Progress in scale-up of R$E$BCO STAR™ wire for canted cosine theta coils and future strategies with enhanced flexibility

We report recent developments in the scale-up of symmetric RE-Ba-Cu-O (R$E$BCO) tapes with 15–22 μm thick substrates. Using these symmetric R$E$BCO tapes, we fabricated up to 10 m long, symmetric tape round (STAR™) R$E$BCO wires, less than 2 mm diameter, using 1.02 mm and 0.81 mm diameter copper formers. The critical current of the long STAR™ wires made in lengths of 2–10 m ranges from 465 A to 564 A at 77 K, self-field. This wire was then used to construct a single-layer, full-depth groove, three-turn canted cosine theta (CCT) coil with a minimum bend radius of 15 mm. This three-turn CCT coil retains 95% of its $I_{c}$ even when wound at a such a small bend radius. This result confirms the capability of fabricating CCT coils with STAR™ wire at a tilt angle of 30º which would yield a dipole transfer function of 0.48 T kA$^{−1}$ at a 15 mm bend radius. Further, the architecture of STAR™ wire was modified for an $I_{c}$ retention of >90% at an even smaller bend radius of 10 mm with the aim of increasing the dipole transfer function. The higher dipole transfer function enabled by STAR™ wire is an important step toward the eventual goal of a 5 T maximum dipole field in a R$E$BCO-based CCT coil. At a bend radius of 10 mm, a six-layer STAR™ wire exhibits a critical current of 288 A at 77 K, self-field, i.e. 94% $I_{c}$ retention and 617 A at 4.2 K in a 15 T background field, which equals a $J_{e}$ of 412.7 A mm$^{−2}$ at a Lorentz force of 9.3 kN m$^{−1}$. This level of flexibility and the high performance of STAR™ wire in high fields at 4.2 K and with a small bend radius underscores its potential use in compact and low-cost high-field magnet and related applications.