Quality Factor Enhancement of 650 MHz Superconducting Radio-Frequency Cavity for CEPC

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 (Q(0)) significantly. In the vertical test (2.0 K), Q(0) of 650 MHz cavity reached 6.4 x 10(10) 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.