REBCO High-Temperature Superconducting Tape
A four-millimetre-wide ribbon of metal that carries hundreds of amps with zero resistance at the temperature of liquid nitrogen — and the only practical material capable of building the magnets for a commercial fusion reactor.
Conducts current with exactly zero resistance below a temperature of about 90 K (–183 °C, well above the 77 K boiling point of liquid nitrogen) and inside magnetic fields up to roughly 45 T at 4 K. Layered into pancake coils, REBCO tape forms the magnet windings for the new generation of compact fusion devices (Commonwealth Fusion SPARC, Tokamak Energy ST40 upgrade, Helion Polaris), for next-generation high-field MRI (10–14 T scanners under development), for compact accelerator magnets, and for high-current bus bars in superconducting power applications.
REBCO tape is a layered structure roughly 0.1 mm thick. The substrate is a 50–75 µm ribbon of Hastelloy C-276 (a nickel-chromium-molybdenum alloy chosen for strength, low magnetism, and matched thermal expansion). On top of this sits a 0.2–0.5 µm stack of textured oxide buffer layers — typically MgO grown by ion-beam-assisted deposition (IBAD) or LaMnO₃ — that align the crystal axes of the substrate to the c-axis of the superconductor. The superconductor itself, REBa₂Cu₃O₇₋ₓ (where RE = Y, Gd, Sm, or rarely Dy), is deposited as a 1–4 µm epitaxial film by metalorganic chemical vapour deposition (MOCVD, used by SuperPower / Furukawa), metalorganic deposition (MOD, AMSC), or pulsed laser deposition (PLD, Bruker, Faraday Factory). A 1–2 µm silver overlayer protects the REBCO film and provides electrical contact; a 20–40 µm copper or stainless-steel stabiliser laminate is added at the end for mechanical strength and quench protection. Engineering current density at 4 K, 20 T reaches 800–1500 A/mm² — roughly an order of magnitude higher than Nb₃Sn at the same field — which is why REBCO is the only practical conductor for the compact, high-field tokamaks being built today.
In plain terms
Imagine an extension cord so good that it doesn't get warm no matter how much current you push through it — and you can pile thousands of them on top of each other to make a magnet a hundred times stronger than a fridge magnet. That cord has to be ice cold to work, but compared to the older superconducting wires it can stay cold using cheap liquid nitrogen instead of a freezer full of liquid helium. That is REBCO tape, and it is what makes new fusion reactors and stronger MRI scanners possible.
Also called: YBCO tape, 2G HTS tape, coated conductor, rare-earth barium copper oxide tape.
Why it matters
- Commonwealth Fusion Systems' SPARC tokamak — under construction in Devens, Massachusetts — uses 10,000+ kilometres of REBCO tape wound into pancake coils generating 12 T on-axis. The same field with Nb₃Sn would require a magnet roughly 4× larger and would dissipate 10× more cryogenic power.
- MIT's TFMC (Toroidal Field Model Coil) test in September 2021 reached 20.1 T inside a 254 mm bore, the highest sustained field ever achieved in a fusion-relevant magnet geometry — and the result that enabled the SPARC reactor construction decision.
- Operating at 20 K (cooled by gaseous helium or a closed-cycle cryocooler) instead of 4 K eliminates the need for a liquid-helium plant: a single closed-cycle cooler replaces an entire cryogenic infrastructure, transforming the economics of high-field magnets.
- REBCO tape is mechanically robust — it can survive Lorentz-force stresses approaching 700 MPa in a fusion magnet, where Nb₃Sn would have shattered. This durability is why compact fusion architectures became plausible only after REBCO became commercially available in 100-metre+ piece lengths in the mid-2010s.
