Nb3Sn critical-current measurements using tubular fiberglass-epoxy mandrels

A systematic study of the effect of sample mounting techniques on the superconducting critical-current measurement was made. A seemingly small change in mandrel geometry can result in a 40% change in the measured critical current of a Nb₃Sn sample at 12 T. This is a result of a change in the conductor prestrain at 4 K caused by variation in thermal contraction between thick-and thin-walled fiberglass-epoxy composite (G-10) tubes. An approximate measure of the vibrations in thermal contraction (from room to liquid-nitrogen temperature) indicate a 0.2% greater contraction for the thick-walled tube. This difference, combined with strain sensitivity measurements, is consistent with the observed decrease in critical current