Speaker
Description
Over almost a decade, the ‘Little Big Coil (LBC)’ program at the Applied Superconductivity Center (ASC) of the National High Magnetic Field Laboratory (NHMFL) has focused on investigating the various limitations of rare-earth barium copper oxide coated conductors (REBCO CCs) in high-field (>40 T) and high-stress (>800 MPa) environments. Under the key philosophy of “no magnets are better than conductors,” we first perform detailed conductor characterizations, choose tapes for coil winding, design and fabricate a coil, test it in the NHMFL 31.1 T Bitter magnet, push it to its limits until the coil quenches, and then conduct a detailed postmortem. The goal is to have a recursive design-build-test loop, which can guide us in refining our conductor and coil simulation models. Over the last three years, our key LBC test findings are: 1) the Jc gradient across conductor width can be used to mitigate screening current stress (SCS) and subsequent wavy plastic conductor damage; 2) a coil wound with a higher Ic conductor shows better survivability from coil quench than one wound with a low Ic conductor; 3) the coil performance can be self-limited by conductor damage initiated at outward edges by excessive SCS, leading to shift of the electric field center after plastic edge damage, and 4) another key consideration in such high-field and high-stress magnet design might be temperature margin. Incorporating all these key findings, we recently made a ninth LBC (LBC9). This coil generated 17.6 T in the 31.1 T, therefore 48.7 T in total, which is a new record of direct-current magnetic field. A very intriguing observation from the coil unwinding was the absence of SCS-initiated wavy plastic deformation that has been so common in earlier LBCs. In this talk, we will present characterization, design, construction, test, and postmortem of recent >40 T ultra high-field, > 800 MPa high-stress REBCO coils, including detailed conductor models and coil simulations. Our goal is to define much better the key parameters controlling the high-stress, high-field behavior of these REBCO test coils.
