Measurements of the low-temperature rf surface resistance of lead at frequencies from 136 to 472 MHz

Abstract

A helically loaded lead-plated cavity has been used to measure the superconducting rf surface resistance of lead at low field levels at frequencies from 136 to 472 MHz and temperatures from 1.5 to 4.2 K. Fits with the BCS theory were found to be consistent with a normal electron mean free path between 350 and 6000 \AA{}. The residual resistance at 136.7 MHz was found to be 4.7 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}9}$ \ensuremath{\Omega}, to vary as ${f}^{1.23\ifmmode\pm\else\textpm\fi{}0.05}$ over the frequency range 136-472 MHz, and to be independent of temperature. Measurements of magnetic field trapping by a lead-plated cavity were also made. The results indicated that the cavity, rather than exhibiting a macroscopic Meissner effect, trapped even weak magnetic fields without significant modification of their magnitude or spatial distribution when the cavity surface was cooled below the critical temperature. Further observations above the critical temperature of lead indicated the presence of transient magnetic fields within the cavity when the cavity was subjected to rapid temperature changes. These magnetic fields are attributed to thermoelectric effects. The magnitude and frequency dependence of the observed residual resistance is compared to that expected from trapped flux, phonon generation, and dielectric losses according to several appropriate theories. No theoretical model predicts the observed frequency dependence. The measured magnitude is in agreement with that expected according to a phonon generation theory due to Passow, and is also in agreement with that expected from a trapped magnetic flux of about 12 mG according to a model due to Pierce. Trapped flux is considered to be the most likely source of the residual resistance measured in this experiment.