Where it lives
- Commonwealth Fusion Systems SPARC — 18 toroidal-field coils, 12 T on-axis, REBCO pancake winding (Devens, MA; first plasma expected 2027)
- MIT PSFC Toroidal Field Model Coil (TFMC) — reached 20.1 T in September 2021 (Phys. Plasmas 30, 070601, 2023)
- Tokamak Energy ST40 — spherical tokamak operating with REBCO TF magnets at Milton Park, UK (record ion temperatures published 2022)
- SuperPower (Furukawa) 2G HTS tape — commercial 4 mm × 100 m+ tape, 500+ A self-field at 77 K (datasheet 2024)
- Helion Polaris — fusion device using REBCO-based pulsed field-reversed-configuration magnets (Everett, WA, expected operation 2024–2025)
- Faraday Factory Japan — laser-deposited REBCO tape used in JT-60SA and ITER R&D coils
Key numbers
- Critical temperature Tc: ~92 K (YBCO); 89–93 K (GdBCO, SmBCO) (Above the 77 K boiling point of liquid nitrogen)
- Upper critical field Hc2 at 4 K: >100 T (In practice limited by mechanical and not magnetic considerations)
- Engineering current density Je: 800–1500 A/mm² (4 K, 20 T) (~10× higher than Nb₃Sn at the same field)
- Operating temperature (fusion magnets): 20 K (Closed-cycle cryocooler practical; eliminates liquid helium)
- Tape cross-section (typical): 4 mm × ~0.1 mm (Hastelloy substrate dominates thickness; REBCO film is 1–4 µm)
- Critical current Ic at 77 K (self-field): 150–500 A per 4 mm tape (SuperPower 2G HTS (FCF) 2024 datasheet)
- Year of REBCO discovery: February 1987 (Wu, Chu et al.) (Triggered cuprate superconductor era)
Surprising facts
- REBCO (yttrium-barium-copper-oxide) was discovered in February 1987 by Maw-Kuen Wu, Paul Chu and colleagues at the University of Houston and the University of Alabama. It was the first material to superconduct above the boiling point of liquid nitrogen (77 K), and the discovery triggered an explosion of cuprate research that earned Bednorz and Müller the 1987 Nobel Prize the same year — possibly the fastest Nobel in physics history.
- A single 4 mm wide REBCO tape can carry 600+ amps at 20 K in a 20 T field. The same cross-section of copper wire at room temperature would carry roughly 12 A before melting from resistive heating — a factor of 50 difference at zero electrical loss.
- Commonwealth Fusion's SPARC reactor will consume roughly 10,000 km of REBCO tape — about a quarter of the global annual production as of 2024. The fusion industry is now the single largest consumer of HTS tape, having overtaken every other application combined within five years.
- The crystal orientation of REBCO must be controlled to within ±5° across the entire kilometre-long tape; a single misaligned grain boundary blocks supercurrent like a dam. Achieving this on a flexible metal substrate via the IBAD-MgO + epitaxial-buffer + MOCVD-superconductor stack is one of the most demanding thin-film deposition processes in any industry.
Open questions
- No-insulation winding instability under fast field ramps: Many REBCO magnets are wound 'no-insulation' (NI) — adjacent turns are in direct metal-to-metal contact, allowing radial current redistribution that improves quench survival. But under rapid field ramps the same radial-current path produces unexpected delayed-charge effects, screening-current-induced fields, and magnetic flux jumps that can shift the field centre by hundreds of ppm — a major problem for MRI applications. The physics is well-understood qualitatively but quantitative predictions still disagree with experiment at the few-percent level.
A short history
The cuprate revolution began in April 1986 when Georg Bednorz and Karl Alexander Müller at IBM Zürich discovered superconductivity at 35 K in La₂₋ₓBaₓCuO₄. Within months, Maw-Kuen Wu, Paul Chu and colleagues at the University of Houston and University of Alabama-Huntsville found that YBa₂Cu₃O₇ (YBCO, the 'Y-123 phase') superconducts at 92 K — the first material to do so above the boiling point of liquid nitrogen. Bednorz and Müller received the 1987 Nobel Prize the same year. For 20 years YBCO was a laboratory curiosity because it could not be drawn into wire — it is a brittle ceramic. The breakthrough came when ion-beam-assisted deposition (IBAD) on flexible Hastelloy substrates was perfected by Y. Iijima and colleagues at Fujikura in the late 1990s, enabling true 'coated-conductor' tape. Commercial 100 m+ piece-lengths reached the market around 2010 (SuperPower, AMSC), and 1 km+ piece-lengths around 2015. The MIT TFMC test in 2021 demonstrated that REBCO could deliver fusion-magnet performance, which in turn unlocked Commonwealth Fusion Systems' Series B and the global compact-fusion industry now under construction.
Sources
- Wu et al. — Phys. Rev. Lett. 58, 908 (1987): Superconductivity at 93 K in YBa₂Cu₃O₇₋ₓ (https://doi.org/10.1103/PhysRevLett.58.908)
- Whyte, D. et al. — Phys. Plasmas 30, 070601 (2023): SPARC and the path to fusion energy (https://doi.org/10.1063/5.0150904)
- Commonwealth Fusion Systems — SPARC magnet program overview (https://cfs.energy/technology/sparc)
- MIT PSFC — TFMC results press release, September 2021 (https://news.mit.edu/2021/MIT-CFS-major-advance-toward-fusion-energy-0908)
- Hahn et al. — Nature 570, 496 (2019): 45.5 T direct current magnetic field generated with REBCO tape (https://doi.org/10.1038/s41586-019-1293-1)
- SuperPower 2G HTS Wire Datasheet (Furukawa Group, 2024) (https://www.superpower-inc.com/specification.aspx)