Mechanically-robust STAR« wires for 20-T Hybrid Dipole Magnets

AMPeers and the University of Houston (UH) have demonstrated round REBCO wires with special REBCO tapes where the superconductor film is positioned near the neutral plane, resulting in the record engineering current density (Je) of 598 A mm-2 at 4.2 K, 30 T at a 15 mm bend radius. Recently, Lawrence Berkeley National Laboratory (LBNL), in collaboration with AMPeers, successfully developed a subscale canted cos? (CCT) dipole magnet using STAR® wires, as a first step to develop STAR®-based magnet technology. The magnet showed a maximum transport current of 8,500 A at 4.2 K, with no obvious wire degradation after winding. The results present timely opportunities to advance STAR® wires into a robust commercial magnet conductor to enable a dipole field of 20 T and above. The mechanical strength of the emerging STAR® wires and cables is critical to enable a dipole field above 20 T. The STAR® conductor must demonstrate a strong tolerance to the electromagnetic forces that are typically transverse to the longitudinal axis of the conductor in high-field accelerator magnets. The conductors need to sustain at least 250 kN/m forces in a magnet generating a dipole magnetic field of 20 T, assuming a 10 kA operating current and a 25 T peak field on the conductor. Although the voids between tape strands in a STAR® wire are necessary to enable bending, they can leave the strands unsupported under high electromagnetic forces. The resulting excessive stress and strain can degrade tapes and wire performance. One option is to support individual tapes via impregnation to distribute and reduce the stress and strain. The REBCO tapes in a STAR® conductor need to demonstrate sufficient strength against the stress that can occur during epoxy impregnation that can delaminate and degrade the REBCO tapes. In the proposed project, AMPeers and the University of Houston will develop symmetric REBCO tapes with enhanced transverse strength and use them to fabricate more robust STAR® wires. LBNL will test impregnate STAR® wires and cables made with strands with enhanced transverse strength. We will identify appropriate media and procedure for vacuum pressure impregnation that are compatible with STAR® conductors. Such resilient STAR® conductors will be tested for critical current under applied transverse load and load cycles at 77 K. The test results will provide effective feedback to the STAR® conductor development, focusing on the transverse mechanical strength that is critical for future high-field accelerator magnets. We will scale up the new technologies in Phase II to demonstrate 15 ľ 30 m long STAR® wires which will be used to construct a full-scale magnet and test in background fields to demonstrate robustness and functionality of the advanced STAR® wires for high-field accelerator magnets.