Test of an 8.66-T REBCO Insert Coil With Overbanding Radial Build for a 1.3-GHz LTS/HTS NMR Magnet

A 1.3-GHz/54-mm LTS/HTS NMR magnet, assembled with a three-coil (Coils 1-3) 800-MHz HTS insert in a 500-MHz LTS NMR magnet, is under construction. The innermost HTS insert Coil 1 has a stack of 26 no-insulation (NI) double pancake (DP) coils wound of 6-mm-wide and 75-mu m-thick REBCO tapes. In order to keep the hoop strains on REBCO tape <0.6% at an operating current I-op of 250 A and in a field of 30.5 T, we overbanded each pancake in Coil 1 with a 6-mm-wide, 76-mu m-thick 304 stainless steel strip: 7-mm-thick radial build for the central 18 pancakes, while 6-mm-thick for the outer 2 x 17 pancakes. In this paper, Coil 1 was successfully tested at 77 K and 4.2 K. In the 77-K test, the measured critical current was 35.7 A, determined by an E-field criterion of 0.1 mu V/cm. The center field magnet constant decreased from 34.2 to 29.3 mT/A, when I-op increased from 5 to 40 A. The field distribution at different I-op along the z-axis was measured. The residual field distributions discharged from 10 and 20 A were recorded. In the 4.2-K test, Coil 1 successfully generated a central field of 8.78 T at 255 A. The magnet constant is 34.4 mT/A, which is same as our designed value. The field homogeneity at the coil center within a +/- 15-mm region is around 1700 ppm. This large error field must be reduced before field shimming is applied.