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

TL;DR

This study demonstrates that melt cast Bi-2212 hollow cylinders can effectively shield magnetic fields at low temperatures, outperforming Bi-2223, and characterizes their E(J) properties across a wide electric field range.

Contribution

The paper provides the first detailed comparison of Bi-2212 and Bi-2223 shielding capabilities and introduces a comprehensive E(J) law covering eight orders of magnitude.

Findings

Bi-2212 cylinders can shield magnetic flux densities up to 1 T at 10 K.

Bi-2212 outperforms Bi-2223 in magnetic shielding at similar conditions.

The E(J) law for Bi-2212 spans eight orders of magnitude of electric field.

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, 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.