Evidence of Kramer extrapolation inaccuracy for predicting high field Nb3Sn properties
01 Jun 2020-Vol. 1559, Iss: 1, pp 012062
TL;DR: In this paper, a variety of both RRP® and PIT Nb3Sn wires were characterized by transport measurements up to 29 T at the Laboratoire National des Champs Magnetiques Intenses (LNCMI), part of the European Magnetic Field Laboratory in Grenoble, to verify whether or not such overestimation is related to the measurement technique.
Abstract: Future applications requiring high magnetic fields, such as the proposed Future Circular Collider, demand a substantially higher critical current density, Jc, at fields ≥16 T than is presently available in any commercial strand, so there is a strong effort to develop new routes to higher Jc Nb3Sn As a consequence, evaluating the irreversibility field (Hirr) of any new conductor to ensure reliable performance at these higher magnetic fields becomes essential To predict the irreversibility field for Nb3Sn wires, critical current measurements, Ic, are commonly performed in the 12-15 T range and the Kramer extrapolation is used to predict higher field properties The Kramer extrapolation typically models the contribution only for sparse grain boundary pinning, yet Nb3Sn wires rely on a high density of grain boundaries to provide the flux pinning that enables their high critical current density However, whole-field range VSM measurements up to 30 T recently showed for Nb3Sn RRP® wires that the field dependence of the pinning force curve significantly deviates from the typical grain boundary shape, leading to a 1-2 T overestimation of Hirr when extrapolated from the typical mid-field data taken only up to about 15 T In this work we characterized a variety of both RRP® and PIT Nb3Sn wires by transport measurements up to 29 T at the Laboratoire National des Champs Magnetiques Intenses (LNCMI), part of the European Magnetic Field Laboratory in Grenoble, to verify whether or not such overestimation is related to the measurement technique and whether or not it is a common feature across different designs Indeed we also found that when measured in transport the 12-15 T Kramer extrapolation overestimates the actual Hirr in both types of conductor with an inaccuracy of up to 16 T, confirming that high field characterization is a necessary tool to evaluate the actual high field performance of each Nb3Sn wire
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TL;DR: In this article , the authors proposed a procedural solution for determining the temperature dependence of transport critical current in Nb3Sn superconducting wires using magnetization measurements, based on experimental data using internal tin-type wire, but also address the theoretical implications that would be generally applicable to other wire designs.
Abstract: Using magnetization techniques to determine the temperature dependence of critical current in Nb3Sn wires is attractive because of the relative ease compared with using variable-temperature transport measurements. However, there is a known mismatch in the temperature scaling characterizations when using magnetization data compared to transport data. From a practical standpoint, it is the transport properties that matter, as performance prediction, margin calculations, and other aspects of magnet designs rely on the knowledge of the amount of transport current the superconductor can carry in a magnetic field. In this paper, we will identify the underlying issues and propose a procedural solution for determining the temperature dependence of transport critical current in Nb3Sn superconducting wires using magnetization measurements. By using one standard transport measurement at 4.2 K as an ‘anchor’, with this procedural solution it becomes possible to combine the simplicity and economy of quick magnetization measurements at different temperatures with the accurate prediction of transport properties in extrapolated temperature ranges. This study is based on experimental data using internal tin-type wire, but we also address the theoretical implications that would be generally applicable to other wire designs. The strength of our analysis is that our work leads to proposed procedures that improve the accuracy of the temperature scaling even if the assumed pinning curve shape does not fit the data as well at some temperatures, whether those temperatures are close to 4.2 K or to T C.
References
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TL;DR: In this paper, it was shown that the peak in Fp scales as [Hc2(T)]2.5 if the temperature is changed; the maximum value of Fp occurred at the same value of reduced field regardless of temperature.
