Longitudinal inhomogeneity of DC current transport properties in Gd-system HTS tapes – Statistical approach for system design

Nakamura, Taketsune; Takamura, Yutaka; Amemiya, Naoyuki; Nakao, Kōichi; Izumi, Teruo

doi:10.1016/j.cryogenics.2014.05.004

Cited by 15 publications

(8 citation statements)

References 25 publications

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“…Fluctuations of the superconductor properties along the tape length can be represented as the variation of both the critical current and the power exponent along the tape length, and inserted in the computation of the U(I) dependence [60]. In some reports, these quantities correlate [29,61], and in some cases the fluctuation of n was found weaker [62] or negligible [63,64]. In order to keep the basic idea simple we neglect here the fluctuations of n and represent the changing tape properties by varying I c only.…”

Section: Overall Critical Current Of Tape With I C Fluctuationsmentioning

confidence: 99%

Impact of critical current fluctuations on the performance of a coated conductor tape

Gömöry

Šouc

Adámek

et al. 2019

Supercond. Sci. Technol.

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Coated conductor (CC) tapes often exhibit critical current, Ic, which fluctuates along the length coordinate, x, leading to an Ic(x) dependence. The need to reduce the price of these tapes does not favor a decrease in the variation of Ic in the near future. Therefore, it is reasonable to develop a method for estimating the possible impact of such fluctuating transport ability on the performance of superconducting devices. Fortunately, providing Ic(x) data together with the delivered conductor is becoming a standard approach among CC tape producers, and in-depth analysis of such data is now possible. Extrapolation of the short sample testing methodology to the case of a device incorporating many meters of CC tape leads to the introduction of an ‘overall critical current’ as a value generating 1 μV cm−1 of electric field over the whole conductor. We show that in the case of Ic(x) fluctuations obeying the Gaussian distribution one can predict the value of the overall critical current from the mean value and the variance of the Ic(x) data set. Deviation from the Gaussian distribution found in real tapes would cause a further reduction of the overall critical current. Conversely, in the case of strong dropouts in the critical current value, a statistical approach is useless and one must analyze the probability of the weakest location developing in a hot spot with a dramatic increase of temperature. Extending the analysis for a single tape to cables and coils results in a rather simple summary: when current sharing is possible, the reduction of overall critical current with respect to its mean value is significantly depressed. This favors the parallel arrangement of tapes with low contact resistance allowing current migration from tape to tape. On the other hand, the insulation of tapes leads to the same overall critical current reduction as observed for a single tape.

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Section: Overall Critical Current Of Tape With I C Fluctuationsmentioning

confidence: 99%

Impact of critical current fluctuations on the performance of a coated conductor tape

Gömöry

Šouc

Adámek

et al. 2019

Supercond. Sci. Technol.

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“…With the help of statistical methods, one can implement the fluctuations of critical current into the design of a superconducting device, provided the conductor properties follow a regular pattern [64][65][66][67][68]. Nevertheless, the focus of this paper is on an 'individual' inhomogeneity like the one shown in figure 1.…”

Section: Introductionmentioning

confidence: 99%

Stability of DC transport in HTS conductor with local critical current reduction

Gömöry

Šouc

2021

Supercond. Sci. Technol.

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A common feature of commercially available conductors based on high-temperature superconducting compounds is the fluctuation of critical current along the length. Fortunately, the practice adopted by manufacturers nowadays is to supply the detailed I c(x) data with the conductor. Compared to knowing just the average of critical current, this should also allow a much better prediction of the conductor performance. Statistical methods are suitable for this purpose in the case when the fluctuations are regular at the low end of critical current distribution. However, a different approach is necessary at the presence of ‘weak spots’ that drop out of any statistics. Because of the strong nonlinearity of the current–voltage curve, such a location could transform into a ‘hot spot’ at transporting direct current (DC), with an abrupt increase of temperature endangering the conductor operation. We present a set of analytical formulas including the prediction of the maximum DC that could be carried sustainably before the thermal runaway appears. It is necessary to know the cooling conditions as well as the properties of the conductor constituents and their architecture. A formula for the voltage appearing on a weak spot, and its dependence on the DC, is also proposed. For this purpose the result of previous theoretical work has been slightly modified after comparing it with numerical iterative computations and finite element modeling. We demonstrate that the derived model allows a powerful analysis of experimental data comprising an estimation of the weak spot parameters i.e. its critical current and the length of the defect zone.

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“…712,14,15) The same situation takes place under the existence of the heterogeneously distributed I c -reducing defects, introduced during the tape-fabrication process. 22,23) It is required to have full knowledge of the relation of heterogeneously distributed defects/cracks to superconducting property for safety design.…”

Section: Introductionmentioning

confidence: 99%

Effects of Size of the Largest Crack and Size Difference among Cracks on Critical Current of Superconducting Tape with Multiple Cracks in Superconducting Layer

Ochiai

Okuda

2020

Mater. Trans.

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The effects of the size of the largest crack and size difference among cracks on critical current of superconducting tape with multiple cracks of different sizes in the superconducting layer were investigated by a model analysis and a Monte Carlo simulation, using the specimens consisting of a series circuit of local sections where each section has one crack of different size from each other. It was revealed that, with increasing distribution-width of crack size, the increase in the size of the largest crack acts to reduce the critical current and the increase in the crack size-difference among the sections acts to raise the critical current, and these conflicting two effects are summed up and determine the critical current value. To describe this feature quantitatively, we expressed the statistics of the size of the largest crack in specimens with the Gumbel's extreme value distribution function. Also we monitored the effect of the difference in crack size among the sections on specimen's critical current, using the number of the sections equivalent to the largest crack-section at the critical voltage for determination of specimen's critical current. In this monitoring, small number of the sections equivalent to the largest crack-section corresponds to large difference in crack size among the sections. With the present approach, the effect of the increase in size of the largest crack, which acts to reduce critical current, and the effect of the increase in difference in crack size among the sections, which acts to raise critical current, with increase in distribution-width of crack size, could be estimated separately. [

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Longitudinal inhomogeneity of DC current transport properties in Gd-system HTS tapes – Statistical approach for system design

Cited by 15 publications

References 25 publications

Impact of critical current fluctuations on the performance of a coated conductor tape

Impact of critical current fluctuations on the performance of a coated conductor tape

Stability of DC transport in HTS conductor with local critical current reduction

Effects of Size of the Largest Crack and Size Difference among Cracks on Critical Current of Superconducting Tape with Multiple Cracks in Superconducting Layer

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