Ultrahigh accelerating gradient and quality factor of CEPC 650 MHz superconducting radio-frequency cavity

Two 650 MHz single-cell superconducting radio-frequency (SRF) cavities used for the Circular Electron Positron Collider (CEPC) were studied to achieve a high accelerating gradient ( E acc ) and high intrinsic quality factor ( Q 0 ). The 650 MHz single-cell cavities were subjected to a combination of buffered chemical polishing (BCP) and electropolishing (EP), and their E acc exceeded 40 MV/m. Such a high E acc may result from the cold EP with more uniform removal. BCP is easy, cheap, and rough, whereas EP is complicated, expensive, and precise. Therefore, the combination of BCP and EP investigated in this study is suitable for surface treatments of mass SRF cavities. Medium temperature (mid-T) furnace baking was also conducted, which demonstrated an ultrahigh Q 0 of 8 × 10 10 at 22 MV/m for both cavities, and an extremely low BCS resistance ( R BCS ) of ~ 1.0 nΩ was achieved at 2.0 K.