Cause of quenching in epoxy impregnated superconducting coils

This paper describes the experimental studies of premature quenching in epoxy impregnated superconducting coils for magnetically levitated trains. It was found that quenching was easily caused by the friction between a coil surface and a spacer. Quenching was induced by the relative displacement of about 10 μm. The insertion of a Kapton film between a coil surface and a spacer improved the resistance against the frictional quenching. It was also experimentally proved that a thermal insulation layer (glass fiber epoxy) stuck on the surface of a coil was effective to prevent the frictional quenching. The effect of a coil deformation and a mechanical stress on quenching was experimentally studied and quantitatively discussed. The limit of a mechanical stress in a winding to induce quenching was found. A full-scale racetrack coil was fabricated on the basis of these test results. The full-scale coil was successfully operated.