Critical currents at high fields in NB3AL multifilamentary wires

Short sample critical currents have been measured on wires of Nb3Al in fields up to 21 tesla. The wires consisted of 7 or 19 filaments of niobium-aluminium foil "jelly rolls" in a copper matrix. They were reacted at temperatures between 750°C and 950°C to form Nb3Al. Critical temperatures were measured inductively and showed transitions with three stages whose onsets were at ∼15K, 10-13K and ∼9.2K. Critical currents measured at low fields reproduced the high values previously reported for this material. Measurements were made at the Francis Bitter National Magnet Laboratory in high fields up to 21 tesla. The magnetic field variation of the critical Lorentz force, JcB, could be fitted to the Kramer relation, b^½(1-b)², for most samples. The bulk Bc2, believed to correspond to the intermediate temperature transition was in the range 10-17 tesla. In addition there was a high field tail with Bc2from 17-21 tesla, related to high (∼15K) transition onset temperature. These results are discussed in terms of the Nb-Al phase diagram.