All-superconducting rotating machines have potentials for meeting the needs of high-power density and high efficiency for electrical aircraft application. However, very high AC loss in superconducting armature windings where superconductors carry AC current and exposed AC magnetic field, hinders its development. Hence, reducing AC loss in superconducting armature windings is one of critical tasks for the application. Considering the achievable filament size of REBCO coated conductors (CC) through laser ablation, critical current degradation due to the laser ablation process, and difficulty in twisting the filaments due to their flat shape, REBCO CCs may not be a suitable conductor choice for the application. On the other hand, multifilamentary MgB₂ wires with filament size lower than 10 mm, and small twist pitches are one of the promising candidates for the stator windings. Therefore, estimating AC loss in multifilamentary MgB₂ wires becomes an urgent task for the aircraft application. As first step, the dependence of critical current on magnetic field and operating temperatures of MgB₂ wires, I_c (B, T), needs to be measured, because it will be a basic input parameter for AC loss simulation.

In this work, we present J_c (B, T), J_e(B, T) measurements in three types of multifilamentary MgB₂ wires. The two non-magnetic low AC loss wires (MgB₂/Nb/CuNi/CuZn) are compared to commercial weakly magnetic sheaths (MgB₂/Nb/Cu/Monel) DC MgB₂ wires. All the MgB₂ wires were manufactured by Hyper Tech Research Inc, USA. A four-probe direct current method is used to measure I_c of the MgB₂ wires at temperatures ranging from 15 K to 35 K with magnetic fields up to 8 T using the SuperCurrent system in Robinson Research Institute, Victoria University of Wellington. The measured J_c (B, T), J_e (B, T), curves and n-values of the MgB₂ wires are compared.

Keywords: AC loss, Ic measurement , MgB2, all-superconducting rotating machines