A circular electron positron collider (CEPC) will adopt hundreds of 650-MHz superconducting cavities with high quality factor (Q) and accelerating gradient (E_acc). 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.

Medium-temperature (mid-T) furnace baking was conducted at 650 MHz superconducting radio-frequency (SRF) cavity for circular electron positron collider (CEPC), which enhanced the cavity unloaded quality factor (Q₀) significantly. In the vertical test (2.0 K), Q₀ of 650 MHz cavity reached 6.4 × 10¹⁰ at 30 MV/m, which is remarkably high at this unexplored frequency. Additionally, the cavity quenched at 31.2 MV/m finally. There was no anti-Q-slope behavior after mid-T furnace baking, which is characteristic of 1.3 GHz cavities. The microwave surface resistance (R_S) was also studied, which indicated both very low Bardeen–Cooper–Schrieffer (BCS) and residual resistance. The recipe of cavity process in this paper is simplified and easy to duplicate, which may benefit the SRF community.