Low field critical current density of titanium sheathed magnesium diboride wires

Magnesium diboride (MgB₂) is replacing some of the conventional superconductors due to its low cost and availability in kilometer lengths. MgB₂ has also been considered for AC applications. In order to model the AC losses and the critical currents of the applications, intrinsic J_c(B)-dependence is an important factor also at low fields. In this work J_c(B)-dependence of an MgB₂ sample is extracted from the standard in field voltage-current measurements. The proposed method is applied to a non magnetic titanium sheathed sample at 16 and 20 K and a simple formula for J_c(B) aligns with the measurements. In the fitting process, the critical current distribution inside the wire is numerically simulated in order to take the self field of the sample into account. Moreover, the same formula aligns with measurements of a different sample. These critical current measurements, performed at 4.2 K, were based on magnetization. In the self field computations, the superconducting cross section must be determined accurately. Therefore, we tailored an image processing tool for MgB₂ wires to obtain the geometry from a photograph.