BaZrO 3 (BZO) nanorods are now incorporated into production IBAD-MOCVD coated conductors. Here we compare several examples of both BZO-free and BZO-containing coated conductors using critical current (I c ) characterizations at 4.2 K over their full angular range up to fields of 31 T. We find that BZO nanorods do not produce any c-axis distortion of the critical current density J c () curve at 4.2 K at any field, but also that pinning is nevertheless strongly enhanced compared to the non-BZO conductors. We also find that the tendency of the ab-plane J c () peak to become cusp-like is moderated by BZO and we define a new figure of merit that may be helpful for magnet design -the OADI (Off-Axis Double I c ), which clearly shows that BZO broadens the abplane peak and thus raises J c 5-30° away from the tape plane, where the most critical approach to I c occurs in many coil designs. We describe some experimental procedures that may make critical current I c tests of these very high current tapes more tractable at 4.2 K, where I c exceeds 1000 A even for 4 mm wide tape with only 1 m thickness of superconductor. A positive conclusion is that BZO is very beneficial for the J c characteristics at 4.2 K, just as it is at higher temperatures, where the correlated c-axis pinning effects of the nanorods are much more obvious.

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REBCO (RE = rare earth) based high temperature superconducting (HTS) wires are now being utilized for the development of electric and electromagnetic devices for various industrial, scientific and medical applications. In the last several years, the increasing efforts in using the so-called second generation (2G) HTS wires for some of the applications require a further increase in their engineering current density (Je). The applications are those typically related to high magnetic fields where the higher Je of a REBCO wire, in addition to its higher irreversibility fields and higher mechanical strength, is already a major advantage over other superconducting wires. An effective way to increase the Je is to decrease the total thickness of a wire, for which using a thinner substrate becomes an obvious and attractive approach. By using our IBAD-MOCVD (ion beam assisted deposition-metal organic chemical vapor deposition) technology we have successfully made 2G HTS wires using a Hastelloy® C276 substrate that is only 30 μm in thickness. By using this thinner substrate instead of the typical 50 μm thick substrate and with a same critical current (Ic), the Je of a wire can be increased by 30% to 45% depending on the copper stabilizer thickness. In this paper, we report the fabrication and characterization of the 2G HTS wires made on the 30 μm thick Hastelloy® C276 substrate. It was shown that with the optimization in the processing protocol, the surface of the thinner Hastelloy® C276 substrate can be readily electropolished to the quality needed for the deposition of the buffer stack. Same in the architecture as that on the standard 50 μm thick substrate, the buffer stack made on the 30 μm thick substrate showed an in-plane texture with a Δϕ of around 6.7° in the LaMnO3 cap layer. Low-temperature in-field transport measurement results suggest that the wires on the thinner substrate had achieved equivalent superconducting performance, most importantly the Ic, as those on the 50 μm thick substrate. It is expected the 2G HTS wires made on the 30 μm thick Hastelloy® C276 substrate, the thinnest and with the highest Je to date, will greatly benefit such applications as high field magnets and high current cables.

The influence of Zr doping in (Gd, Y)-Ba-Cu-O ((Gd, Y)BCO) tapes made by metal-organic chemical vapor deposition has been studied with a specific objective of uniform and reproducible enhancement in in-field critical current (I c ) over long lengths. 50 m long tapes with 7.5 and 10 at.% Zr doping in 1 µm thick (Gd, Y)BCO films have been found to exhibit a sharply enhanced peak in I c in the orientation of field parallel to the c-axis and retain 28% of their self-field I c value at 77 K and 1 T. BaZrO 3 (BZO) nanocolumn density in the cross-sectional microstructure was found to increase with increasing Zr addition. The end segments of the 50 m long tapes were found to display nearly identical angular dependence of critical current at 77 K and 1 T, indicative of the uniformity in in-field performance over this length. A 610 m long tape was fabricated with 10% Zr doping and a 130 m segment showed a 3.2% uniformity in critical current measured every meter in the orientation of B c-axis. A retention factor of 36% of the zero-field I c value measured at 0.52 T over the 130 m is consistent with that obtained in short samples.

Next generation particle accelerators and fusion machines will greatly benefit from the development of low-inductance magnets capable of generating magnetic fields in excess of 16 T. Such magnets require high-temperature superconductors capable of carrying very high currents exceeding 5 kA at current densities of 400-600 A mm −2 , such as Conductor on Round Core (CORC ® ) cables and wires wound from RE-Ba 2 Ca 3 O 7-δ (ReBCO, Re=rare earth) coated conductor tapes. CORC ® wires containing ReBCO tapes with 30 μm thick Hastelloy ® substrates have previously been demonstrated as a viable high-field magnet conductor that can be produced at long lengths. Further improvement of the performance and flexibility of CORC ® wires would benefit from the development of ReBCO tapes with even thinner substrates. SuperPower Inc. recently demonstrated ReBCO tapes based on 25 μm thick Hastelloy ® substrates that allow the development of thinner and more flexible CORC ® wires that meet the stringent performance requirements of high-field magnets. Several tapes containing 25 μm thick substrates were produced and analyzed, exhibiting critical current and cabling performance in-line with the current production level tapes with 30-50 μm thick substrates. Tape critical current was measured at 4.2 K and applied magnetic fields up to 31.2 T. Several CORC ® wires incorporating these tapes were manufactured by Advanced Conductor Technologies using similar winding procedures that previously resulted in high-quality magnet-grade CORC ® wires based on tapes with 30 μm thick substrates. The CORC ® wires were tested in an applied magnetic field up to 12 T after bending to a 63 mm diameter. A critical current as high as 6231 A (12 T, 4.2 K) was measured with an engineering current density (J e ) of 678 A mm −2 , which extrapolates to over 450 A mm −2 at 20 T and is the highest current density reported in a CORC ® conductor to date. The combination of ReBCO tapes produced using 25 μm thick substrates and the ability to wind them into longlength, high-quality CORC ® magnet wires brings the development of low-inductance accelerator and fusion magnets that operate at magnetic fields exceeding 20 T closer to fruition.