Flexible, compact, flat REBCO cables for high-field accelerator magnets

RE-Ba-Cu-O (REBCO, RE = rare earth) coated conductor is one of the most promising superconductors for high-field accelerator magnets. It is in form of a tape, which is not directly useable for coil fabrication because of the relatively low critical current (Ic) of a tape strand. Hence, multiple tapes must be combined in a single cable that should still be relatively flexible at least in one plane with a minimum bending diameter ~30 mm to fabricate non-planar (e.g. cosine-theta) coils with the bore size of around 60 mm, which are needed for future collider applications. AMPeers has developed a special type of REBCO tape – symmetric REBCO tape – that can be bent to a diameter as small as 0.7 mm without Ic degradation. Such a remarkable bend performance is feasible by positioning the superconductor film near the neutral plane. Such tapes have been used to fabricate STAR® wires with record engineering current density (Je) of 598 A mm-2 at 4.2 K, 30 T at a 15 mm bend radius. Recently, Fermilab demonstrated another accelerator magnet design – Conductor-on-Molded-Barrel (COMB) using STAR® wires. The wires retained over 90% of their critical current after winding into the COMB structure with 60 mm bore diameter. However, to generate higher magnetic fields, multiple layers of the wire should be used, which complicates the fabrication and increases the risk of damaging the conductor in the process. Also, the increased number of turns, and the magnet inductance will complicate the energy extraction from the coil in case of a quench and may require several parallel extraction circuits, which is typical for standalone solenoid magnets, but problematic for multiple accelerator magnets connected in a string. We propose an alternate REBCO cable design to increase the cable current: switch from a solid round former to a flexible flat former. This concept has been considered to some extent for fusion applications where REBCO tapes were wrapped around a flat aluminum bar with rounded edges and a 5-mm thickness. The former thickness in that work was limited by the Ic degradation due to tape bending around the edge. For typical accelerator magnets, the cable thickness should be no more than 2 mm, which means that the former thickness needs to be 1 mm or less. AMPeers is uniquely positioned to produce such flexible, flat cables using the unique bend properties of our symmetric REBCO tapes. In the Phase-I project, several cables will be fabricated by wrapping our symmetric tapes on 1-mm-thick Rutherford cable former made of copper with rounded edges. The critical currents of these cables will be tested at 77 K before and after bending them to 30 mm diameter. A small (e.g. double-pancake solenoid or racetrack) coil will also be fabricated and tested. A larger cosine-theta magnet with ~60 mm bore will be fabricated in the Phase II project and tested in liquid helium. Our proposed project will open a pathway for substantially increasing the magnetic field in the REBCO accelerator magnets while minimizing the number of layers and using the well-proven coil fabrication techniques developed for Nb-Ti and Nb3Sn accelerator magnets.