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.

A procedural solution for determining the temperature dependence of transport critical current in Nb3Sn superconducting wires using magnetization measurements

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

Iole Falorio

Papers

Evidence of Kramer extrapolation inaccuracy for predicting high field Nb3Sn properties