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. A challenge with the use of RE-Ba-Cu-O (REBCO, RE=rare earth) superconductors for accelerator magnets is associated with their flat rather than round geometry. So, the desired multi-strand transposed cable design has been difficult so far for REBCO conductors. AMPeers has demonstrated REBCO round wires as small as 1.3 – 1.9 mm in diameter 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 Phase I project, AMPeers and Lawrence Berkeley National Laboratory (LBNL) successfully demonstrated 2-meter-long, 6-around-1 cables with STAR® wires with good critical current retention and wound the cable into a dipole magnet structure. Evidence of current sharing was also observed, which is a significant advantage of a multi-strand cable architecture. In the Phase II project, AMPeers will work with LBNL and University of Houston to optimize the design of multi-strand cables with STAR® wires and demonstrate the feasibility of a transposed STAR® cable as a conductor for high-field REBCO dipole magnets. We will focus on three goals: 1) develop and demonstrate compact STAR® cables with a diameter of 4 mm or less for a minimum bend radius of 25 mm or less, 2) increase the critical current of the STAR® cables to 10 kA at 4.2 K, 15 T with high current REBCO tapes 3) scale up cable length to at least 10 meters to develop a subscale dipole magnet to acquire feedback on further conductor development. In addition to meeting the needs of High Energy Accelerators that would operate at 20 T and beyond, 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. The resulting commercial STAR® cable will be of great interest also to the fusion community for applications such as stellerator devices that require flexible, high-current conductors.