Field dependence of the critical current density of MgB2 conductors between 4.2 K and 30 K up to 20 T

The critical current density (J_c) of the monofilament Monel/Fe/MgB₂ wire was measured between 4.2 K and 30 K with a Variable Temperature Insert (VTI) in a 20 T superconducting magnet using the analog DC pulse four-wire method with 1 μv/cm criterion. The critical current density, J_c was measured to be 0.16×10⁵ A/cm² at 4 T at 21 K for a MgB₂ sample which was annealed at 700°C for 5 min and at 900°C for 5 min under Ar gas flow. Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) analysis were used to characterize the microstructure of the samples.