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The 'Little Big Coil (LBC)' magnet test bed investigates key coated conductor characteristics in a high field and high stress environment by testing a REBCO coil in the bore of the NHMFL 31.1 T Bitter magnet. Indeed, LBC3 first demonstrated excessive screening current stress (SCS) resulting in wavy plastic conductor damage, while obtaining the record-high DC magnetic field of 45.5 T by generating 14.4 T in its 31.1 T background field. The important result of LBC3 was that single slit tapes with slit edges pointing towards the coil center were barely damaged while those with opposite orientation showed large damage. To explore this apparent paradox, LBC4 was wound completely from single-slit tapes whose slit edges were all pointed towards the coil center. Indeed, almost no damage was seen on LBC4 attaining 44.0 T. Our post mortem showed that the key reason was a specific peculiarity of the MOCVD tapes used in our LBC coils. The edges of the tapes have a markedly higher density of current-blocking CuO and a-axis grains that markedly lower the edge $J_c$ below that of the more central slit edge even with its cracked edge. Measurement of the transverse $J_c$ variability and implementation of the local edge $J_c$ into the SCS model yielded stresses consistent with the transition from large SCS-initiated damage in LBC2 (42.5 T) and 3 (45.5 T) to much lower SCS damage in LBC4. We are now investigating new Little Big Coils using recent laser-slit, thicker REBCO, and PLD conductors made for compact superconducting fusion magnets to obtain a direct comparison with mechanically slit MOCVD conductors that we have used until now.
Acknowledgment: A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement No. DMR-2128556 and the State of Florida. This work is primarily supported by the DOE Office of Fusion Energy Sciences Grant DE-SC0022011. Many aspects of this work have been done in collaboration with the groups of Professor Seungyong Hahn at Seoul National University and Professor So Noguchi at Hokkaido University.
