Electromagnetic coupling in small grain YBa₂Cu₃O_7-δ superconducting ceramics: Full and hollow cylinders

This paper describes a method to assess the electromagnetic characteristics of a superconducting ceramic material. YBa₂Cu₃O_7-d ceramic samples have been made in the form of hollow cylinders with the standard 4 terminal configuration for measurement of the critical current (I_c) attached to the outer surface. An additional wire carries a second internal current (I_int) along the central axis. I_c can thus be modulated by I_int. In particular, an increase in I_c can be achieved by directing I_int in the opposite direction to the cylinder current in order to compensate the self-generated magnetic field. The I_c-I_int characteristic can be described by a critical state model where we assume a linear dependence between the local critical current density (j_c) and the local magnetic field (H) for the superconducting polycrystalline material. Based on this linear model the experimental data for a given sample can be analyzed to extract two parameters: the maximum local critical current density (j₀) and the local magnetic field (H₀) for which j_c should vanish. The best sample yielded j₀ = 1023 A cm^-2 and H₀ = 4450 A m^-1.