Exploration of the Copper-Niobium Composite Superconducting Cavities for Pursuing Extremely High Operational Stability at IMP

Theoretically, the copper-niobium composite superconducting cavities have excellent potential for high thermal and mechanical stability, which can make full use of the high-gradient surface processing recipes developed for the bulk niobium cavity, the thick copper layer’s high thermal conductivity and rigidity, thereby enhancing the operational stability of the bulk niobium cavities. This paper provides a global review of the technical approaches employed for copper-niobium composite superconducting cavities. We present the exploration of the copper-niobium composite superconducting cavities based on two technologies at IMP, including their manufacturing processes, RF characteristics, and mechanical performance. The cavities studied exhibit robust mechanical stability. Firstly, the investigation of several 1.3 GHz single-cell elliptical cavities using the copper-niobium composite sheets indicates that the wavy structure at the copper-niobium interface influences the reliable welding of the copper-niobium composite parts. We observed the generation and trapping of magnetic flux density during the Tc crossing of niobium in cooldown process. The cooling rates during the Tccrossing of niobium were found to have a substantial impact on the performance of the cavities. Furthermore, we measured and analyzed the surface resistance attributed to the trapped magnetic flux induced by the Seebeck effect after quench events. Secondly, a low-beta bulk niobium cavity has been plated with copper on its outer surface by electroplating technology for the first time. Achieving a high peak electric field Epk of ~ 88.8 MV/m at 2 K, the unloaded quality factor Q0 at the Epk of 88.8 MV/m exceeding 1E10  demonstrates that the electroplating copper on the bulk niobium cavity is a practical way to develop the copper-niobium composite superconducting cavity with superior thermal stability. The outcomes presented here provide valuable insights for applying the copper-niobium composite superconducting cavities in the superconducting accelerator with stringent operational stability requirements.