Experimental studies of the microwave properties of 8.6 GHz superconducting niobium, niobium-tantalum, and tantalum cavities

In this dissertation measurements on the surface impedance of superconducting niobium, tantalum, and a 30 at% niobium-70 at% tantalum alloy are reported. The measurements were made on X-band mode cavities at a nominal frequency of 8.6 GHz. The surface resistance and the change of surface reactance for the three materials were measured from 1.3 K up to their respective transition temperatures. These data were compared to computer calculations based on the Mattis-Bardeen expressions for the surface resistance and reactance of superconductors. Evidence is also presented concerning the effect of thermal expansion of niobium on the resonant frequency of an X-band cavity as a function of temperature below 2K. The influence of trapped magnetic flux on the microwave surface resistance and reactance of niobium is also reported.