Design of a large-scale vertical open-structure cylindrical shield employing magnetic shaking

The shield developed consists of four concentric magnetic shells positioned on the outer surfaces of paper pipes of similar to 2.7 m length, similar t o 1 cm thickness, and with outer diameters of 67, 72, 82.2, and 97.4 cm, re spectively. The first (innermost) shell is a Permalloy shell of 2.1 mm thic kness and 1.8 m length. The second, third, and fourth shells are made of si milar to 50 mm wide, similar to 22 mu m thick Metglas 2705M amorphous ribbo ns. The second shell, which is a 2.2 m long helical structure, consists of 48 layers of Metglas ribbon divided into four equal sections by similar to 1 cm thick flexible Styrofoam sheets. The third shell, 2.43 m in length, an d fourth shell, 2.7 m in length, consist of 26 and 30 layers, respectively. A thin polyethylene film is tightly wound on each section of the second sh ell as well as on the third and fourth shells. It increases the friction be tween the Metglas ribbons and prevents them from sliding down; there is no foreign material in between the layers of the ribbon. All shells are enclos ed by toroidal coils which are used to demagnetize the Permalloy shell and to apply magnetic shaking to the amorphous magnetic shells. The gross weigh t of the shield is similar to 400 kg including similar to 65 kg of Permallo y and similar to 110 kg of Metglas. An similar to 10(5) transverse shieldin g factor and a relatively large similar to 380 axial shielding factor, desp ite the effect of the openings, are achieved for a 10 mu T external field i n the extremely low frequency region. The measured shaking leakage and magn etic noise field strengths at the shield's center are less than 1 nT. As th ese low field strengths, it is possible to operate highly sensitive SQUID m agnetometers for biomagnetic measurements. (C) 2000 American Institute of P hysics. [S0021-8979(00)82108-8].