Design and test of SX-FEL cavity BPM

TL;DR

This paper presents the design and cold testing of a cavity beam position monitor for SX-FEL, achieving a resolution of 0.1 micrometers through optimized damping and mode suppression techniques.

Contribution

The paper introduces a novel cavity BPM design with enhanced mode damping and reduced excitation of unwanted modes, improving measurement resolution.

Findings

Isolation of TM010 mode is about 117dB.

Shunt impedance is about 200Ω at 4.65GHz.

Resolution of the CBPM is about 0.1μm.

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{\mu}m, even 0.1{\mu}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 117dB, and the shunt impedance is about 200{\Omega}@4.65GHz 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 90mV/mm when the source was 6dBm, and the accomplishable minimum voltage was about 200{\mu}V. The resolution of the CBPM was about 0.1{\mu}m from the linear fitting result based on the cold test.