Development and vertical tests of 650 MHz 2-cell superconducting cavities with higher order mode couplers

650 MHz 2-cell superconducting cavities are proposed for the main ring of the Circular Electron–Positron Collider (CEPC). The cavity is designed with two detachable double-notch higher order mode (HOM) couplers to suppress the fundamental mode and extracted the HOM power. The present paper describes the design, fabrication, surface treatment and vertical tests of the cavities and HOM couplers. The performances of the cavity at 2 K is not affected after the installation of the HOM coupler. The maximum intrinsic quality factor $Q_0$ of the cavity dressed with the HOM coupler reached $3.1\times 10^{10}$ at operation accelerating gradient. The vertical tests results showed that the fundamental mode external quality factor $Q_e$ of all HOM couplers is an order of magnitude larger than $Q_0$ of the cavity. The HOM damping results for the 650 MHz 2-cell cavity were also measured at cryogenic temperature and compared with the simulated and measured results at room temperature. Two 650 MHz 2-cell cavities each with two HOM couplers will be assembled in the CEPC 650 MHz test cryomodule after helium vessel dressing.