Subjects: Other Disciplines submitted time 2025-05-24
Abstract： Abstract The Shanghai Hard X-ray Free Electron Laser facility requires two 3.9 GHz superconducting crymodules (CMs), containing a total of sixteen 3.9 GHz nine-cell superconducting radio frequency (SRF) cavities. Due to small number of the 3.9 GHz cavities, maintaining a sufficient accelerating gradient margin is crucial to compensate for potential failures of individual cavities within the accelerator. In recent years, a two-step baking process has been developed world widely, which has been proven effective in enhancing the accelerating gradient of 1.3 GHz superconducting cavities. However, its effect on 3.9 GHz cavities remains unclear. In this study, we conducted the two-step baking process on 3.9 GHz 9-cell SRF cavities for the first time. A dedicated low-temperature baking apparatus was designed and constructed specifically for 3.9 GHz cavities. After optimization, the system achieved excellent temperature uniformity and was subsequently used for batch processing of 3.9 GHz 9-cell cavities for the SHINE project. Vertical test results demonstrate that the maximum accelerating gradient of 3.9 GHz 9-cell SRF cavities is significantly improved after the two-step baking process. Moreover, this treatment effectively eliminates the high field Q-slope observed at high accelerating gradients. In addition, the failure experience of abnormal baking, such as local temperature up to 260 ℃ on cell surfaces, and its influence on cavity RF performance are also reported. In the paper, the newly built baking apparatus and the extension of the two-step baking process onto 3.9 GHz 9-cell cavities, provides new insights and experience for the two-step low-temperature baking on other types of superconducting cavities.
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