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Plastic damage of REBCO (REBa 2 Cu 3 O 7-x , where RE=rare earth) coated conductors by screening current stress (SCS) is a significant concern for ultra-high-field superconducting magnets. Indeed, the third Little Big Coil (LBC3), a REBCO magnet that generated a record, high field of 45.5 T, showed wavy plastic damage produced by excess SCS in all pancakes except two made with single-slit conductors having their slit edges pointing inward towards the magnet center. Reasons for this slit edge orientation-dependent damage mitigation having not yet been presented, we made it the central issue of this new Little Big Coil (LBC4). Accordingly, we constructed and tested LBC4 by replicating LBC3, except that only single-slit tapes were used and every slit edge pointed inward towards the magnet center. LBC4 reached 44.0 T without quench but with some dissipation. After a small lowering of the current without disappearance of the dissipation, the current was charged again, resulting in a quench at 43.5 T due to excess heating in one pancake-to-pancake joint. Indeed, LBC4 exhibited much less wavy conductor damage than LBC3, demonstrating significant SCS mitigation. Detailed post mortem showed a transverse variation of critical current density ( ) across the LBC4 conductor, being highest at the slit edge and lowest at the not-slit edge. Our computed screening current stresses were markedly lowered by this gradient. This paper shows the importance of considering such transverse variability, which has not previously been considered, in the precise stress analysis of ultra-high-field REBCO magnets.

The magnetic flux pinning capabilities of YBa 2 Cu 3 O 7-x (YBCO) coated conductors vary strongly across different regions of the magnetic field–temperature phase diagram and with the orientation of the magnetic field θ . Here, we determine the optimal pinning landscape for a given region of the phase diagram by investigating the critical current density J c ( H , θ , T ) in the 5–77 K temperature range, from self-field to high magnetic fields of 35 T. Our systematic analysis reveals promising routes for artificially engineering YBCO coated conductors in any region of interest of the phase diagram. In solution-derived nanocomposites, we identify the relevance of coexisting high amounts of short stacking faults, Cu-O vacancy clusters, and segmentation of twin boundaries, in combination with nanoparticles, for enhanced pinning performance at high magnetic fields and low temperatures. Moreover, we demonstrate that twin boundaries preserve a high pinning energy in thick YBCO films, which is beneficial for the pinning performance at high magnetic fields and high temperatures. Optimizing the microstructure of YBa 2 Cu 3 O 7-x coated conductors across the magnetic field–temperature phase diagram is important for strengthening vortex pinning and thereby enhancing the critical current. Here, a systematic microstructural investigation identifies the most relevant vortex pinning contributions in a broad range of temperatures and magnetic fields.

This chapter contains sections titled: Introduction Introduction of Low‐temperature Scanning Laser Microscopy and Its Application in Defect Studies in Superconductors Case Studies of Using LTSLM to Study Defects in Superconductors Conclusions

Residual resistance ratio (RRR) of Cu stabilizer in REBCO coated conductor is an important design parameter for REBCO magnets. In this work, we measured RRR of electroplated Cu stabilizer in commercial REBCO tapes. Over 130 samples were measured for the quality assurance programs of REBCO magnet projects at the National High Magnetic Field Laboratory, USA (NHMFL). The average RRR value was above 50. In order to investigate the factors that influence RRR, several samples were analyzed by using scanning electron microscopy, secondary ion mass spectroscopy, and inductively coupled plasma mass spectroscopy. We found that, in our samples, RRR was strongly correlated with the grain size. We demonstrated that RRR was primarily determined by grain boundary resistivity. Lower RRR was also strongly correlated with higher concentration of chlorine impurity. This is explained by that higher chlorine impurity hindered the grain growth in the room temperature self annealing process resulting smaller grain. Smaller grain resulted in lower RRR. In addition, thermal annealing significantly enhanced RRR. An activation energy of 0.4 eV was obtained from the annealing experiment which corresponds to the activation of Cu grain growth.

The residual low angle grain boundary (GB) network is still the most important current-limiting mechanism operating in bi-axially textured rare earth barium copper oxide (REBCO) coated conductors. While Ca-doping is well established to improve super-current flow across low angle GBs in weak fields at high temperatures, Ca-doping also depresses Tc, making it so far impractical for high temperature applications of REBCO coated conductors. On the other hand, high field magnet applications of REBCO require low temperatures. Here we systematically evaluate the effectiveness of Ca-doping in improving the GB transparency, r\\(^{GB}\\)= Jc\\(^{GB}\\)/Jc\\(^{grain}\\) , of low angle Yb1-xCaxBaCuO [001] tilt bi-crystal films down to 10K and with magnetic fields perpendicular and parallel to the film surfaces, while varying the Ca and oxygen doping level. Using Low Temperature Scanning Laser Microscopy (LTSLM) and Magneto-Optical Imaging (MOI), we found rGB to strongly depend on the angle between magnetic field and the GB plane and clearly identified regimes in which Jc\\(^{GB}\\) can exceed Jc\\(^{grain}\\) (r\\(^{GB}\\)>1) where the GB pinning is optimized by the field being parallel to the GB dislocations. However, even in this favorable situation, we found that r\\(^{GB}\\) became much smaller at lower temperatures. Calculations of the GB Ca segregation profile predict that the high Jc channels between the GB dislocation cores are almost Ca-free. It may be therefore that the positive effects of Ca doping seen by many authors near Tc are partly a consequence of the higher Tc of these Ca-free channels.

The magnetic flux pinning capabilities of YBa2Cu3O7-x (YBCO) coated conductors (CCs) vary strongly between different regions of the magnetic field-temperature (H-T) diagram and with the orientation of the magnetic field (θ). Here, we determine the optimal pinning landscape for a given H-T region by investigating the critical current density Jc(H,θ,T) in the 5-77 K temperature range, from self-field to very high magnetic fields (35 T). Our systematic analysis reveals the best directions to target to artificially engineer CCs in any region of interest. In solution-derived nanocomposites, we identify the relevance of coexisting high amounts of short stacking faults, Cu-O vacancy clusters and segmentation of twin boundaries, in combination with nanoparticles, for enhanced pinning performance at very high magnetic fields and low temperatures. Moreover, we demonstrate that twin boundaries preserve a high pinning energy in thick YBCO films, which is beneficial for the pinning performance at high magnetic fields and high temperatures.