RF design of 650-MHz 2-cell cavity for CEPC

In order to meet the requirements of the Circular Electron Positron Collider (CEPC), a 650-MHz 2-cell superconducting cavity was proposed and designed. In this study, we show the progress in the cavity design for the CEPC. The radio frequency and mechanical designs are ready for the first design version, called CEPC-V1. A cryomodule, which consists of two 650-MHz 2-cell cavities, was scheduled the last year. Therefore, prototypes of the 2-cell cavity based on this design were fabricated and tested the following year. The vertical test results show that multipacting (MP) exists from 10 to 19 MV/m. This will increase the operational risk of the CEPC, while the continuous-wave operation for the 650-MHz cavity is 19.7 MV/m. Two trapped modes were also found, which exist because of the identical half-cell design. Therefore, a few improvements regarding MP and more efficient extraction of higher-order mode power are discussed. A new type of cavity design, called CEPC-V2, is also discussed in this paper.