Dependence of the residual surface resistance of superconducting RF cavities on the cooling dynamics around $T_\mathrm{c}$

摘要

We report a strong effect of the cooling dynamics through $T_\mathrm{c}$ onthe amount of trapped external magnetic flux in superconducting niobiumcavities. The effect is similar for fine grain and single crystal niobium andall surface treatments including electropolishing with and without 120$^\circ$Cbaking and nitrogen doping. Direct magnetic field measurements on the cavitywalls show that the effect stems from changes in the flux trapping efficiency:slow cooling leads to almost complete flux trapping and higher residualresistance while fast cooling leads to the much more efficient flux expulsionand lower residual resistance.