Production of a 11 km long jelly roll processed Nb3Al strand with high copper ratio of 4 for fusion magnets

When building high field, large scale magnets such as those used in fusion applications where large electromagnetic forces are involved in the superconductor, Nb₃Al conductor provide a great advantage, because the strain sensitivity of the critical current for Nb₃Al is substantially smaller than that of Nb₃Sn. A co-project carried out between the Japan Atomic Energy Research Institute (JAERI) and Hitachi Cable, Ltd., has established a procedure for finishing with a long one piece length of Nb₃Al strands with the high Cu ratio of 4 through the Jelly Roll process. The hydrostatically extruded rod shows excellent workability, and a 0.74 mm dia Nb₃Al strand with the high Cu ratio of 4 has been successfully drawn in one continuous piece length of 11 km without wire breakage. The strand shows a Jc and n value of 590 A/mm² and 45 at 12 T and 1984 A/mm² and 70 at 7.4 T which is the maximum field experienced in the innermost conductor of the toloidal field coil winding for the modification of JT-60 fusion device being planned in JAERI. A 30 m long Nb₃Al cable with a cabling scheme 3 × 3 × 3 × 3 × 4 was fabricated from the strand.