Large-volume conventional magnetic shields

2014 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-03BC 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-03BC 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. REVUE DE PHYSIQUE APPLIQUÉE TOME 5, FÉVRIER 1970, PAGE 1 will divide this talk into three parts. Part I will briefly deal with some general ideas and concepts of magnetic shields; part II will deal with some details ofboth the theory and practice of large-volume shields; part III will be a description of the shielded room now under construction at the Francis Bitter National Magnet Laboratory. I. General Ideas and Concepts. Most of us know that there is an increasing demand for volumes or regions which are shielded against both de and ac magnetic fields. By magnet fields we here mean the B-vector. (At high frequencies the E-vector always accompanies the B-vector ; hence, magnetic shielding and electrostatic shielding are one and the same thing. At very low frequencies these two vectors can exist almost independently and are shielded separately.) This increasing demand is due to magnetometer testing, checking demagnetization of spacecraft components, (1) Supported by the U.S. Air Force Office of Scientific Research. (2) Partially supported by the U.S. Office of Naval Research. measurements on geophysical samples, extremely sensitive physics experiments involving general relativity, measurements of magnetic fields from natural ion currents in humans, and other needs in the scientific and engineering community. Now without using superconductivity there are three phenomena which can be used to produce these regions of low magnetic fields; these phenomena can be used separately or together. The first phenomena is ferromagnetism. The region of interest is partially or completely enclosed by one or more ferromagnetic shells; this type of shielding is effective in the frequency range from de to > 1 MHz. The second phenomenon is that of induced eddycurrents. The region of interest is partially or completely enclosed by one or more shells of good conductor; this type of shielding acts only on ac, and increases in efficiency with increasing frequency. The third phenomenon may be called "active shielding" and involves the production of fields from artificial currents; these fields act in a way which oppose or counterbalance the external fields. A magnetometer, usually 3-dimensional, at the position of best shielding provides amplifield output currents which are passed through coils Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:019700050105300