The in-vacuum undulator (IVU) exhibits exceptionally strong trapped-mode impedance due to its distinctive ridge-loaded waveguide structure and narrow magnetic gap design, which may lead to beam instability issues. This study primarily use the CST wakefield solver to investigate the trapped-mode impedance of the IVUs of the Hefei Advanced Light Facility (HALF). The trapped-mode impedance in the vertical direction is evaluated for both structures, with and without pump ports. Based on the impedance results, two mitigation strategies are proposed: ferrite damping and transition section optimization. Simulation results demonstrate that both strategies effectively reduce the impedance, with the transition section optimization strategy showing superior suppression performance.
Yao et al. (Tue,) studied this question.