3D simulations of total AC loss in a twisted multifilamentary MgB₂ wire carrying AC currents under AC magnetic fields at 20 K

Abstract In the pursuit of enabling the application of all-superconducting rotating machines in electric aviation, high AC loss in the armature windings where superconductors carry AC currents and exposed to AC/rotating magnetic fields is a critical stumbling block. For lowering AC loss, multifilamentary magnesium diboride (MgB 2 ) wires with fine filaments and tight twist are one promising candidate for aviation applications. In this paper, 3D AC loss simulations of a 54-filament MgB 2 wire with a non-magnetic matrix at 20 K are carried out based on the H -formulation. The transport loss carrying AC current without external field, Q t0 , of 12-, 30- and 54-filament wires is firstly obtained, where the current amplitudes range from 20% to 90% of its self-field critical current I c0 . Then the magnetization loss exposed to field amplitudes up to 2 T without current, Q m0 , is presented, where the operational frequency, the twist pitch and resistivity of the matrix are varied to investigate their impacts on Q m0 and its three loss components (hysteresis loss Q h , coupling loss Q c and eddy current loss Q e ). Lastly, the total loss, Q total , of the 54-filament wire with various twist pitches and frequencies is compared, where the current amplitudes vary from 30% to 70% of I c0 and the field amplitudes are up to 2 T. All simulations use the measured J c ( B , 20 K) and n ( B , 20 K) data of the 54-filament wire. Simulation results show that, the use of the 5 mm twist pitch wire can significantly reduce Q m0 due to the decoupling of the filaments, where the simulated Q h matches well with the analytical hysteresis loss for a cylindrical superconductor multiplied by 54 (the number of filaments). With increasing twist pitch, the filaments become coupled, resulting in a greater increase in both Q c and Q h . Surprisingly, the simulated Q total values in the wires with different twist pitches agree well with the sum of Q m0 and Q t0 for all different current levels. This implies that Q total in an MgB 2 wire carrying an AC current exposed to an AC magnetic field can be accurately predicted by knowing Q m0 and Q t0 values which are more easily obtained.