The Optimization of Jc, Birr, n value, Wires Diameter, and Research of the Dominant Pinning Mechanism for Applications of Undoped MgB2 in Superconducting Multi-section Coils

This article discusses the influence of MgB2 wire diameter on critical parameters, dominant pinning mechanism, and n value. Studies were conducted on multifilament MgB2 wires of the 0.84 mm and 0.64 mm diameter. The wires were annealed under 1 GPa isostatic pressure, between 650 and 730 °C. Studies show that the best results (Jc, Birr, n value, and point pinning mechanism) for a 0.64-mm wire were obtained with annealing temperature of 650 °C. However, the best results for wires with a 0.84-mm diameter were obtained with annealing temperature of 730 °C. This research was conducted for exploring the use of MgB2 wires in multi-section superconducting coils, e.g., main coils generating magnetic fields and compensation coils—increasing the homogeneity of the magnetic field. Our research indicates that the main coil made with larger diameter wires should be heated at higher temperature (730 °C). In contrast, compensating coils with smaller diameter wires should be heated at a lower temperature (650 °C).