Dependence of trapped-flux-induced surface resistance of a large-grain Nb superconducting radio-frequency cavity on spatial temperature gradient during cooldown through T c

Recent studies by Romanenko et al. revealed that cooling down a superconducting cavity under a large spatial temperature gradient decreases the amount of trapped flux and leads to reduction of the residual surface resistance. In the present paper, the flux expulsion ratio and the trapped-flux-induced surface resistance of a large-grain cavity cooled down under a spatial temperature gradient up to 80 K/m are studied under various applied magnetic fields from 5 to 20 mu T. We show the flux expulsion ratio improves as the spatial temperature gradient increases, independent of the applied magnetic field: our results support and enforce the previous studies. We then analyze all rf measurement results obtained under different applied magnetic fields together by plotting the trapped-flux-induced surface resistance normalized by the applied magnetic field as a function of the spatial temperature gradient. All the data can be fitted by a single curve, which defines an empirical formula for the trapped-flux-induced surface resistance as a function of the spatial temperature gradient and applied magnetic field. The formula can fit not only the present results but also those obtained by Romanenko et al. previously. The sensitivity r(fl) of surface resistance from trapped magnetic flux of fine-grain and large-grain niobium cavities and the origin of dT/ds dependence of R-fl/B-a are also discussed.