Analysis of the medium field Q-slope in superconducting cavities made of bulk niobium

The quality factor of superconducting radio-frequency cavities made of high purity, bulk niobium decreases with rf field in the medium field range (peak surface magnetic field between 20 and about 100 mT). The causes for this effect are not clear yet. The dependence of the surface resistance on the peak surface magnetic field is typically linear and quadratic. This contribution will present an analysis of the medium field Q-slope data measured on cavities at different frequencies treated with buffered chemical polishing (BCP) at Jefferson Lab, as function of different treatments such as post-purification and low-temperature baking. The data have been compared with a model involving a combination of heating and of hysteresis losses due to “strong-links” formed on the niobium surface during oxidation.