Advances in development of Nb3Sn superconducting radio-frequency cavities

A 1.3 GHz Nb3Sn superconducting radio-frequency cavity prepared with a modified annealing step reached B-pk > 50 mT, well above B-c1 = 25 +/- 7 mT, without the strong Q-slope observed in previous Nb3Sn cavities. At 4.2 K, it has a Q(0) of approximately 1 x 10(10) at > 10 MV/m, far outperforming Nb at useable gradients. At 2 K, quench occurred at similar to 55 mT, apparently due to a defect, so additional treatment may increase the maximum gradient. Material parameters of the coating were extracted from Q vs T data, including a T-c of 18.0 +/- 0.1 K, close to the maximum literature value. High power pulses were used to reach fields far higher than in CW measurements, and near Tc, quench fields close to the superheating field were observed. Based on a review of previous experience with Nb3Sn cavities, a speculative mechanism involving weak link grain boundaries is presented to explain how the modified annealing step could be the cause of the absence of strong Q-slope. Finally, an analysis of the progress to date provides hints that the path forward for Nb3Sn cavities should focus on minimizing defects.