Low-Temperature Baking Effect of the Radio-Frequency Nb_3Sn Thin Film Superconducting Cavity

The Nb_3Sn thin film cavity, having the potential to be operated at a higher temperature and higher gradient compared to the cavity made from bulk niobium, is one of the most promising key technologies for the next-generation radio-frequency superconducting accelerators. In our work, several 1.3 GHz single-cell TESLA-shaped Nb_3Sn thin film cavities, coated by the vapor diffusion method, were tested at Peking University and Institute of Modern Physics, Chinese Academy of Sciences. It was observed that the performance of the Nb_3Sn thin film cavities in the tests without the slow cooling down procedure and the effective magnetic field shielding was significantly improved by using a low temperature baking at 100 ℃ for 48 hours. Although the peak electric field of the cavity remained unchanged, the rapid drop of the unloaded Q value (Q_0) with the increasing accelerating field (Q-slope) was effectively eliminated, resulting in an improvement of the Q_0 in the intermediate field region by \sim 8 times. Furthermore, under better test conditions with the shielded magnetic field less than 5 mG and the slow cooling down procedure in the temperature range of 25–15 K, the Q_0 was still improved by about 20%. Our study shows that the low temperature baking can be an effective supplement to the effective post-treatment for the Nb_3Sn thin film cavity.