The mechanical design, fabrication and tests of dressed 650 MHz 2-cell superconducting cavities for CEPC

650 MHz 2-cell superconducting radio-frequency (SRF) cavities have been adopted for the collider ring of the Circular Electron Positron Collider (CEPC). By optimizing the wall thickness and geometry, the mechanical properties of the dressed cavity have been significantly improved with a focus on stress, tuning capabilities, pressure sensitivity, and Lorentz-force detuning, while microphonics and buckling were carefully monitored. Three cavities were later fabricated, post-processed and received vertical tests. The maximum unloaded quality factor (Q0) of the cavities has reached 3.2 × 1010 at 19.7 MV/m, which exceeded the design goal during operation. The favored small coefficient of Lorentz-force detuning was validated, too. Pressure sensitivity was measured and consistent with simulation prediction. Mounted with tuners, fundamental and high-order-mode (HOM) couplers, two dressed 650 MHz 2-cell cavities were finally integrated into a test cryomodule for a beam test facility. Both cavities were successfully tuned to the operating frequency of 650.000 MHz at 2.0 K. This article covers the mechanical aspects in the development and cryogenic tests of the 650 MHz 2-cell cavities.