Shielding efficiency and E(J) characteristics measured on large melt cast Bi-2212 hollow cylinders in axial magnetic fields

摘要

We show that tubes of melt cast Bi-2212 used as current leads for LTS magnets canalso act as efficient magnetic shields. The magnetic screening properties under an axialDC magnetic field are characterized at several temperatures below the liquid nitrogentemperature (77 K). Two main shielding properties are studied and compared withthose of Bi-2223, a material that has been considered in the past for bulk magneticshields. The first property is related to the maximum magnetic flux density that canbe screened, Blim; it is defined as the applied magnetic flux density below which thefield attenuation measured at the centre of the shield exceeds 1000. For a cylinder ofBi-2212 with a wall thickness of 5 mm and a large ratio of length over radius, Blim isevaluated to 1 T at T = 10 K. This value largely exceeds the Blim value measured atthe same temperature on similar tubes of Bi-2223. The second shielding property thatis characterized is the dependence of Blim with respect to variations of the sweep rate ofthe applied field, dBapp/dt. This dependence is interpreted in terms of the power lawE = Ec(J/Jc)^n and allows us to determine the exponent n of this E(J) characteristicsfor Bi-2212. The characterization of the magnetic field relaxation involves very smallvalues of the electric field. This gives us the opportunity to experimentally determinethe E(J) law in an unexplored region of small electric fields. Combining these resultswith transport and AC shielding measurements, we construct a piecewise E(J) lawthat spans over 8 orders of magnitude of the electric field.