Magnetic field leakage from a 45-degree-angle magnetic shell and its reduction - Toward a high-performance magnetic shield

摘要

A cylindrical magnetic shield having a large transverse shielding factor (TSF) (≈40,000) has been built in a rugged form for heavy duties or to avoid any unexpected damage to delicate magnetic shells. In this case, magnetic shells made of thin amorphous tapes (5cm in width) and a toroidally wound shaking coil were composited with fiber-reinforced plastic (FRP) [1]. It has been known that a large TSF can be best obtained by using helically wound (wound in close to 90 degree angle to the axis of the cylinder) shells and that a large axial shielding factor (ASF) can be best obtained by using axial shells (wound in 0 degree angle to the axis). A practical problem for combining these two shells to operate with a single shaking coil is that optimum shaking current for the axial shell is ten times larger than that for the helical one [2]. One way to avoid this difficulty is to use 45-degree-angle shells (wound in 45 degree angle to the axis) in place of axial ones, because the required shaking current should be larger by a factor of only 1.3～2. However, a known problem with this shell is that a circumferential shaking magnetic field (preferably 400Hz～1kHz) is partly guided along its length direction due to the shape magnetic anisotropy of the amorphous magnetic tape, resulting in a field leakage from edges near open ends of the cylinder.