Superconducting Test Cavity With Dielectric Concentrator for High Q, High Surface Field

A superconducting test cavity and cryostat are being developed to support the testing of round wafer samples of new superconducting materials. The cavity is designed to test these samples in surface fields up to and beyond the BCS limit of Nb. The design is optimized to produce maximum surface field on the sample compared with that elsewhere in the cavity. It operates in the TE₀₁₁ mode. A dielectric hemisphere of low-loss sapphire is located just above the wafer surface and serves to concentrate the surface field. The sapphire is cooled by a column of superfluid He, with sufficient heat transport to maintain high Q throughout the rf pulse. It should be possible to test a sample to twice the BCS limit of Nb while measuring the rf surface resistance of the sample.