It is well known that the highest current densities in MgB 2 superconducting wires can be achieved using the internal magnesium diffusion (IMD) method. We present a square 19-filament MgB 2 wire made by the IMD process with dominant rolling deformation, which yields a high MgB 2 filling factor of 14.2% and, consequently, a high engineering current density. Non-insulated wind-and-react coils were made from 1.1 mm and 0.68 mm wire, and their critical currents were compared with corresponding short wire samples. The threshold engineering current density of the present 19-filament MgB 2 /Nb/CuNi wire , J et = 10 4 A cm -2 , was measured at an external field of 6.5 T and a temperature of 4.2 K, representing the best performance reported in the literature for undoped filamentary wires made by the IMD process. The presented results demonstrate progress in the quality of filamentary MgB 2 wires produced by the IMD process, which could be of interest for future DC coil applications.
Kováč et al. (Fri,) studied this question.