Solenoidal coils made from monofilamentary and multifilamentary MgB2 strands

Three solenoids have been wound with MgB2 strands and tested for transport properties. One of the coils was wound with a Cu-sheathed monofilamentary strand and the other two with a seven-filament strand with Nb-reaction barriers, Cu stabilization, and an outer monel sheath. The wires were first S-glass insulated, then wound onto an OFHC Cu former. The coils were then heat treated at 675 °C/30 min (monofilamentary strand) and 700 °C/20 min (multifilamentary strand). Smaller (1 m) segments of representative strand were also wound into barrel-form samples and HT along with the coils. After HT the coils were epoxy impregnated. Transport Jc measurements were performed at various taps along the coil lengths. Measurements were made initially in liquid helium, and then as a function of temperature up to 30 K. Homogeneity of response along the coils was investigated and a comparison to the short sample results was made. Each coil contained more than 100 m of 0.84–1.01 mm OD strand. One of the seven-strand coils reached 222 A at 4.2 K, self-field, with a Jc of 300 kA cm−2 in the SC and a winding pack Je of 23 kA cm−2. At 20 K these values were 175 and 13.4 kA cm−2. Magnet bore fields of 1.5 and 0.87 T were achieved at 4.2 and 20 K, respectively. The other multifilamentary coil gave similar results.