Next-generation highly flexible round REBCO STAR wires with over 580 A mm⁻² at 4.2 K, 20 T for future compact magnets

Abstract

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.

Dipole Magnets Above 20 Tesla: Research Needs for a Path via High-Temperature Superconducting REBCO Conductors

“…Understanding how strain develops in REBCO layer during the cabling and magnet winding processes will be critical [32,104,106,[108][109][110]195,196]. The design tools based on the understanding of the conductor mechanics will help guide the development of more flexible round REBCO conductors and the optimized REBCO tapes with thinner substrates and narrower widths [34,38,160]. It is also important to systematically measure the impact of electromagnetic loads on conductors and to understand and mitigate potential conductor degradation.…”

“…Although one can make CCT magnets using stacked-tapes or Roebel cables [104,110], we focus on round conductors that are under active development in the U.S. [31,34,38]. With multiple layers of tapes wrapped helically around a round former, the tapes are transposed within each layer.…”

“…Other cable configurations such as twisted-stack [32] and exfoliated REBCO cable [33] may also be viable for accelerator magnet applications. [35] 26 [8] 105 [8] 110 [36] Upper critical field at 20 K T 0 0 9 [8] 100 [36] Typical wire diameter mm 1.065 [24] 0.85 [27] 0.80 [28] 12×1.5 [37] 3.7 [38] 1.3 [34] J e at 16 T, 4.2 K a A mm −2 0 474 [39] 1300 [28] 964 b [40] 310 c [38] 695 d [34] J e at 20 T, 4.2 K A mm −2 0 123 [39] 1180 [28] 821 b [40] 267 c [38] [43] 30 [38] 15 [34] Effective filament diameter f µm 7 [24] 55 [27] 130 [44] 2-5500 g [22] 2000 g [38] 1400-2500 g [34] Magnetization at 4.2 K, 1 T h mT 10 i [45] 270 [46] 60 [47] 716 j [48] 502 [49] TBD Strand transposition -Full Full Partial Peak heat treatment temperature…”

“…• C N/A 665 [27] 892 [50] N/A Irreversible tensile strain limit ->1% [51] 0.4% [52] 0.3% [53,54] 0.36% [55] 0.85% [56] TBD Irreversible transverse stress limit MPa >200 [24] 150-260 [57,58] 60-TBD [59] 370-440 [37] 99 k [60] TBD Stabilizer fraction -62% [24] 55% [27] 78% [61] 20% [37] 57% l [38] 17% l [34] Stabilizer RRR -200 [24] ≥150 [27] 90-440 [61,62] 13-69 m [63,64] g When the magnetic flux lines are in parallel with the broad surface of the REBCO tape, the REBCO layer thickness becomes the characteristic length scale for an effective filamnet size.…”

“…The data for the CORC R wire at a bending radius of 30 mm with projected performance at 20 T are from [38]. The data for the STAR wire at a bending radius of 15 mm are from [34].…”