The Circular Electron Positron Collider (CEPC) at IHEP made an important breakthrough recently. Two of its 650 MHz 1-cell cavities (650S4 and 650S7) reached 2.7E10@35MV/m and 3.6E10@32MV/m, respectively, in a vertical test at 2.0 K. The results have broken China's gradient record for low-frequency (<1 GHz) elliptical cavities and now rank in the top level worldwide.

Based on the successful experience of 1.3 GHz cavities at IHEP, these two 650 MHz 1-cell cavities have achieved both high quality factor (Q) and high accelerating gradient (E_acc). 650S4, which is made of fine-grain niobium, received bulk buffered chemical polishing (BCP) and light electro-polishing (EP). 650S7, which is made of large-grain niobium, received flexible polishing to repair surface defects, followed by light EP.

In the future, relevant techniques will be optimized to further improve the Q and E_acc of 650 MHz 1-cell cavities and applied to 650 MHz 2-cell cavities.

This work was supported by the Platform for Advanced Photon Source Technology R&D and pre-research project of CEPC.