Design and test of SX-FEL cavity BPM

Abstract：

This paper reports the design and cold test of the cavity beam position monitor (CBPM) for SX-FEL to fulfill the requirement of beam position measurement resolution of less than 1 u m,even 0.1 u m. The CBPM was optimized by using a coupling slot to damp the TM₀₁₀ mode in the output signal. The isolation of TM₀₁₀ mode is about 117 dB,and the shunt impedance is about 200Ω@4.65 GHz with the quality factor 80 from MAFIA simulation and test result. A special antenna was designed to load power for reducing excitation of other modes in the cavity. The resulting output power of TM₁₁₀ mode was about 90 mV/mm when the source was 6 dBm,and the accomplishable minimum voltage was about 200 uV. The resolution of the CBPM was about 0.1u m from the linear fitting result based on the cold test.