A prototype of 650 MHz single cell cavity for CEPC was tested at the Institute of High Energy Physics (IHEP) on May 3^rd, 2017, which is for high-Q research of CEPC cavities.

The test results proved that the cavity Q₀ reached 4x10¹⁰ at a voltage gradient of 19.4 MV/m, with corresponding surface resistance of 7.1 nΩ. The cavity was processed by bulk BCP, followed by outgassing at 750°C for 3 hours (annealing), fine BCP, HPR and then baking at 120°C for 48 hours.

The main ring of Circular Electron Positron Collider (CEPC) will use 336 650 MHz 2-cell superconducting cavities operating in continuous-wave mode. These cavities need to have a high quality factor (Q₀), to reduce the construction and operation cost of the cryogenic system.

This promising result is the first step towards the CEPC vertical test goal of Q₀ > 4 x10¹⁰ at 22 MV/m, while the operation target is 2 x10¹⁰ at 16 MV/m. Nitrogen-doping and electro-polishing (EP) will be used to further increase Q₀.

The cavity was designed by IHEP, and fabricated by OTIC Ningxia. The material is high-purity fine grain niobium provided by OTIC. Design and fabrication of the CEPC prototype 650 MHz 2-cell cavity with full end groups is now underway.