Large-volume conventional magnetic shields

Abstract

Increasing demand for low-field regions of large volumes has resulted in the construction of many magnetically-shielded walk-in rooms. Most have been of the cubic, two-layer high-μ type, but several have used three layers, including one built by this author about three years ago. This room used several innovations, including "shaking" to increase the permeability of the magnetic sheets. The shielding characteristics and performance of this room were measured in some detail, and the results of these measurements will be discussed. These results indicate the improvements in room shielding which could profitably be made if one wished to push to the shielding limit for conventional (non-superconducting) walk-in shields. At MIT, we decided to build such a limiting low-field room in which we hope eventually to reach a B-vector amplitude of 1 × 10-8 gauss (rms, including dc) at the room center. This room is now under construction (2) and will be a national facility. It is roughly spherical in shape, with 26 sides, and has three layers of high-μ sheets and two layers of pure aluminium. The inner and outer diameters are 2.5 meters and 4.0 meters. The concepts and design details of this room will be discussed, along with the planned uses of this room for the next several years