Analysis of local burnout in a sub-scale test coil for the 32 T magnet after spontaneous quenches during fast ramping

Industrial production of REBa2Cu3O7-delta (REBCO) coated conductors has made it possible to construct the 32 T magnet, the first successful all-superconducting user magnet to exceed 30 T, which now serves users as SCM4 (superconducting magnet) at the NHMFL. Here, we present an analysis of the damage that occurred in late-stage proof testing of the 32 T prototype coil after many essential facets of the design had been proven through more than 100 intentionally triggered quenches at fields up to 24 T. This prototype coil was then subjected to accelerated charge-discharge cycles at a rate 44 times faster than its design ramp rate in an attempt to address its fatigue tolerance. The extra hysteresis loss of the fast ramps led to heating of the end pancakes, which was induced after 55 fatigue cycles, three spontaneous quenches at progressively lower currents. Recognizing that the coil was damaged, the pancakes were then unwound and their REBCO tapes run through our continuous in-field transport I-c and remnant-field magnetization monitoring device, YateStar, which revealed three highly localized zones of low I-c in the end pancake that induced quench. Careful examination of these zones, especially the most intensely damaged one, revealed that the worst hot spot reached at least 779 degrees C during the quenches. Magneto-optical imaging showed that this damaged zone was about 5 mm in diameter and indeed the perpendicular damage length induced in neighboring turns by this localized quench heating was almost as great. Although there is much present concern about fatigue crack propagation from edge defects, we actually attribute this damage not to fatigue but to fluctuations in vortex pinning density due to imperfect BaZrO3 (BZO) nanorod growth that locally reduced the critical current I-c. These localized low-I-c regions then had to shed their excess current into the copper stabilizer, producing intense heating. We provide transmission and scanning electron microscopy evidence for local fluctuations of the BZO pinning structure and relate it to recent work that shows significant variations of 4 K, high field I-c values due to apparent production fluctuations of the growth conditions of the Zr-doped metal-organic chemical vapor deposition REBCO used for this test magnet.