Multi-Strand Transposed Cables of Round REBCO Wires for Accelerator Magnets

Superconducting coils built for dipole and quadrupole magnets for particle accelerators operate at currents as high as 10 to 20 kA. Coils operating at such high currents are wound with multi-strand cables of Nb-Ti and Nb3Sn wires, each 0.6-1.2 mm in diameter. Such multi-strand cables provide many benefits: substantial flexibility that enables compact coils with small bending radius, twisted geometry that can reduce losses during ramping and perturbations of the magnetic field, possible current sharing between wires, fewer turns compared to a coil made with a single wire which reduces the required strand length as well as lowers magnet inductance which in turn decreases voltages during magnet ramping and enables a faster discharge of current during quench. Even higher gains in beam energy and luminosity of particle accelerators can be obtained by using High Temperature Superconductor (HTS) dipoles which are the only option for field strengths in the vicinity of 20 T. RE-Ba-Cu-O (REBCO, RE=rare earth) coated conductors are a leading candidate for HTS dipoles. A challenge with the use of REBCO for accelerator magnets and Bi-2212 wires is associated with their flat rather than round geometry and a wide (~ 12 mm) profile rather than a multifilamentary architecture. So, the desired multi-strand transposed cable design has been difficult so far for REBCO coated conductors. The flat REBCO tape geometry can be converted to round wire by helical winding of narrow tapes on a round former. AMPeers and the University of Houston have demonstrated REBCO round wires as small as 1.3 – 1.9 mm in diameter with engineering current density of 586 A/mm2 at 4.2K, 20T using a special type of REBCO tape where the superconductor film is positioned near the neutral plane. These Symmetric Tape Round (STAR) wires exhibit excellent tolerance to bend strain and can retain over 95% of their critical current even when wound over a 15 mm bend radius. In the proposed program, AMPeers LLC will work with Lawrence Berkeley National Laboratory and University of Houston to develop STAR wires specifically to meet the requirements of compact, transposed cable designs. Specifically, the STAR wires will be designed to be mechanically robust to withstand the significant stresses in cable winding at small bend diameters. Prototype cables will be fabricated with STAR REBCO wires and tested for critical current retention and bend performance. The optimized cables will be tested in a dipole magnet. In addition to meeting the needs of High Energy Accelerators that would operate at 20 T and beyond for High Energy Physics, the new technology would be highly impactful in the commercial market for accelerators which includes proton beam therapy in medicine, ion implantation for digital electronics, for safer use of nuclear energy, treatment of water and flue gases, pharmaceutical drug development and nuclear forensics and cargo screening for security. Additionally, our round REBCO wires can enable commercialization of HTS in power applications such as generators, motors and transformers.