Fermilab is developing high-temperature superconductor (HTS) accelerator magnets based on the Conductor on Molded Barrel (COMB) magnet technology. This technology, proposed at the beginning of 2019, constrains the cable 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 causes degradation of the conductor properties. It is particularly well suited for round conductors and offers an elegant solution for producing HTS inserts to boost the magnetic field in the low-temperature superconducting (LTS) accelerator magnets. Dipole magnet is the main and the most challenging component of any circular accelerator. 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, the dipole coil with 50 mm aperture, which is optimum for a future hadron collider, has a minimum pole diameter of ~25 mm. This brings the first important requirement for the conductor it should be bendable around that pole diameters with no (or minimal) bending degradation. The second important requirement is that for HTS coils to be used as inserts in the LTS magnets, they need to be powered in series, which means they should have comparable currents. The currents used are in 10-15 kA range at 15 T, 4.2 K for the LTS coils. Several manufacturers in the U.S., Europe, and Asia are producing commercial REBCO tapes and cables. While there has been a considerable progress in increasing the engineering critical current density of these conductors in the past decade, Symmetric Tape Round (STAR) wire produced by AMPeers LLC is currently the only conductor bendable to sub-50 mm diameters without a significant performance degradation. STAR wire is directly compatible with the COMB magnet design and would allow coil apertures below the present limit of 100 mm. The goals of the proposed Phase I project include magnetic and structural designs of the COMB dipole magnet with 50-60 mm aperture, manufacturing of 10-20 m of STAR wire to meet the design requirements, fabrication of a small COMB dipole coil, and its test in liquid nitrogen to demonstrate that winding the STAR wire into the COMB structure does not degrade it by more than 10% (proof of principle). The Phase-II project goals will include manufacturing of 100-200 m of STAR wire, fabrication of a multi-layer COMB dipole magnet and testing it in liquid helium to demonstrate 5-8 T fields in 50-60 mm aperture generated by the HTS coil (performance demonstration). A successful project completion will open the pathway to creating the REBCO HTS magnets with the aperture dimensions relevant for the future collider applications.
