Characteristics of Round and Extracted Strands of Nb3Al Rutherford Cable

Long Nb₃Al strands with copper stabilizer are promising for future high field accelerator magnets. A 1.2 kilometer Nb₃Al strand with Cu stabilizer was fabricated at the National Institute for Materials Science in Japan. Using this strand a 30 meter Cu stabilized Nb₃Al Rutherford cable was made for the first time by a collaboration of NIMS and Fermilab. The Nb₃Al strands extracted from cable with a relatively low packing factor showed almost no J_c degradation. But the extracted strands from the highly compacted cable showed some degradation in both J_c and n value, which may be caused by local separation of the copper stabilizer. Still, its J_c degradation is lower than that of typical Nb3Sn strands. The current limit due to magnetic instability in low field is about 500 A at 4.2 K. The magnetization of the strands, which was measured with balanced coils at 4.2 K, showed large flux jumps, usually around 1.5 T. This value is much larger than the B_c2 (4.2 K) of the Nb matrix, which is around 0.4 Tesla. The magnetic instability of the Nb₃Al strand at low field is not completely understood, but it might be explained by the superconducting coupling current through the Nb matrix.