Direct measurement of inter-filament resistance in various multi-filamentary superconducting NbTi and Nb₃Sn strands

Abstract

For a proper characterization of multi-filamentary NbTi and Nb 3 Sn strands and a better understanding of their performance in short sample tests, as well as for increased understanding of inter-strand current redistribution in cabled conductors, a quantitative knowledge of the inter-filament transverse resistance is essential. In particular, in the case of strain or crack distributions among and along filaments in strain-sensitive superconductors such as Nb 3 Sn cable-in-conduit conductors, a much better understanding of the voltage-current transition is required as a basis for the analysis of full-size cables.Two particular four-probe voltage-current methods are developed to measure the transverse inter-filament resistance distribution directly, both in well-established and in state-of-the-art superconductors that are presently applied in the ITER, JT-60SA and LHC magnets. To extract values of the filament-to-matrix contact resistance from these direct experiments, some further assumptions are needed. These assumptions are based on FEM simulations and on measurement of the longitudinal strand resistance.An overview is given of a wide range of measurements on various NbTi and Nb 3 Sn strands, performed at temperatures below 10 K and at various applied magnetic fields. We present the results of the experiments and simulations and demonstrate how the extracted characteristic parameters provide a better insight into the current flow patterns within the strands.

Spatial Periodic and Homogeneous Transverse Stress Loading on ITER TF ${\rm Nb}_{3}{\rm Sn}$ Bronze and Internal Tin Strand

“…This will be evaluated with the strand model presently under development and the ongoing studies on intrastrand resistance distribution and current transfer length [9,10] and additional voltage taps. The interstrand current transfer length in CICCs depends on the loading history and void fraction but is expected to go with relatively long interstrand current transfer lengths.…”

“…The aim of this work is to quantify the influence of the transverse stress and in particular investigate the influence of stress distribution in terms of periodicity. This may lead to a better understanding of the role of the interfilament current redistribution on the average strand electric field and its distribution [9,10].…”

Intra wire resistance and strain affecting the transport properties of Nb3Sn strands in cable-in-conduit conductors

“…As illustrated in Figure 2-10, the 3D strand model is fed with input data from a series of experiments developed at the University of Twente: the bending strain dependence if I c measured with the TARSIS set-up [87], [89], [106]; the axial strain dependence of I c from the PACMAN [80] , [87]; the transverse intra-strand resistance measured with the new four-micro-pointcontact setup [113]; and The data which are fed into the model are summarized as follows: 1. The strand dimensions, d f and t, are determined from SEM images showing the detailed wire cross-section; 2.…”

“…The strand dimensions, d f and t, are determined from SEM images showing the detailed wire cross-section; 2. The resistive parameters (described in chapter 3) are determined from direct intrastrand resistance measurement, AC loss measurements, from Energy-dispersive Xray spectroscopy (EDX) and from the Residual Resistivity Ratio (RRR) data [113] as introduced in section 2.3; 3. The spatial filament strain distribution is calculated which a 3D finite element method (FEM) presented in chapter 4, a COMSOL simulation based on known elastic properties; 4.…”

“…Recently, direct approaches have been developed by ENEA [133], [134] and by the University of Twente (UT) [113]. In this work the probe-station described in chapter 2, section 2.3 is used with four micro-point-contact needles serving as voltage 3.2 Intra wire resistance measurements tips and current leads, allowing for a standard four-point voltage-current (V-I) measurement to extract intra-wire resistance values, including filament-to-matrix resistance and matrix resistivity [113]. In sections 3.2 to 3.7, an overview is given of such data for a wide range of NbTi, Nb 3 Sn, MgB 2 and Bi-2212 wires, as well as Bi-2223 or YBCO tapes, collected at various temperatures and applied magnetic fields.…”