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

Abstract

We show that tubes of melt cast Bi-2212 used as current leads for LTS magnets can alsoact as efficient magnetic shields. The magnetic screening properties under an axial DCmagnetic field are characterized at several temperatures below the liquid nitrogentemperature (77 K). Two main shielding properties are studied and compared with those ofBi-2223, a material that has been considered in the past for bulk magnetic shields. The firstproperty is related to the maximum magnetic flux density that can be screened,Blim; it is defined as the applied magnetic flux density below which the field attenuation measuredat the centre of the shield exceeds 1000. For a cylinder of Bi-2212 with a wall thickness of5 mm and a large ratio of length over radius,Blim is evaluated to 1 Tat T = 10 K. This valuelargely exceeds the Blim value measured at the same temperature on similar tubes of Bi-2223.The second shielding property that is characterized is the dependence ofBlim with respect to variations of the sweep rate of the applied field,dBapp/dt. This dependence is interpreted in terms of the power lawE = Ec(J/Jc)n and allows us todetermine the exponent n of this E(J) characteristics for Bi-2212. The characterization of the magnetic field relaxation involves verysmall values of the electric field. This gives us the opportunity to experimentally determine theE(J) law in an unexplored region of small electric fields. Combining these resultswith transport and AC-shielding measurements, we construct a piecewiseE(J) law that spans over eight orders of magnitude of the electric field.