| Summary: | A circular electron positron collider (CEPC) will adopt hundreds of 650-MHz superconducting cavities with high quality factor (<i>Q</i>) and accelerating gradient (<i>E</i><sub>acc</sub>). 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 × 10<sup>10</sup> 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 (<i>Q</i><sub>0</sub>) of 4.5 × 10<sup>10</sup> at 22.0 MV/m, which was higher than the CEPC spec. The other cavity obtained a lower <i>Q</i><sub>0</sub> of 3.4 × 10<sup>10</sup> at 22.0 MV/m, which may have resulted from the cancellation of high-temperature annealing.
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