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High-temperature superconducting REBa{2} Cu{3}O{7-x} (rebco) conductors have the potential to generate a high magnetic field over a broad temperature range. The corresponding accelerator magnet technology, still in its infancy, can be attractive for future energy-frontier particle colliders such as a multi-TeV muon collider. To help develop the technology, we explore the requirements and potential characteristics of a rebco magnet, operating at 4.2 or 20 K, with a dipole field of 8-10 T in a clear aperture of 150 mm. We use the canted \cos \theta magnet configuration to reduce the electromagnetic stresses on the conductors. We present the resulting dipole fields, field gradients for combined-function cases, conductor stresses, magnet dimensions and conductor lengths. We also discuss the conductor performance that is required to achieve the target dipole field at 4.2 and 20 K. The information can provide useful input to the development of rebco magnet and conductor technology for collider-ring magnets in a muon collider.

Superconducting magnets designed for high energy physics and nuclear fusion require mechanical and electrical integrity to perform at high currents and magnetic fields. Vacuum Pressure Impregnation (VPI), a process of curing epoxy in and around the superconducting wires, is often used to support and consolidate a magnet. However, the heat and mechanical stresses associated with the process can degrade the wires, significantly lowering their critical current. This study explores different methods of potting and curing CORC wire with the aim of reducing wire performance degradation to less than 3% measured at 77 K, self-field. The wires were 2.9 mm in diameter consisting of a total of six REBCO tapes (three layers of two tapes). Two bending radii (20 mm and 50 mm) were tested to mimic the winding shape of a magnet. Mix 61 epoxy was used in preliminary tests for potting. For each test, two wires were used, and their critical currents were measured simultaneously in liquid nitrogen at 77 K - in their straight form, then bent, followed by the heat treatment used for Mix 61 but without epoxy and finishing with the full epoxy impregnation test. Later tests were performed using CTD-528 to explore a room temperature cure, limiting possibility of degradation from thermal expansion and prolonged exposure of the REBCO tapes to elevated temperature. Here we report the experimental results with multiple CORC wires and different curing schedules. The results obtained are the first steps toward identifying the VPI process with minimum degradation in critical current to be implemented in high-field magnets using CORC wires.

Although the high-temperature superconducting (HTS) REBa2Cu3O x (REBCO, RE-rare earth elements) material has a strong potential to enable dipole magnetic fields above 20 T in future circular particle colliders, the magnet and conductor technology needs to be developed. As part of an ongoing development to address this need, here we report on our CORC® canted cosθ magnet called C2 with a target dipole field of 3 T in a 65 mm aperture. The magnet was wound with 70 m of 3.8 mm diameter CORC® wire on machined metal mandrels. The wire had 30 commercial REBCO tapes from SuperPower Inc. each 2 mm wide with a 30 µm thick substrate. The magnet generated a peak dipole field of 2.91 T at 6.290 kA, 4.2 K. The magnet could be consistently driven into the flux-flow regime with reproducible voltage rise at an engineering current density between 400-550 A mm-2, allowing reliable quench detection and magnet protection. The C2 magnet represents another successful step towards the development of high-field accelerator magnet and CORC® conductor technologies. The test results highlighted two development needs: continue improving the performance and flexibility of CORC® wires and develop the capability to identify locations of first onset of flux-flow voltage.

REBCO coated conductors maintain a high engineering current density above 16 T at 4.2 K. That fact will significantly impact markets of various magnet applications including high-field magnets for high-energy physics and fusion reactors. One of the main challenges for the high-field accelerator magnet is the use of multi-tape REBCO cables with high engineering current density in magnet development. Several approaches developing high-field accelerator magnets using REBCO cables are demonstrated. In this paper, we introduce an alternative concept based on the canted cos θ (CCT) magnet design using conductor on round core (CORC®) wires that are wound from multiple REBCO tapes with a Cu core. We report the development and test of double-layer three-turn CCT dipole magnets using CORC® wires at 77 and 4.2 K. The scalability of the CCT design allowed us to effectively develop and demonstrate important magnet technology features such as coil design, winding, joints and testing with minimum conductor lengths. The test results showed that the CCT dipole magnet using CORC® wires was a viable option in developing a REBCO accelerator magnet. One of the critical development needs is to increase the engineering current density of the 3.7 mm diameter CORC® wire to 540 A mm-2 at 21 T, 4.2 K and to reduce the bending radius to 15 mm. This would enable a compact REBCO dipole insert magnet to generate a 5 T field in a background field of 16 T at 4.2 K.