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.

REBCO coated conductors are now being used for building very high-field magnets with large electromagnetic stresses, both expected ones due to transport current ⃗ J × ⃗ B stresses and additional stresses resulting from the large screening currents inherent in wide tapes. Post mortem analyses of several recent test coils operated above 40 T show that significant conductor plastic deformation occurs, even for JBR stresses well below the ∼1 GPa yield of the Hastelloy substrate of the conductor. To investigate these deformation mechanisms, conductors were unwound after coil test and carefully examined with respect to their length-wise I c which revealed many areas of local damage. Regions of interest were examined by metallographic cross-section, Hall microscopy, magneto-optic imaging and scanning electronic microscopy. Important damage frequently occurred to the outer edges of pancakes in the coil ends, which were often plastically deformed over the whole turn circumference, especially when this outer edge was a slit edge. Internal conductor damage was also seen, especially delamination between the buffer and REBCO layers at slit edges. Careful sectioning of the tape at ∼10 mm intervals showed that the plastic deformation of the turns was complex and variable around the turn circumference, with tape cross-sections that exhibited continuous shape change in the outer turns. The bending center line of tapes often shifted from the tape center line toward the edge closest to the coil center, indicating asymmetric effects of transport and screening current stresses across the conductor width. A surprising and vital result is that damage was prevalent when the slit edge was also the edge at which transport current flowed. This damage was absent when the transport current flowed at the not-slit edge, implying great sensitivity of the effect of screening current stresses to localized conductor damage.

Conductor on Round Core (CORC®) wires and cables, constructed from helically wound layers of REBCO tapes. An important feature of HTS cables is the ability to share current between conductors, allowing current to bypass drops in Ic and minimizing the risk of hot spot formation, which could lead to potential burnout in the superconductor. In CORC® the current transfer at discrete tape crossovers. The tape-to-tape contact resistance, Rc, is critical to current sharing and current distribution in cables. Special CORC® wires were manufactured using different winding parameters to investigate variations in Rc. Variations included different winding lubricants as well as pre-tinning. Cables were tested as straight lengths, followed by bending to a 10 cm diameter. In straight cables Rc values ranged from 1 to over 1,000 μΩ·cm2, depending on cabling parameters, with the highest values occurring for cables made by the present “standard” process. Bending the cables to a 10 cm diameter decreased Rc by a factor 2–5. Tinning with PbSn decreased Rc by 3 orders of magnitude compared to standard CORC® wires, and heat treating the tinned wires resulted in only a small decrease in Rc. Based on the measured Rc at an electric field of 1 μV/cm the resulting current transfer length between layers ranges from a few millimeters to a tens of centimeters. Examination of contacts with a laser confocal microscope showed plastic deformation of the copper at the edges of the crossovers, caused by thicker plating at tape edges, suggesting that only a fraction of the total crossover may actually be in contact. Images of the PbSn coated tapes indicated that application of solder produces a more uniform contact surface. CORC® cross-sectional images depict regions no contact between tapes in the non-tinned cables, in contrast the PbSn cable shows significantly more contact between the tapes.