A new method of surface resistance measurement with a niobium triaxial cavity working at 2 K

A 1.5-GHz superconducting niobium triaxial cavity has been fabricated to study residual surface resistance of superconducting materials at 2 K. Unlike many other structures where the entire test samples have to be placed in strong magnetic field locations, we have the edge of a 25.4 mm or larger diameter sample outside of the strong field region, a procedure which will greatly reduce edge effects and image current losses between the thin film and substrate. A calorimetric method is used to measure the sample losses, and is designed to resolve a 10-muK temperature change using 16 carbon resistor sensors. A detection limit of 0.05-muW power dissipation has been determined with a calibration heater, which corresponds to a surface resistance of 0.02 nOMEGA at a maximum cavity magnetic field of 250 G. Initial cavity testing was performed in a magnetically unshielded cryostat, yielding a 2.2-muOMEGA residual resistance which was measured by both the rf measurement and the calorimetric measurement.