Electric surface resistance R/sup E/(T, f, E/sup /spl perp//) of Nb/Nb/sub 2/O/sub 5-y/-interfaces and Q-drop of superconducting Nb cavities

The RF losses, especially actual level and increase with RF fields limit most stringently the application of superconducting RF cavities. This is due to the needed cooling power to be supplied locally to the high field region and due to the nonlinearities causing harmonics and RF breakdown. The separation of RF residual losses R/sub res/(T,f) from the intrinsic losses R/sub BSC/(T,f) yields the quasi-exponential increases of the electric surface resistance with the electric field E/sup l/ perpendicular to the surface /spl delta/R/sup E/(E/sup /spl perp//)/spl prop/exp (-c/E/sup /spl perp//) and the power law increases of the magnetic surface impedances with the magnetic field H/sup /spl par// parallel to the surface /spl delta/R/sup H/(H/sup /spl par//)/spl prop/(H/sup /spl par//)/sup 2n/ (n=1, 2..). By the Nb/Nb/sub 2/O/sub 5-y/ interfaces at external and internal surfaces R/sup H//sub res/(T,f) and R/sup E//sub res/(f,E/sup /spl perp//) can be explained quantitatively. Especially, the drop of Q/sub 0/(E/sup /spl perp//)/spl prop/1/R/sup E//sub res/(E/sup /spl perp//) and its reduction by EP- and BCP-smoothening and by better interfaces by UHV anneal are well accounted for by interface tunnel exchange.