Very powerful RF cavities are now being developed for future large-scale particle accelerators from high purity sheet niobium (Nb) superconductor. Today’s advanced prototype resonator cavities operate in peak RF surface magnetic fields of up to 180 mT at quality factors Q > 1010. This is the result of a successful worldwide technology development effort over the last decades.
The basic model for Q-slope in SRF cavities, i.e. the reduction of the cavity quality factor with increasing operating electric and magnetic fields, is the so-called thermal feedback model (TFBM). The exponential dependence of the BCS surface resistance Rs(T) of the superconductor on the temperature T provides a positive feedback with the RF power dissipation ultimately leading to thermal runaway (thermal quench) of the RF exposed surface. Most important for the agreement between the model and experimental data, however, is which different surface resistance contributions are included in the TFBM calculation. This paper presents an attempt to further clarify if the non-linear pair-breaking correction to the BCS resistance [1,2] is among those essential surface resistance contributions, through a comparison of TFBM calculations with experimental data from bulk Nb cavities. The discussion encompasses a wide variety of cavities from DESY, CEA-Saclay, J-Lab, Cornell University and Fermilab.
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