Magnetic shielding effects for the superposition of multi-layered soft-iron cylinders over a BPSCCO cylinder: relationship between the shielding effect and the layer number of the soft-iron cylinder

摘要

For practical use, it is often necessary to improve the value of the maximum shielded magnetic flux density B_s, of high-critica temperature superconductor (HTS) vessels. Accordingly, the present authors have improved the value of B_s for a Bi-Pb-Sr-Ca-Cu-O (BPSCCO) cylinder used as the shielding vessel, by the superposition of multi-layered soft-iron cylinder over the BPSCCO cylinder. The behavior of the magnetic flux density B_(in) within the superimposed cylinder, however, for an applied excitation magnetic flux density B_(ex) having values less than that of B_s, has previously been unknown. To rectify this, the present research examines and clarifies the magnetic behavior at the center of the superimposed cylinder, measured with the use of an HTS dc-SQUID magnetometer and spectrum analyzer. Experimental results reveal several characteristics of the magnetic shielding within the superimposed cylinder, including the effects of a region B_(BF) where the values of B_(in) are not dependent on the applied B_(ex), but are dependent on the layer number n of the soft-iron cylinder, and the effect of these independent regions B_(NP) of the magnetic noise power on the value of B_(ex). The results indicate an important criterion that is determined for use in the design of large-sized vessels with an effective magnetic shield for practical applications.