Design Evolution of the Higher Order-Mode Damping in 166.6-MHz Quarter-Wave \beta=1 Superconducting Cavities for HEPS

High-energy photon source is a 6-GeV diffraction-limited synchrotron light source with ultralow beam emittance. A 166.6-MHz quarter-wave superconducting \beta=1 structure was proposed as the main accelerating cavities for the storage ring. The high beam current of 200 mA requires heavy damping of higher order modes (HOMs) in the 166.6-MHz cavities to avoid coupled multibunch instabilities. The low HOM frequencies and the limited space of the straight sections for cavities impose great challenges on the HOM damping design. Several hybrid design concepts have been exploited, combining dedicated HOM couplers for the first few HOMs and a beam-line HOM absorber for the remaining high-frequency modes. An enlarged beam pipe scheme was also examined, adopting a large ferrite HOM absorber on the beam line. The design optimizations and limitations of these damping schemes are elaborated and compared. This article presents the design evolution of the HOM damping schemes in the 166.6-MHz superconducting cavities and finally determines the baseline design.