Nitrogen doping/infusion of 650 MHz cavities for CEPC

The nitrogen doping/infusion of 650 MHz cavities for the circular electron positron collider (CEPC) is investigated in this study. Two 650 MHz 1-cell cavities are first treated via buffered chemical polishing (BCP), followed by nitrogen doping. A “2/6” condition is adopted, similar to that for 1.3 GHz cavities of Linear Coherent Light Source II. The quality factor of both cavities improved to 7 × 1010 in low fields, i.e., higher than that obtained from the baseline test. One 650 MHz two-cell cavity is nitrogen infused at 165 °C for 48 h with a BCP surface base. The intrinsic quality factor (Q0) reached 6 × 1010 at 22 MV/m in the vertical test, and the maximum gradient is 25 MV/m, which exceeds the specification of the CEPC (4 × 1010 at 22 MV/m).