by Dr. Lance Cooley, Technical Division–Fermilab, ldcooley@fnal.gov; and Dr. David Larbalestier, Director, Applied Superconductivity Center–Florida State University, larbalestier@asc.magnet.fsu.edu

Our close friend David Sutter, long-time leader of Advanced Accelerator R&D in the Office of High Energy Physics, once remarked that superconductivity would never be used unless there was absolutely no other choice. Past and present superconducting magnet technology suggests that cheap, strong, available-by-the-ton, Nb-Ti alloy conductors will always be used unless it’s absolutely necessary to use another superconducting material. In this decade “absolutely necessary” has taken on a clearer definition, thanks to ITER, the upgrade of LHC, 1 GHz NMR magnets, the desire for >30 T frontier-science user magnets, as well as superconducting magnet systems operating at well above the ~5 K limit of Nb-based superconductors. Especially the attractive temperature space above 20-30 K, now mainly the province of demonstrations, could show that helium-free magnets, rotating electric machines, electric utility devices and the like are possible and perhaps capable of making totally new technologies for superconducting magnet devices in the oft-predicted but, so far, not yet commercial liquid nitrogen domain.

ITER and LHC have established Nb3Sn as a true workhorse. The 600-ton ITER strand procurement developed new manufacturing infrastructure essential to the coming 50-ton procurement for the High-Luminosity Upgrade of the LHC and beyond. “Hi-Lumi” strand for accelerator magnet production at nearly 12 T could become the starting point for further conductor development, perhaps enabling ~16 T main ring magnets and a very large collider at ~100 TeV.

This is not assured, however: Demand for Nb3Sn has no analog of the MRI industry that continued to pull Nb-Ti after the Tevatron and its successors. So, potentially transformative ideas to improve performance further, as well as manufacturing innovations that might drive Nb3Sn toward commodity rates, could struggle to find sufficient support to fully emerge and displace present production conductors. Perhaps this will be the driver that makes HTS materials “absolutely necessary” to move beyond the present limit of Nb3Sn.

Magnet technology prefers round, isotropic, multi-filamentary, strong conductors. It will be exciting to watch the competition between round Bi-2212 and flat tape Bi-2223 and REBCO for next-up status, since none so far has a complete set of desirable characteristics and all are several times more expensive than Nb3Sn. Bi-2212 can be made in 2 km single pieces as twisted multi-filament conductor and then into Rutherford cables. But strict requirements of a silver matrix, high quality powder and high-pressure melt-texturing are challenging requirements without obvious alternatives. Bi-2223 comes as a mature, multi-filamentary tape in variously laminated forms, but the lack of pathways for cables will probably restrict it to solenoids and pancake coils. Both Bi-2212 and Bi-2223 benefit from a higher stability provided by much higher critical temperatures (Tc) than for Nb-Ti and Nb3Sn, while also, at high fields, not having too high a difference between 4.2 K and the critical surface, a great benefit for magnet protection.

REBCO is evolving quickly as an ultrahigh field magnet conductor. It is clearly the simplest route to fields above the 23 T maximum of Nb3Sn and the achievement of a little over 40 T with test coils sets an impressive mark. With up to seven manufacturers worldwide vying to supply conductor, engineering of both the vortex pinning in the REBCO layer and the substrate is leading to distinctive property baskets from one manufacturer to another.

A dramatic reduction of substrate thickness to as little as 30 mm in SuperPower’s product, coupled with Conductor on Round Core (CORC) cables at Advanced Conductor Technology, has transformed REBCO from a mono-filament monolith into a macroscopically isotropic, round, multi-filament high current density conductor. It is anticipated that such a transposed, cabled conductor will be able to share current around drop-outs and defects. We anticipate that reduction of the substrate thickness toward ~20 µm could provide ~3 mm diameter CORC “wires” with 50 individual strands that could greatly stimulate REBCO magnet development. The no-insulation approaches for magnet technology used by Seungyong Hahn’s NHMFL team to achieve 40 T in a resistive background of 31 T reclaims substrate and normal metal stabilizer real estate for magnet protection, allowing a several times increase in magnet operating current density to values well above 500 A/mm2.

A very exciting development cycle may now be at hand, in which conductor and magnet construction innovations result in more conductor choices and frontier test magnets, inciting new magnet demands that will pull further conductor innovation. Especially vital will be manufacturing innovation to make thicker REBCO in longer piece lengths with higher yields that can finally deliver on the promise of the (almost) Ag-free, low cost conductors that REBCO conductor promoters have so widely promised.

At the moment, “absolutely necessary” applications beyond 20 T have not taken solid form. Development pull is presently coming from user magnets and advanced NMR systems. Fusion science and compact reactors with fields on the plasma of twice those of ITER would be a huge pull for REBCO. A wild card that could emerge in the meantime is a conductor based on (Ba,K)Fe2As2, an almost isotropic compound with Tc of almost 40 K, an upper critical field (Hc2) of almost 90 T and capability to be made like a Bi-2212 round wire with Cu rather than Ag stabilizer.

We close by speculating a bit about the future of low-field magnets at >5 K temperature. MgB2 has life here, having been displaced from high field consideration because its Hc2 is still less than that of Nb-Ti at 4.2 K. MgB2 must deliver on the basis of low cost, since it is threatened by REBCO at 30 to 50 K where cryogenic costs are lower. Wind turbines are an interesting prospect for strong end-use pull for large-scale production of MgB2 and HTS field coils. If the economics make it “absolutely necessary” to achieve flux density higher than what can be attained with permanent magnets, iron or induction coils, then magnet and conductor development could repeat the beneficial development cycle that Nb-Ti accelerator magnets and MRI systems went through to create commodity-rate conductors. The EcoSwing presentations at the Denver ASC showed the first full-size REBCO field coils for turbines. Perhaps the 2026 ASC will look back on this as a pivotal application.