AC losses in monofilamentary MgB2 round wire carrying alternating transport currents

Abstract

AC losses in a monofilamentaryMgB2 round wire with niobium and copper metal sheaths and carrying alternating transportcurrents are evaluated at several temperatures and frequencies. First, the transport currentlosses are observed electrically using a lock-in amplifier. Experimental results show that theAC losses decrease with an increase in the temperature if the amplitude of the transportcurrent normalized by the corresponding critical current is maintained constant. On theother hand, the AC losses increase slightly with the frequency. Next, the AC losses arecalculated numerically by a finite difference method. The numerical results for thesuperconductor filament show a good agreement with the results of the conventionaltheoretical expression formulated using the Bean model over a wide range of currentamplitudes. It is also found that the AC losses in the niobium sheath are negligible whereasthose in the copper sheath are comparable with those in the superconductor. Onthe basis of the numerical calculations, an expression is analytically derived forestimating the eddy current loss occurring in a metal sheath. The derived expressionwell reproduces the AC loss properties of both the copper and niobium sheaths.