Magnetic Shielding by Superconducting Niobium Tin Plates

Abstract

This work experimentally explores the effectiveness of magnetic shielding between superconducting Nb₃Sn plates at 4.2°K in transverse magnetic fields up to 19.6 kG. At 19.6 kG the field exclusion obtained in the center between two 3.0×2.5×0.3‐cm thick plates, separated from each other by 1.0 cm, was 17.9 kG. It has been found that flux jumps not only degrade the shielding capability of the plates but also give rise to nonsymmetrical field distributions. A relationship has been established between the applied field B₁ and its critical rate of increase (dB₁/dt)_c, below which flux jumps do not occur for pairs of plates of various thicknesses. Flux jumps have been completely eliminated by keeping dB₁/dt<(dB₁/dt)_c. Bean's model of current penetration into a high‐field superconductor and the assumption of induced rectangular current loops have been successfully applied in calculating field distributions between the plates at medium and high applied fields. This model is not as successful at the early stages of plate magnetization where the current penetration is a small fraction of the total depth of the superconductor and the surface currents are appreciable. It has been found that the condition of field reversal external to the shielding volume can be minimized by the use of shunt iron in close proximity to the superconducting plates. Such a feature is desirable for the application of superconductor plate shields to a deflector system for a high‐intensity variable‐energy cyclotron.