Homogeneity and accuracy of the second critical field B_c2 of the NIST SRM-1457 standard NbTi/Cu wire

In the frame work of the VAMAS intercomparison activities the second critical field Bc2 of the standard KIST NbTi/Cu superconductor was investigated in several laboratories in the world. In this paper the specific results obtained in the UT laboratory are reported. On a single piece of wire of 1.5 meter length the voltage is measured across 10 straight (crossing the magnet axis) and 10 bend sections (on radius 25 mm in the magnet bore). A detailed analysis of all possible errors in field, temperature and voltage is presented. The bend sections provide the best results. As a characteristic result we mention a Bc2 (at 50%) at 4.2 K of 11.31 T with an error of 10 mT only. The variation in Bc2 over the sections is about the same as the variation found for various runs of the same section. The Bc2-homogeneity of the wire over a length of 1.5 meter is extremely good (<10¿3). This accuracy is such that the Bc2 of the standard wire could be used as a field calibrating method.