Investigation on the Trapped Modes of CPMU at HEPS

The Cryogenic Permanent Magnet Undulator (CPMU) is a crucial component in synchrotron radiation sources. Due to the small magnet gap of CPMU, the interaction between the beam and its surroundings is strong, which can result in a significant contribution to coupling impedance. In this work, the influence of CPMU on coupling impedance was investigated using wakefield and eigenmode solvers. The results indicated that some of the transverse impedance resonances in CPMU were much stronger than the impedance threshold determined by synchrotron radiation damping, which could cause vertical beam instability. To address this issue, different types of damping materials were investigated through simulations to suppress the resonances.