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 12 mm wide manufactured high-temperature superconductor tapes are compared with those obtained after slitting the tapes to 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 the case of fluctuating critical current, Ic ,ovrl is lower than the average critical current, Ic ,aver . Also, the dissipation concentrates in ‘weak spots’ with reduced critical current. A simple model allows us to estimate the thermal runaway current, I tr , at which the weakest location would convert to a ‘hot spot’ with rapid increase in temperature. In the prediction of I tr , the absolute minimum of the Ic value, I cmin, is essential. Then, by comparing the I cmin /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. It is also interesting to check if the relation between I tr and Ic ,ovrl remains unchanged, because the lower of these two quantities represents the limit of transportable current. We have found a reduction of both I cmin /Ic ,aver and I tr /Ic ,ovrl ratios in those tapes containing the edges of the originally manufactured 12 mm tape. Contrary to our expectations, the most serious drops in slit tapes were not in the same location as in the original tape. However, the systematically better performance of the central strips (the tapes with both edges cut) is a strong indication that the most serious defects are created during manufacturing, close to the tape edges. We conclude that tape cutting by femtosecond laser has not introduced additional non-uniformity. The analysis also shows that the criticality of defects in applications is largely governed by the width of said defects, the cooling conditions and the thickness of the stabilizing layer.