HOM Coupler Design and Optimization for the FCC-ee W Working Point

A 2-cell 400 MHz superconducting radio-frequency cavity with improved damping has been designed as an alternative to the baseline 4-cell cavity for the W working point of the future circular lepton collider (FCC-ee). For this cavity, the longitudinal higher-order modes’ (HOMs) impedance stay under the beam stability threshold value. This paper, therefore, focuses on the design of the HOM couplers for the purpose of damping the high-impedance transverse HOMs. The resulting impedance of the HOM-damped cavity is then calculated and compared with the impedance limit set by synchrotron radiation.