Abstract: For all hard high‐field superconductors examined to date, there is a maximum in the pinning force density Fp as a function of the reduced magnetic field h. Fietz and Webb first demonstrated in dilute Nb alloys that the peak in Fp scales as [Hc2(T)]2.5 if the temperature is changed; the maximum value of Fp occurred at the same value of reduced field regardless of temperature. Recent data on the temperature dependence of pinning in Nb3Sn, Nb–25% Zr and a Nb–Ti alloy, which exhibits the ``peak effect'', are analyzed to show that similar scaling laws are obeyed by these materials. All presently available evidence indicates however that the reduced field hp at which the maximum Fp occurs, as well as the height and shape of this maximum, can be altered by metallurgical treatment. Apparently weak pinning defects, or widely spaced ones, produce a small peak in Fp(h) at high h whereas strong closely spaced pins produce a large peak in Fp(h) at low h without producing much change in Fp(h) at high h. A model which p...
1,104 citations
TL;DR: In this article, the authors examined the upper critical field of a large and representative set of present multifilamentary Nb3Sn wires and one bulk sample over a temperature range from 1.4 K up to the zero-field critical temperature.
Abstract: We have examined the upper critical field of a large and representative set of present multifilamentary Nb3Sn wires and one bulk sample over a temperature range from 1.4 K up to the zero-field critical temperature. Since all present wires use a solid-state diffusion reaction to form the A15 layers, inhomogeneities with respect to Sn content are inevitable, in contrast to some previously studied homogeneous samples. Our study emphasizes the effects that these inevitable inhomogeneities have on the field-temperature phase boundary. The property inhomogeneities are extracted from field-dependent resistive transitions which we find broaden with increasing inhomogeneity. The upper 90%–99% of the transitions clearly separates alloyed and binary wires but a pure, Cu-free binary bulk sample also exhibits a zero-temperature critical field that is comparable to the ternary wires. The highest μ0Hc2 detected in the ternary wires are remarkably constant: The highest zero-temperature upper critical fields and zero-fiel...
114 citations
CERN1
TL;DR: In this paper, the authors provide a summary of desirable properties and performance targets for Nb3Sn to satisfy the challenging magnet specifications for upgrades of existing and future HEP accelerators, which is the only practical candidate material offering the required high field performance.
Abstract: High Energy Physics has been consistently pushing the performance of technical superconductors, for the benefit of high field magnet technology. So far the workhorse for particle accelerators has been Nb-Ti, but the practical performance limit has been attained with the LHC. Calls for higher beam luminosity (e.g., HL-LHC), and higher beam energy (e.g., FCC), demand a transition from Nb-Ti to Nb3Sn, presently the only practical candidate material offering the required high field performance. This paper provides a summary of desirable properties and performance targets for Nb3Sn to satisfy the challenging magnet specifications for upgrades of existing and future HEP accelerators.
100 citations
TL;DR: In this article, the authors summarized the strategy and actions being undertaken in the framework of the FCC 16 T Magnet Technology Program and the Work Package 5 of the EuroCirCol.
Abstract: A key challenge for a future circular collider (FCC) with centre-of-mass energy of 100 TeV and a circumference in the range of 100 km is the development of high-field superconducting accelerator magnets, capable of providing a 16 T dipolar field of accelerator quality in a 50 mm aperture. This paper summarizes the strategy and actions being undertaken in the framework of the FCC 16 T Magnet Technology Program and the Work Package 5 of the EuroCirCol.
88 citations
TL;DR: Nb3Sn strands fabricated using Nb-Zr alloy can be internally oxidized, provided that oxygen is properly supplied via an oxide powder, which allows the formation of fine intragranular and intergranular ZrO2 particles in a Nb3 Sn matrix.
Abstract: Nb3Sn strands fabricated using Nb-Zr alloy can be internally oxidized, provided that oxygen is properly supplied via an oxide powder. This allows the formation of fine intragranular and intergranular ZrO2 particles in a Nb3Sn matrix. These particles can refine the grain size by a factor of three and thereby greatly enhance the Nb3Sn critical current density.
70 citations