Measurements of the magnetic noise spectra in HTS cylinders for magnetic shielding

Idealized magnetic shielded vessels can be realized by use of high-critical temperature superconductors (HTS). It is, however, generally, very difficult to evaluate the limit of the value of magnetic shielding and the fluctuation of the magnetic flux within the HTS vessel. In this present research, the magnetic noise power spectra (NPS) within the Bi-Pb-Sr-Ca-Cu-O (BPSCCO) cylinder was measured using a HTS dc-SQUID, under an application of a magnetic field B/sub cx/ less than the value of B/sub s/ (the maximum shielded magnetic flux density, 3.2/spl times/10/sup -4/ T at 77 A K). The value of NPS was determined as 5.0/spl times/10/sup -6//spl phi/o/sup 2//Hz at 1 kHz excluding B/sub cx/ which was the background noise of the SQUID control system, and 1.0/spl times/10/sup -5//spl phi/o/sup 2//Hz at 1 kHz for a B/sub cx/ of 3.0/spl times/10/sup -4/T. The value of NPS at 1 kHz increased linearly with values of B/sub cx/ in the region of 0.0 T to 3.0/spl times/10/sup -4/ T. Also shown are the important criteria for the design of effective shielded vessels for low magnetic measurements.