Investigation of two-step baking technology for 3.9 GHz superconducting RF cavities

BackgroundThe Shanghai Hard X-ray Free Electron Laser facility (SHINE) requires two 3.9 GHz superconducting cryomodules, containing a total of sixteen 3.9 GHz nine-cell superconducting radiofrequency (SRF) cavities, and the high design accelerating gradient of each cavity provides sufficient margin to compensate for individual cavity failures. The two-step baking process (75 °C/4 h + 120 °C/48 h) has been internationally recognized as an effective method to enhance the accelerating gradient of 1.3 GHz SRF cavities; however, its impact on the RF performance of 3.9 GHz SRF cavities and the control of associated baking parameters remain largely unexplored.PurposeThis study aims to investigate the effects of two-step baking on the RF performance of 3.9 GHz SRF cavities and to establish effective temperature control strategies for the associated baking process.MethodsFirstly, a dedicated low-temperature baking apparatus was designed and constructed for 3.9 GHz 9-cell SRF cavities, and the two-step baking process was conducted on prototype cavities for the first time. Then, the temperature control scheme was optimized by adding an independent bottom heating belt, achieving a temperature uniformity within ±3 °C across all nine cells. Finally, the optimized process was applied to batch processing of fifteen SHINE 3.9 GHz 9-cell cavities, with protective argon gas continuously introduced to prevent surface oxidation during baking. In addition, two cavities subjected to abnormal baking with local cell surface temperatures up to 260 °C were investigated to evaluate the impact on cavity RF performance.ResultsVertical test results demonstrate that the maximum accelerating gradient of the 3.9 GHz 9-cell SRF cavities is significantly improved after two-step baking: the nine batch-processed cavities achieve an average maximum accelerating gradient of 22.8 MV·m-1, all exceeding the SHINE design specification of 16.5 MV·m-1. Moreover, this treatment effectively eliminates the high-field Q-slope at high accelerating gradients. The abnormally baked cavities exhibit RF performance characteristics similar to those of medium-temperature baking, which can be largely recovered after subsequent BCP treatment and a second normal two-step baking cycle.ConclusionsThe two-step baking recipe proposed in this study markedly enhances the usable accelerating gradient (average 22.8 MV·m-1 for batch-processed cavities) and eliminates the high-field Q-slope in 3.9 GHz SRF cavities, demonstrating its suitability for large-scale cavity production in SHINE. This research provides new insights and practical experience in applying two-step low-temperature baking to other types of superconducting cavities.