Design and simulation of cavity BPM for BEPCII

Abstract

In the linac of Beijing Electron Positron Collider II (BEPCII), the resolution of existing BPMs is several hundred microns. But the requirement of transverse position resolution of BPM for future top-up injection needs to be more precise. This paper is mainly concerned about the design and simulation of the cavity beam position monitor (CBPM) for BEPCII linac and free electron laser (FEL). The requirement of beam transverse position resolution for linac is several microns, even a few nanometers for FEL. When the beam bunch goes through the CBPM, it can excite different electromagnetic field modes in resonator. In these modes, TM110 is the most sensitive to beam transverse position offset. The design of CBPM is aim to analyze the relationship between TM110 and beam transverse position by CST Studio and Ansys HFSS. In this research, we design a pair of rectangular slots for position cavity to revise the direction of electromagnetic field. The simulation result of theoretical resolution is able to reach 15 nm@1nC with the operating frequency 2500 MHz. It is obviously that CBPM has more excellent resolution than all of operating BPM in BEPCII. Meanwhile, the research of CBPM improves the application of beam detection technology in BEPCII and accumulates technology for future design of HEPS and FEL. The mechanical design of the prototype is being fabricated.