Large shielding factor obtained by a multiple-shell magnetic shield having separate magnetic shaking

Large shielding factors obtained by magnetic shaking techniques are reported. A cylindrical magnetic shield with both ends open (length=120 cm, inner diameter=52 cm) was constructed as a one-half model of an actual size. Metglas 2705M amorphous tapes of 5 cm in width and about 22 μm in thickness were wound helically on the outer surface of a tube (outer diameter=52 cm) to form a thin multiple-shell structure of five shells. The outer three shells, each made of 4 layers of tapes, were subjected to a common magnetic shaking (200 Hz). The fourth shell made of 8 layers was subjected to a different magnetic shaking (536 Hz). The innermost one was a passive shell. The shielding factor in the center of the shield measured for a transversely applied magnetic field (100 mG) showed 1/f-like behavior and extremely large values of 90 900 at 1 Hz and 21 000 at 10 Hz when the opening compensation was on. The shielding factor was much lower and constant at the level of 7000 at frequencies of less than 10 Hz when opening compensation was off. The residual dc magnetic field at the center was as small as 5 μG. It is shown by the experiments that a multiple-shell structure with even narrow spacings exhibits large shielding factors when a magnetic shaking technique is applied. It is also shown that the opening compensation technique is a key to building an open-structure magnetic shield with a large shielding factor.