Basic Behavior of the Contact Resistivity of an Intra-Layer No-Insulation (LNI) REBCO Coil

Abstract

The intra-layer no-insulation (LNI) method is effective for protecting a layer-wound REBCO coil from a quench. The most influential parameter is the electrical contact resistivity <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ρ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ct</sub> between the conductors and the copper sheets for current bypassing inside the winding. In previous work, an LNI-REBCO coil was protected from a 31.4 T quench, owing to a high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ρ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ct</sub> value of 10,000 μΩcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Thus, achieving the desired <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ρ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ct</sub> is of great importance for designing and fabricating an LNI-REBCO coil. In the present work, for obtaining basic knowledge on the behavior of the value of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ρ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ct</sub> , we made a series of experiments; contact model experiment and coil experiment. The comparison between the experimental results in combination with a structural analysis suggests that the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ρ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ct</sub> value of a coil does not follow a general electrical contact theory dominated by surface pressure; it is substantially affected by layer-to-layer contact conditions, resulting in being sensitive to external effects such as thermal cycling. The knowledge obtained in this work suggests that a solution to “freeze” the contact condition is required to control the value of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ρ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ct</sub> .