Development of a large-scale vertical setting cylindrical magnetic shield - Realization of low-level, low-gradient magnetic field with current superposition

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(1) We designed a large-scale vertical open-structure cylindrical magnetic shield and constructed it employing magnetic shaking. The shield developed consists of four concentric magnetic shells positioned on the outer surfaces of paper pipes of 〜 270 cm length, 〜 1 cm thickness, and with outer diameters of 67, 72, 82, and 97 cm, respectively. The first (innermost) shell is a passive Permalloy shell of 2.1 mm thickness and 180 cm length. The second, third and fourth shells are made of 〜 5 cm wide and 〜 22 μm thick Metglas 2705M amorphous ribbons. The second shell is a 220 cm long helical structure of 48 layers of the amorphous ribbons. The third shell, 243 cm in length and fourth shell, 270 cm in length, are axial structure consisting of 26 and 30 layers, respectively. The gross weight of the sheild is 〜 400 kg including 〜 68 kg of Permalloy and 〜 110 kg of Metglas ribbons. An 〜 10ィイD15ィエD1 transverse shielding factor and a relatively large 〜 380 axial shielding factor, despite of the e … More ffect of openings, are achieved for a 10 μT external magnetic field in the extremely low frequency region. The shaking leakage measured by a fluxgate at the shield's center is less than 1 nT and the level of magnetic noise field measured by a SQUID magnetometer at the same position is less than 1 pT.(2) The optimum geometry of the open-ended cylindrical magnetic shield is presented for the maximum axial shielding factor and for the maximally flat distribution of the magnetic field inside the shield. With the geometrical parameter including the diameter, D, the length, L, and the thickness, t, and with the material parameter, the permeability μ, a maximum shielding factor for the axial magnetic field is obtained when L/D = 1 + log (μt/D), and a maximally flat distribution is obtained when L/D = 1 + log (μt/D) + C where C = 0.75 〜 1.0.(3) The effect of magnetic anisotropy on the shaking enhancement is made clear. The magnetic field to be shielded should be parallel to the easy axis of magnetization, whereas shaking magnetic field does not have such preference in direction. However, the amplitude of the shaking magnetic field required is ten times larger when the shaking magnetic field is applied perpendicularly to the easy axis than that required when applied in parallel. Less