Shielding with superconductors in small magnetic fields

2014 The shielding properties of a superconducting lead shell have been investigated in fields of the order of 10-4 gauss. The small ambient fields were obtained with a room size 03BC-metal shield. Fields of less than 2 microgauss were obtained over a sphere of 7.5 cm diameter. This region is at room temperature and is suitable for magnetometer testing. REVUE DE PHYSIQUE APPLIQUÉE TOME 5, FÉVRIER 1970, PAGE The testing and calibration of magnetometers for space applications requires the development of field free regions. The Meissner effect in a superconducting shield in the form of a lead shell in a small ambient field ( 10-4 gauss) has resulted in a final static magnetic field of less than 2 X 10-6 gauss over a region of 7.5 cm diameter and 15 cm length. The Meissner effect arises in a superconductor such as lead due to a smaller free energy of the superconductor when the flux is expelled from the bulk material rather than being frozen in as would be the case for perfect conductivity alone [1] as shown in figure 1. The small initial fields of less than 100 microgauss were obtained with a room size double wall (J.-metal shield as shown in figure 2 [2]. A brief account of this work FIG. 1. Meissner effect in a superconducting sphere cooled in constant applied field. The lines of induction B are expelled from the sphere on passing below the transition temperature. has been given elsewhere [3]. A shield similar to that shown has recently been constructed in our laboratory [4]. The shield can be disassembled and moved if necessary. The wood battens supply mechanical pressure on the three inch wide pL-metal strips in order to make a magnetic contact between two adjacent panels. The inside room dimensions are those of a six foot cube. A partial vacuum of .2 PSI between the inner and outer door provides a stable magnetic contact. The ambient field of such a room immediately after assembly can be as large as 5,000 microgauss. This large ambient field can systematically be reduced either (1) Supported by National Aeronautics and Space Administration under NASW-6 and NASA Grant 44005-022. FiG. 2. y-metal shielded room. (1), Alum-handle; (2), 1" plywood with .060" y-mental and gasket supports; (3),1" plywood; (4), .060" [L-metal; (5), outside door support (wooden) with alum-guide for casters ; (6), wooden platform for inside box ; (7), fl-metal with 1/2"R ; (8), wooden battens with y-metal strips ; (9), f plywood, .060"" li-inetal; (10),1"plywood sides ; (11), aluminium brackets ; (12), outer box rests on wooden battens ; (13), wooden dowling ; (14), brass "T" nut. by moving tape degaussers over the walls or with a large A.C. demagnetizing coil. It has been found that a 25 ampere 60 cycle per second current in a 70 cm diameter 300 turn coil is sufficient to demagnetize the inside of the shield. The advantage of a two wall shield over a three wall shield is that the demagnetization is always possible with inside and outside coils. The variation of the ambient field with temperature at an ambient field of 100 microgauss is about 10 microgauss per OC per axis. This temperature coefficient requires that the surroundings be temperature controlled and the walls thermally insulated. A typical gradient of such a room at the center is .5 microgauss per cm over a 7.5 cm diameter sphere. The small gradient is the most important feature of a large shielded room. A reduced field at a point is always possible, but the attainment of a reduced field over a large region in small shields requires consideArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:019700050104900