A circular electron positron collider (CEPC) will adopt hundreds of 650-MHz superconducting cavities with high quality factor (Q) and accelerating gradient (Eacc). Two 650-MHz single-cell cavities made of fine-grain niobium were first treated via buffered chemical polishing (BCP), which was easy and convenient. However, the vertical test results could not meet the specification of the CEPC (4 × 1010 at 22 MV/m). Therefore, electro-polishing (EP) of 650-MHz single-cell cavities was conducted, which was complicated but remarkably effective. Both 650-MHz single-cell cavities achieved state-of-the-art gradients of 35 MV/m after the EP process, which is extremely high for large elliptical cavities (frequency < 1 GHz). One cavity achieved an intrinsic quality factor (Q0) of 4.5 × 1010 at 22.0 MV/m, which was higher than the CEPC spec. The other cavity obtained a lower Q0 of 3.4 × 1010 at 22.0 MV/m, which may have resulted from the cancellation of high-temperature annealing.
