Measurement of the microwave properties of superconducting Nb30-Ta70

The low field and high field microwave characteristics of a superconducting 8.6 GHz cavity which was machined from a rod of 30 at % Nb and 70 at % Ta were investigated. The low field temperature dependence of the surface resistance was in agreement with the surface resistance calculated from the Mattis-Bardeen expression for a reduced energy gap of 3.61. The maximum critical rf field attained was 6.1 mT which is well below the value of Hclfor this alloy. Thermal calculations based on a line defect model for the surface of the superconductor were made and compared with the experimental measurements. This comparison indicates a field independent defect model does not adequately represent thermal magnetic breakdown observed for the Nb30-Ta70cavity.