Impedance of in-air undulator vacuum chamber in HEPS

Undulators with small gap have been widely used in various light sources all over the world for the demand of high brilliance. A large number of in-air undulators with small gap will be installed in the storage ring of the High Energy Photon Source, and become one of the important impedance contributors. Antechambers are adopted for the bypass of the synchrotron radiations, as well as to reach high vacuum in the beam pipe with small aperture. Due to the asymmetric structure of the vacuum chamber, monopole wakefield in the transverse planes will be excited by the beam passage. In addition, photon absorbers will be installed in the vacuum chamber of the in-air undulators to shield the downstream components, which will introduce extra resonances in the transverse planes. In this paper, a series of numerical simulations are performed to investigate the impedance of the in-air undulator vacuum chamber. The high order modes generated by the photon absorber and their mitigations are also investigated.