Flux pinning in superconducting Pb-Na

Abstract

Some of the reversible properties of type II superconductora are considered in tens of the Ginzberg-Landau equations. The discussion is extended to consider flux pinning in the bulk and on the surface in the mixed state.
The apparatus used to measure the a.c. flux penetration into a Pb-Na cylindrical single crystal is described together with the experimental techniques employed.
The measurements of other workers on the Pb-Na system are reviewed and the parameters for the specimen calculated.
Results obtained for the specimen as functions of the d.c. field, temperature and frequency of operation are presented. The bulk pinning in the peak region is analysed in tens of the Bean model and the characteristics compared with those of other workers on other bulk pinning materials. The initial penetration of the critical state is also discussed.
The surface pinning below the peak field is considered and a surface critical state model developed which gives good agreement with the results. End effects and the bulk viscous drag are considered and their effects on the model discussed. The characteristics are analysed as a function of the d.c. field and temperatureS.I. units are used throughout.