Upper critical fields in multifilamentary NbTi alloy superconducting wires

In order to improve the high field performance of superconducting magnets, the upper critical field Bc2 for practical multifilamentary alloy wires of NbTi, NbTiTa and NbTiHf were examined in respect with the usage of a pressurized superfluid cooling technique. The addition of Ta or Hf to NbTi enhanced by 0.5 T for Bc2 at 1.8 K. Although the addition of a heavy element such as Ta or Hf has been regarded as suppressing Pauli-paramagnetism so far, it was found that the mechanism for Bc2 enhancement by Hf addition is different from that by Ta addition. (author)