Modeling and optimization of transverse trapped-mode impedance for in-vacuum undulators of HALF

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.