Conductor on Molded Barrel (COMB) magnets using Symmetric Tape Round (STAR®) Wires

Conductor on Molded Barrel (COMB) magnet is a recently-proposed, elegant technology for hightemperature superconductor (HTS) accelerator magnets. This technology constrains the conductor in dedicated channels on the surfaces of 3D-printed metallic support structures. COMB design ensures precise conductor positioning to produce magnetic fields with high spatial uniformity necessary for accelerator applications, and to prevent accumulation of Lorentz forces and mechanical stresses, which cause degradation of conductor properties. A general “rule of thumb” applicable to the cosine-theta type dipoles, which have been used in every collider since Tevatron, is that the minimum bending diameter of the pole turn is equal to ~1/2 of the coil aperture. Hence, a dipole coil with 50-60 mm aperture, which is optimum for a future hadron collider, has a minimum pole diameter of ~25-30 mm. This brings the first important requirement for the conductor – it should be bendable around that pole diameter with minimal degradation. While several conductor designs have been developed for REBCO superconductor, Symmetric Tape Round (STAR®) wire is currently the only conductor bendable to sub-50 mm diameters without a significant performance degradation. An objective of the Phase I project was to evaluate if COMB magnets can be successfully made with STAR® wires. In the Phase I project, STAR® wires of ~ 2.5 mm diameter using 11-12 strands of high current REBCO tapes using Advanced Metal Organic Chemical Vapor Deposition (A-MOCVD) were fabricated. A 1-m-long, 2.5 mm diameter STAR® wire showed 98% retention in critical current of 980 A at 77 K even when bent into the pole channel of the inner layer COMB support structure – the tightest bend in the coil with a diameter of 33 mm. The n-value of 20 of the wire did not change after bending. A COMB magnet with 60-mm clear bore was then successfully fabricated using 10-meter-long STAR® wires. The critical current retentions for the two coils comprising the COMB magnet were in 94-99% range, which exceeded the ultimate Phase-I project target of >90% critical current retention after winding the STAR® wires into the magnet with a 33 mm diameter in the innermost turn. In the Phase II project, the COMB magnet model produced in Phase-I will be tested in liquid helium to study the resistive transition characteristics (quench or thermal run-away) and fine tune the instrumentation and the data acquisition system to be able to detect resistive transitions and protect the magnet that will be fabricated in the Phase II project. 100+ meters of 2.5 mm diameter STAR® wire will be manufactured to fabricate a multi-layer COMB dipole magnet to demonstrate 5+ T in a 60-mm clear bore in liquid helium. The COMB magnet tests will include several thermal cycles, quench studies, ramp-rate studies and magnetic measurements. A successful completion of the project will open the pathway to creating REBCO HTS magnets with aperture dimensions relevant for future collider applications.