroforming an elliptical cavity causes substantial plastic deformation, which degrades the surface roughness. We are investigating how improving the surface finish of this substrate affects RF performance. In the cavities previously manufactured, several approaches were taken: one was coated with a niobium thin film without any prior reduction of surface roughness, while the other involved polishing the copper surface before deposition using several methods. Mechanical polishing of the copper tube prior to hydroforming has been applied to two cavities; however, this treatment did not appreciably improve the roughness. RF measurements revealed that one cavity achieved an excellent result, maintaining $Q_0 > 1 \times 10^{10}$ at 18 MV/m at 1.85 K, while the second exhibited markedly poorer performance. In addition, a method of mechanically polishing the copper surface after hydroforming has also been investigated. This technique yielded significant improvement in surface roughness, and RF performance measurement is scheduled in the near future. We will present the ongoing status of our collaboration and the results with a focus on the roughness improvement.