Analysis of critical current fluctuations as a means of checking the quality of high-temperature superconductor tape slitting

Abstract

The patterns of longitudinal Ic fluctuations in the 12 mm wide production HTS tapes are compared with those obtained after slitting them to the widths of 6 mm, 4 mm and 3 mm, respectively. In the statistical analysis of Ic(x) data, the overall critical current, Ic,ovrl, was first calculated. In case of fluctuating critical current, Ic,ovr is lower than the average of critical current, Ic,aver. Also, the dissipation concentrates in the “weak spots” with reduced critical current. Simple model allows to estimate the thermal runaway current, Itr, at which the weakest location would convert to a “hot spot” with rapid increase of temperature. In the prediction of Itr, essential is the absolute minimum of Ic value, Icmin. Then, by comparing the Icmin/Ic,aver ratio in the slit tapes to that established previously for the manufactured tape, one can deduce if some new defects have been introduced during slitting. Interesting is also to check if the relation of Itr vs. Ic,ovrl remained without changing, because the lower of these two quantities represents the limit of transportable current. We have found a reduction of both Icmin/Ic,aver and Itr /Ic,ovrl ratios in some tapes, containing the edges of the original 12 mm production tape. Contrary to our expectations, the most serious drops in slit tapes were not in the same location as in the original tape. However, systematically better performance of the central strips (the tapes with both edges cut) is a strong indication that the most serious defects are created already during manufacturing, close to the tape edges. We conclude that the tape cutting by femtosecond laser has not introduced an additional non-uniformity. The analysis also shows that the criticality of defects in applications is largely governed by the width of said defect, the cooling conditions and the thickness of the stabilizing layer.

Winding angle optimization and testing of small-scale, non-planar, high-temperature superconducting stellarator coils

“…These show a clear degradation of the critical current with many dropouts to I C = 0. This shows the much better mechanical stability of laser-slit tape; tape slit with a rolling knife has cracks in the microstructure of the HTS crystal that open under strain that would be tolerable for an undamaged tape [24,25].…”