Construction and operation of the 12-T superconducting coils for the mirror fusion test facility

We have successfully constructed and tested a pair of high-field coils that is part of the magnet set of the Mirror Fusion Test Facility (MFTF-B) at the Lawrence Livermore National Laboratory. Each coil consists of a multifilamentary Nb3Sn magnet nested inside a multifilamentary NbTi magnet. During our test, these coils produced a central field of 12 T, with a peak conductor field of 12.5 T. The dimensions of the Nb3Sn insert coil are: 0.36-m bore, 1.32-m outer diameter, and 1.14-m overall length. These coils were designed to be fully cryogenically stabilized and cooled by pool-boiling liquid helium. The operating current density of the Nb3Sn coils is 2000 A/cm²and 2400 A/cm²for the NbTi magnet. In this paper, we present design considerations and details, construction techniques, and operational results of these coils.