Study on the Superconducting Radio-frequency Performance of the Copper-Niobium Composite Cavities Based on a Heat Transfer Computational Model

hermal stability is a significant indicator for evaluating the long-term operational reliability of superconducting radio-frequency (SRF) cavities. Theoretically, the copper-niobium (Cu-Nb) composite cavity has the potential for high mechanical and thermal stability by combining the exceptional mechanical rigidity and thermal conductivity of the thick Cu layer with the excellent SRF performance of the Nb layer. However, the existence of the Cu-Nb bimetal structure as the cavity wall complicates the heat transfer process compared to that of bulk niobium cavities. Further understanding of the influence of physical property parameters on the thermal transfer efficiency is essential for high-quality application of the Cu-Nb composite cavity. In this paper, we proposed a comprehensive heat transfer computational model for analyzing the RF performance of the Cu-Nb composite cavities, incorporating key parameters including the measured copper-niobium thermal boundary resistance (2-410 mK/W). The model was applied to three types of cavities to investigate the effect of the material layer thickness, the thermal conductivity of Cu, and the hot island diameter on the RF performance of the Cu-Nb composite cavities at 4.2 K and 2 K. Moreover, the model was used to analyze the RF test data of a Cu-Nb composite half-wave resonator with an optimal value of 0.3 (labeled as the HWR030 Cu-Nb composite cavity) at 4.2 K and 2 K. The simulation results indicate that the defects may be the primary cause of performance degradation of the HWR030 cavity. Finally, the dynamic process simulations of hot island on the RF surface of the cavity were conducted, revealing the expansion behavior and the size limit of the hot island, providing insights into performance degradation of the SRF cavities compared to their theoretical performance limit. The proposed heat transfer model can guide the design of the Cu-Nb composite cavities and help to understand the underlying mechanism of cavity performance degradation.