Evidence for non-linear BCS resistance in SRF cavities

Very powerful RF cavities are now being developed for future large-scale particle accelerators such as the International Linear Collider (ILC). The basic model for the cavity quality factor Q-slope in high gradient SRF cavities, i.e. the reduction of Q with increasing operating electric and magnetic fields, is the so-called thermal feedback model (TFBM). Most important for the agreement between the model and experimental data, however, is which different surface resistance contributions are included in the TFBM. This paper attempts to further clarify if the non-linear pair-breaking correction to the BCS resistance [A. Gurevich, in: Pushing the Limits of RF Superconductivity Workshop, ANL, September 2004; A. Gurevich, This conference.] is among those surface resistance contributions, through a comparison of TFBM calculations with experimental data from bulk Nb cavities built and tested at several different laboratories.