Development of a 12-Tesla Multifilamentary Nb₃Sn Magnet for MFTF-B

Abstract

Two 12 tesla superconducting insert coils are being designed by General Dynamics Convair Division for the axicell regions of MFTF-B for Lawrence Livermore National Laboratory. A major challenge of this project is to ensure that combined fabrication and operational strains induced in the conductor are within stringent limitations of the relatively brittle Nb₃Sn superconductor filaments. These coils are located in the axicell region of MFTF-B (Figure 1). They have a clear-bore diameter of 36.35 cm (14.31 inches) and consist of 27 double pancakes (i.e., 54 pancakes per coil) wound on an electrically insulated 304LN stainless bobbin/helium vessel. Each pancake has 57 turns separated by G-10CR insulation. The complete winding bundle has 4.6 million ampere-turns and uniform current density of 1985 A/cm². In conjunction with the other magnets in the system, they produce a 12 tesla central field and a 12.52 tesla peak field. A multfilamentary Nb₃Sn conductor was selected to meet these requirements. The conductor consists of a monolithic insert soldered into a copper stabilizer. Sufficient cross-sectional area and work-hardening of the copper stabilizer has been provided for the conductor to self-react the electromagnetic Lorenz force induced hoop stresses with normal operational tensile strains less than 0.07 percent. This conductor configuration also provides adequate stabilizer area for unconditional cryostability in pool boiling liquid helium. The monolithic insert has a geometric aspect ratio of 3.2:1 that was selected to minimize the strain induced during winding without excessive critical current anisotropic degradation. The resulting cross sectional area of the insert has a 1,500-ampere operational current.