Abstract
AC transport current loss (self field loss) of ex-situ-processed <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ MgB_2$</tex> superconducting tapes with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ Ni$</tex> sheath was measured under different conditions of transport current value, transport current frequency, and temperature. Experimental results were compared with Norris' theoretical curves and previous measured losses of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ MgB_2$</tex> tapes with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ CuNi$</tex> and stainless-steel sheaths, which were larger than the theoretical ones and previous data. The loss vs. normalized transport current (transport current normalized by critical current) curves varied with temperature and the loss at 9 K was larger than that at 14 K. The measured loss approached Norris' curves in the high normalized transport current region. In order to gain insight into these origins, FEM analysis of magnetic flux concentration by <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ Ni$</tex> and estimations of hysteresis loss of ferromagnetic nickel-sheath and of homogeneity of the core were carried out. As a result, the hysteresis loss of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ Ni$</tex> was very small based on the estimation and the loss measurement of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ Ni$</tex> . Main contributions to large loss are considered to be self-field enhancement due to magnetic flux concentration by the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ Ni$</tex> -sheath and critical current density distribution of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ MgB_2$</tex> core caused by the inhomogeneity of critical current density and critical temperature.
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