Strain in YBCO Double-Pancake Coil With Stainless Steel Overband Under External Magnetic Field

This paper deals with the mechanical strain issue in a high-temperature superconducting (HTS) insert for a GHz-class (
$>$
23.5 T) LTS/HTS NMR magnet. We present results, experimental and analytical, of hoop strains in a double-pancake (DP) test coil, wound with 6-mm wide YBCO coated conductor (CC) and equipped with strain gauges at their innermost and outermost turns. To keep the YBCO CC to within a 95%
$-Ic$
retention, the conductor tensile strain must be limited to 0.6%. To satisfy this strain limit in our test DP coil, we wrapped 0.08-mm thick, 6-mm wide stainless steel strip over its outermost turn of an 4.8-mm overband radial build deemed sufficient by our stress analysis based on force equilibrium and generalized Hooke's law with plane stress approximation. A control test DP coil, actually the same test DP coil, without overbanding, was run under the same experimental condition. In each case the test DP coil was energized up to 350 A at 4.2 K in a background field of 4 T. We report the experiment and analysis, with discussion on the merit of overbanding as a means to limit hoop strain in high-field HTS inserts.