TEMPERATURE DEPENDENCE OF MAGNETIC FIELDS TRAPPED AXIALLY INSIDE SUPERCONDUCTING SOLENOIDS.

Experimentation on the thermal and magnetic properties of superconducting solenoids in magnetic fields is reported. This includes measurements on the temperature dependence of the intensity of axial magnetic fields trapped in the center bore of the solenoids. Solenoids wound of niobium, niobium-titanium and niobium-zirconium have been investigated. The major contribution to the temperature dependence of field intensity is attributed to penetration-depth changes in combination with a flux-pinning effect. As the author reported for the case of superconducting tubes (ibid. , vol. MAG-21, no. 2, pp. 606-609, 1985), anamolously large values of penetration depth have been determined for the wire samples. However, wider deviations from the Gorter-Casimir temperature dependence have been observed for the solenoids than for the tubes. It is believed that these deviations arise from material inhomogeneities and flux pinning. Wire diameter appears to be a critical parameter with the temperature dependence increasing as the diameter decreases. By means of a thermal switch, the effect of flux pinning has been quantified. Both reversible and irreversible flux-pinning effects have been observed.