Fundamental Characteristics of a Large-Scale Cylindrical Magnetic Shield with Shaking.

A large-scale cylindrical magnetic shielding case was developed. It was made of Permalloy (outer diameter = 68 cm. length = 180 cm. thickness = 1.6 mm, weigt≈60Kg)with an opening at one end and Metglas® 2705M amorphous ribbons (width=5 cm, thickness ≈ 22 μm), which were wound spirally around the Permalloy case in 16 layers (total weight = 12.1 kg) to cover the surface. A toroidal coil (36 turns) was wound around the amorphous layers to allow the creation of a magnetic shaking field. An external magnetic field was applied to the shielding case by means of a large rectangular Helmholtz coil, to evaluate the shielding factor. This was about 2700 at a frequency of 0.5 Hz, when the shaking frequency was 200 Hz. At higher frequencies, it gradually dropped. The shielding factor was almost constant relative to the amplitude of the disturbing field down to 3 mG; beyond that point, a residual field could not be detected because of the limit of resolution of the flux gate magnetometers used. The leakage of the shaking field was about 30,μG at the center of the shielding case.