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Abstract

In particle accelerators superconducting RF cavities are widely used to achieve high accelerating gradients and low losses. Power consumption is proportional to the surface resistance R_S which depends on a number of external parameters, including frequency, temperature, magnetic and electric field. Presently, there is no widely accepted model describing the increase of R_S with applied field. In the frame of this project the 400MHz Quadrupole Resonator has been extended to 800 and 1200MHz to study surface resistance and intrinsic critical RF magnetic field of superconducting samples over a wide parameter range, establishing it as a world-wide unique test facility for superconducting materials. Different samples were studied and it was shown that R_S is mainly caused by the RF electric field in the case of strongly oxidized surfaces. This can be explained by interface tunnel exchange of electrons between the superconductor and localized states in adjacent oxides. For well prepared surfaces, however, the majority of the dissipation is caused by the magnetic field and R_S factorizes into field and temperature dependent parts. These different loss mechanisms were correlated to surface topography of the samples and distribution of oxides by using ultrasonic force microscopy and X-ray photon spectroscopy.