We report the latest developments of next-generation flexible round RE–Ba–Cu–O (REBCO, RE = rare earth) wire, driven by the needs of compact accelerator magnets requiring round isotropic wire with an engineering current density (Je) of 600 A mm−2 at 4.2 K, 20 T at a bend radius of 15 mm. We have developed a Symmetric Tape Round (STAR) REBCO wire using multiple layers of REBCO tapes specifically developed for this architecture, featuring a mechanically symmetric geometry with a 10–18 μm thick substrate wherein the superconductor film is positioned near the tape’s neutral plane for superior bend strain tolerance. Furthermore, each layer of REBCO tape is individually optimized for maximum bend strain tolerance. These ultra-thin substrate REBCO symmetric tapes enabled us to fabricate next-generation isotropic round wires of just 1.3 mm diameter and a critical current equivalent to commercial 12 mm wide REBCO tapes. The in-field performance of STAR wires of several configurations has been tested at National High Magnetic Field Laboratory to identify the most suitable architecture to meet the needs of high-field compact accelerators. At a bend radius of 15 mm, a six-layer STAR wire exhibits critical current of 778 A at 4.2 K in 20 T background field, which equals Je of 586.4 A mm−2 at a Lorentz force (FL) of 15.5 kN m−1 which is the highest reported Je value for REBCO wire in round geometry at this magnetic field. Similarly, a 12-layer STAR wire shows an Ic of 1156 A at 31.2 T, 4.2 K which corresponds to a Lorentz force of 36 kN m−1. Multiple tests of STAR wires at high magnetic field confirm a <0.1% variation in measured Ic. This level of reproducibility of the high performance of STAR wire in high magnetic fields at 4.2 K and small bend radius underscores the potential of STAR REBCO wire for use in compact accelerator magnet and related applications.
We report recent developments in the scale-up of symmetric RE-Ba-Cu-O (REBCO) tapes with 15-22 µm thick substrates. Using these symmetric REBCO tapes, we fabricated up to 10 m long, symmetric tape round (STAR™) REBCO 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 REBCO-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.
Symmetric Tape Round (STAR) wire fabricated with an ultra-thin RE-Ba-Cu-O (REBCO, RE=rare earth) coated conductor has a high engineering current density (J e ) and remarkable flexibility. It is a competitive candidate for future canted cosine theta magnets with smaller winding radii that operate at fields above 20 T at 4.2 K. STAR wires consisting of six or eight REBCO coated conductor layers have been fabricated with an outer diameter smaller than 2 mm. We performed in-field tests of the critical current of STAR wires at 4.2 K in background fields up to 31.2 T. At a 15 mm bending radius, J e values of 438 A mm −2 at 20 T and 299 A mm −2 at 31.2 T were obtained. These are the highest J e values measured in any reported round-geometry REBCO wires at such ultra-high background magnetic fields. This exhibits the potential of STAR wires in future compact accelerator magnets.
A dipole magnet generating 20 T and beyond will require high-temperature superconductors such as Bi2Sr2CaCu2O8-x and REBa2Cu3O7-x (RE = rare earth, REBCO). Symmetric tape round (STAR®) wires based on REBCO tapes are emerging as a potential conductor for such a magnet, demonstrating a whole-conductor current density of 580 A mm-2 at 20 T, 4.2 K, and at a bend radius of 15 mm. There are, however, few magnet developments using STAR® wires. Here we report a subscale canted cosθ dipole magnet as an initial experiment for two purposes: to evaluate the conductor performance in a magnet configuration and to start developing the magnet technology, leveraging the small bend radius afforded by STAR® wires. The magnet was wound with two STAR® wires, electrically in parallel and without transposition. We tested the magnet at 77 and 4.2 K. The magnet reached a peak current of 8.9 kA, 78% of the short-sample prediction at 4.2 K, and a whole-conductor current density of 1500 A mm-2. The experiment demonstrated a minimum viable concept for dipole magnet applications using STAR® wires. The results also allowed us to identify further development needs for STAR® conductors and associated magnet technology to enable high-field REBCO magnets.
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