Parasitic mode loss measurement in BEPCⅡ e^- ring

In the design phase of BEPCII, the impedance is limited in terms of beam instabilities and parasitic mode loss. However, the parasitic mode loss has been one of the unfavorable factors affecting high beam current operation of BEPCII. This paper describes a measurement of the parasitic mode loss in BEPCII e^- ring, which is based on a small change of synchronous phase shift with beam current, the beam power measurement and the power of higher order mode absorber. The measurement results with two different methods of the total parasitic mode loss are repetitive and reliable. Moreover, the total parasitic mode loss of e^- ring is 4-5 times as large as the parasitic mode loss of the superconducting cavity.