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

Abstract

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, B lim ; 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, B lim is evaluated to 1 T at T = 10 K. This value largely exceeds the B lim value measured at the same temperature on similar tubes of Bi-2223. The second shielding property that is characterized is the dependence of B lim with respect to variations of the sweep rate of the applied field, dB app /dt. This dependence is interpreted in terms of the power law E = E c (J/J c ) 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.

Magnetic Shielding Above 1 T at 20 K With Bulk, Large Grain YBCO Tubes Made by Buffer-Aided Top Seeded Melt Growth

“…The value is smaller than that expected for single-grained YBCO samples in the form of disk pellets (of the order of 10 4 A/cm²), which can be ascribed to the fact that some parts of the tube are not perfectly textured especially those far away from the seed. This Jc value, however, is much higher than that obtained previously in polycrystalline YBCO [36] or BSCCO hollow cylinders (of the order of 350 A/cm² at 77 K and 20 mT [8,19]). Knowing Jc, the threshold induction Blim for a four times longer tube is estimated to be 47 mT, a value similar to the Blim = 51 mT measured at the center of the closed tube.…”

“…These values are comparable to those achieved with using Nb-Ti multifilamentary composite wires at the same temperature [18]. At T = 10 K, bulk Bi2Sr2CaCu2O8 (BSCCO-2212) tubes can be used for shielding fields up to 1 T [19]. Solenoid magnets made of bulk melt-textured YBCO prepared by various techniques [20][21][22][23][24][25][26] show significant potential for magnetic shielding.…”

Magnetic shielding properties of a superconducting hollow cylinder containing slits: modelling and experiment

“…The equations have to be solved with the boundary (Dirichlet) conditions at infinity: 0, = A (9) and 0, = φ (10) that have been implemented by using a Jacobian transformation for sending the outer surface of a spherical shell to infinity [38]. The geometry studied in the present work is a hollow cylinder of finite height with two slits made in a vertical plane along a diameter.…”

“…The present work deals with enclosures made of bulk high temperature superconductors (HTS). They can be used either at liquid nitrogen temperature (77 K) -at which magnetic shielding occurs up to a few tens of mT -or at low temperature (10 K and below) where shielding of magnetic fields in excess of 1 tesla was demonstrated recently [8,9]. Several materials are candidates for efficient magnetic screens, including MgB 2 [8,10], YBa 2 Cu 3 O 7 [3,11] or Bi-based compounds [12][13][14][15].…”

Magnetic shielding of an inhomogeneous magnetic field source by a bulk superconducting tube

“…HTS materials can be used either at liquid nitrogen temperature, where shielding remains effective up to a few mT, or at lower temperatures (e.g. typically 10 K or lower), where shielding of magnetic inductions in excess of 1 tesla has been successfully demonstrated using hollow cylinders made of Bi-2212 [12] or MgB2 [13].…”