New higher-order mode damping scheme for passive harmonic superconducting radio frequency cavity in fourth-generation synchrotron-radiation source

In order to reduce the effects of Touschek scattering in the beamline and extend the beam lifetime of the fourth-generation synchrotron-radiation source, a harmonic radio frequency cavity will be employed. In this paper, we propose a new damping scheme based on a 1500 MHz passive third harmonic superconducting cavity to effectively extract the higher-order modes (HOM). The new scheme utilizes a combination of a fluted beam pipe and a large beam pipe for the HOM extraction, allowing all higher-order monopole and dipole modes within the cavity to propagate to the absorber and be damped, especially the TE₁₁₁ and TM₀₁₁ mode. The new HOMs extraction structure is novel and efficient. We also optimized the structure of the absorber, resulting in a high stability of its performance with minimal dependence on its installation position. Additionally, we proposed an efficient water-cooling structure to ensure that the maximum temperature of the ferrite does not exceed 60 degrees Celsius during operation.