No abstract

We present a broad study by multiple techniques of the critical current and critical current density of a small but representative set of nominally identical commercial RE123 (REBa2Cu3O7−δ, RE = rare Earth, here Y and Gd) coated conductors (CC) recently fabricated by SuperPower Inc. to the same nominal high pinning specification with BaZrO3 and RE2O3 nanoprecipitate pinning centers. With high-field low-temperature applications to magnet technology in mind, we address the nature of their tape-to-tape variations and length-wise Ic inhomogeneities by measurements on a scale of about 2 cm rather than the 5 m scale normally supplied by the vendor and address the question of whether these variations have their origin in cross-sectional or in vortex pinning variations. Our principal method has been a continuous measurement transport critical current tool (YateStar) that applies about 0.5 T perpendicular and parallel to the tape at 77 K, thus allowing variations of c-axis and ab-plane properties to be clearly distinguished in the temperature and field regime where strong pinning defects are obvious. We also find such in-field measurements at 77 K to be more valuable in predicting 4.2 K, high-field properties than self-field, 77 K properties because the pinning centers controlling 77 K performance play a decisive role in introducing point defects that also add strongly to Jc at 4.2 K. We find that the dominant source of Ic variation is due to pinning center fluctuations that control Jc, rather than to production defects that locally reduce the active cross-section. Given the 5–10 nm scale of these pinning centers, it appears that the route to greater Ic homogeneity is through more stringent control of the REBCO growth conditions in these Zr-doped coated conductors.

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.

A no-insulation (NI) pancake coil was wound with a 23 m long REBCO tape containing multiple ‘defects’, at which local critical currents are substantially lower (<80%) than the tape’s lengthwise average. During a charging test up to 60 A in a bath of liquid nitrogen at 77 K, the test coil showed ‘defect-irrelevant’ behaviors, i.e. coil terminal voltages and center fields in steady-state operations were barely discernible from those of an ideal ‘defect-free’ coil. Also, the critical current (Ic) of the coil was measured to be 68 A, close to 72 A estimated from the measured Ic angular dependency data of a defect-free sample tape that was used for construction of the coil. The results demonstrate a potential to build a pancake coil with REBCO tapes regardless of their defects, which may lead to a substantial reduction in the construction cost of high field NI REBCO magnets.