Improvement of critical current density and residual resistivity on jelly-roll processed Nb/sub 3/Al superconducting wires

The residual resistivity of copper stabilizer (/spl rho/), is deteriorated by the diffusion of chromium plated on the strand surface during heating of Nb/sub 3/Al strands. To improve this degradation, the copper stabilizer has been located on the center of the cross-section of the strand rather than near the strand surface. As a result, /spl rho/ is improved to one-sixth of previous value achieving 1.6/spl times/10/sup -10/ /spl Omega/m in a laboratory scale strand. The critical current density (J/sub c/) of this strand was 715 A/mm/sup 2/ at 12 T. This technique was applied to prototype strands to be used in a Nb/sub 3/Al Insert Coil developed in ITER R&D programme resulting in a /spl rho/ of 1.5/spl times/10/sup -10/ /spl Omega/m, which meets ITER specification. In addition, the improvement of J/sub c/ was also studied by two-stage reaction including a continuous heating procedure, which resulted in a J/sub c/ of 1130 A/mm/sup 2/ at 12 T and good longitudinal uniformity.