Theoretical design study of higher-order-mode absorber for 500 MHz spare superconducting cavity module of SSRF

Background The higher-order mode (HOM) absorber of the 500 MHz spare superconducting cavity module of Shanghai Synchrotron Radiation Facility (SSRF) is a key component of the cavity chain. It can damp heavily the HOMs excited by beam-cavity interaction by reducing the shunt impedance of HOMs to a lower level, play an important role in avoiding longitudinal and transverse bunch instabilities, and improving the quality and lifetime of the beam.

Purpose This paper is dedicated to carrying out a relatively systematic investigation and arriving at an optimized theoretical structure model for the HOM absorbers.

Methods Firstly, based on the beam operation parameters of SSRF storage ring, requirements on longitudinal and transverse shunt impedance thresholds of the HOMs in 500 MHz spare superconducting cavity module are proposed. Secondly, the monopole and dipole HOMs are calculated and analyzed by CST, their geometric shunt impedance and corresponding requirements on loaded quality factor are given theoretically. Thirdly, the power distribution of HOMs on each component of the cavity chain is calculated by energy attenuation method (EAM). Fourthly, the structure of the substrate and water-cooling pipeline of absorption unit are optimized by thermal-structural stress analysis.

Results It is shown that the type and structural dimensions of absorbing material as well as its longitudinal position in the cavity chain will influence greatly on the loaded quality factor of HOMs including the two lowest-order TM₀₁₁ HOMs, based on which, simulation optimization for radio frequency performance of absorber are carried out. A full set of optimized geometric parameter sets of absorbing material is ultimately acquired. It is derived that over 94% of the HOM power will be absorbed by absorbers according to the results calculated by EAM. When the absorption unit is subjected to maximum absorption power, the maximum stress in absorbing material remains always below its tensile strength, decreasing the risk of damage.

Conclusions It can be anticipated that all main HOMs will be deeply suppressed by optimally designed HOM absorbers, and the beam bunch stability will be strongly guaranteed. The optimization design method for HOM absorber presented in this paper can also be applied to other microwave absorption structure design of similar facilities.