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

We show that tubes of melt cast Bi-2212 used as current leads for LTS magnets can
also act as efficient magnetic shields. The magnetic screening properties under an axial
DC magnetic field are characterized at several temperatures below the liquid nitrogen
temperature (77 K). Two main shielding properties are studied and compared with
those of Bi-2223, a material that has been considered in the past for bulk magnetic
shields. The first property 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 measured at the centre of the shield exceeds 1000. For a cylinder of
Bi-2212 with a wall thickness of 5 mm and a large ratio of length over radius, Blim is
evaluated to 1 T at T = 10 K. This value largely 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 of Blim with respect to variations of the sweep rate of
the applied field, dBapp/dt. This dependence is interpreted in terms of the power law
E = Ec(J/Jc)^n and allows us to determine the exponent n of this E(J) characteristics
for Bi-2212. The characterization of the magnetic field relaxation involves very small
values of the electric field. This gives us the opportunity to experimentally determine
the E(J) law in an unexplored region of small electric fields. Combining these results
with transport and AC shielding measurements, we construct a piecewise E(J) law
that spans over 8 orders of magnitude of the electric field.