Non quadratic RF losses in niobium sputter coated accelerating structures

Low field Q-values of more than 10/sup 10/ and maximum accelerating gradients E/sub a/ between 10 and 15 MV/m have been obtained in superconducting mono-cell and multi-cell accelerating cavities between 350 and 1500 MHz. The superconductor is niobium, which is magnetron-sputtered as a thin film (/spl sim/1-2 /spl mu/m) on a cavity made from copper sheet. The dependence of the slope of Q vs. E/sub a/ on temperature and frequency can be explained by RF magnetic flux trapped within intrinsic defects.>