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 present results on the in-field critical current (I c ) performance of 4.0 µm thick REBCO film with 15% Hf addition with fields up to 31.2 T and field orientations in the B∥ab plane and B∥c axis. Unlike the behavior at B∥c, the critical current at B∥ab is only very weakly dependent on field, decreasing from self-field to 31.2 T by only 22%, i.e. from the self-field value of ∼7700 A/4 mm width to ∼6300 and 5812 A/4 mm width at 14 and 30 T, respectively. These values are remarkably 3 and 5.7x higher than the corresponding critical currents at B∥c. The in-field behavior of the present 15% Hf sample at field orientation B∥c axis is nearly identical to the previously reported record values found in 4.3 and 4.6 µm thick 15% Zr samples in terms of critical current density. In contrast to the pinning force behavior in the B∥c orientation, which saturates to a constant value of 1.7 TN m −3 above ∼5-6 T, the pinning force in the B∥ab orientation increases near-linearly, reaching a remarkable value of over 11.5 TN m −3 at 31.2 T. These results demonstrate the potential of thick REBCO conductors at 4.2 K for high field and energy density applications, in particular where the magnetic field is contained near the ab-plane.

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Due to an error in the production process, the title of this article should read 'Symmetric Tape Round (STAR) REBCO wire with J e (4.2 K, 15 T) beyond 450 A mm −2 at 15 mm bend radius: a viable candidate for future compact accelerator magnet applications'. Furthermore, all references to 'symmetric tape round wires' should be 'Symmetric Tape Round (STAR) wires'.In the 'Tape preparation' subsection 'The REBCO tape was then slit from a width of 12-2.5 mm using a reel-to-reel laser slitting machineK' should read 'The REBCO tape was then slit from a width of 12 to 2.5 mm using a reel-to-reel laser slitting machineK'.

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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.