Natural Vibration Frequencies of an SRF Module at Liquid-Helium Temperature

As a superconducting radio-frequency (SRF) cavity is a thin-shell structure with several thin bellows to bridge to its cryostat, any external excitation readily induces oscillations. The operation of a SRF module becomes unstable when the RF resonant frequency varies, due to mechanical vibrations, to a critical point. In this investigation we illustrate the mechanical vibration behavior of the 500-MHz SRF module of KEK type. An experimental test with an RF technique was used to assess the computational modes with models according to a finite-element method, for which material properties at various temperatures from 4.4 K to 300 K were assigned to the respective components. The calculated modes of vibration of each major component are shown to approach the computed results for the full structure, although some combined modes can be predicted only with the full model.