Method for expanding the uniformly shielded area in a short-length open-ended cylindrical magnetic shield

A compensation method is proposed by which the uniformly shielded area of the axial magnetic field in a relatively short, open-structure axial magnetic shield can be extended. An open-ended cylindrical magnetic shield of 120 cm in length, 52 cm inner diameter, and a ;0.5 mm total thickness of the shielding material is used to demonstrate the idea. The shield axis is oriented along the horizontal component ~;320 mG! of the Earth’s magnetic field. A simple way to increase the axial shielding factor is to use a pair of compensating coaxial ring coils set at both open ends of the shield. This increases, however, the radial gradient of the shielded field since the axial compensation field is stronger towards the shield axis. In order to decrease the radial gradient, an additional ring coil is wound around the middle part of the outer surface of the shield. The compensating field generated by this central ring coil is stronger towards the inner surface of the shield, and it helps, therefore, to unify the axial resultant field over a wider area inside the shield. The axial shielding factor obtained with this compensation according to the proposed method is 128, in contrast to only 16.4 obtained with compensation by a set of two ring coils. The field gradients observed are 1.2 mG/cm along the length direction and 2.7 mG/cm along the radial direction, in contrast to the 14 mG/cm axial and 78 mG/cm radial gradients obtained with compensation by a set of two ring coils. © 1999 American Institute of Physics. @S0021-8979~99!55508-4#