Elastoplastic Tuning on a Two-Cell 1.5-GHz Superconducting Radio-Frequency Cavity

The National Synchrotron Radiation Research Center (NSRRC) is developing a 2-cell 1.5-GHz superconducting radio-frequency (SRF) module. This SRF cavity's fundamental pi-mode frequency must be adjusted within a specific tolerance by applying a longitudinal displacement to its structure, extending it into the elastoplastic range. A one-quarter symmetric model is established to simulate the frequency tuning process, taking into account the structural elastoplastic behavior. A multi-physics computing process is also employed to calculate this SRF cavity's pi-mode frequency after structure deformation. The resonance frequency at every tuning step, along with the overall frequency shift after pre-tuning, can be computed, in addition to the structural behavior and stress distribution. Initially this SRF cavity is tuned with a small elastoplastic deformation following it's construction. It then undergoes a complete tuning to reach the proper resonance frequency after being electropolished and annealed. This work represents the first successful demonstration of tuning an SRF cavity with its interior in vacuum, effectively eliminating the disturbance caused by air's permittivity on the frequency shift.