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 magnetscan also act as efficient magnetic shields. The magnetic screening propertiesunder an axial DC magnetic field are characterized at several temperaturesbelow the liquid nitrogen temperature (77 K). Two main shielding properties arestudied and compared with those of Bi-2223, a material that has been consideredin the past for bulk magnetic shields. The first property is related to themaximum magnetic flux density that can be screened, Blim; it is defined as theapplied magnetic flux density below which the field attenuation measured at thecentre of the shield exceeds 1000. For a cylinder of Bi-2212 with a wallthickness of 5 mm and a large ratio of length over radius, Blim is evaluated to1 T at T = 10 K. This value largely exceeds the Blim value measured at the sametemperature on similar tubes of Bi-2223. The second shielding property that ischaracterized is the dependence of Blim with respect to variations of the sweeprate of the applied field, dBapp/dt. This dependence is interpreted in terms ofthe power law E = Ec(J/Jc)^n and allows us to determine the exponent n of thisE(J) characteristics for Bi-2212. The characterization of the magnetic fieldrelaxation involves very small values of the electric field. This gives us theopportunity to experimentally determine the E(J) law in an unexplored region ofsmall electric fields. Combining these results with transport and AC shieldingmeasurements, we construct a piecewise E(J) law that spans over 8 orders ofmagnitude of the electric field.