Jack W. Ekin

Magnetic-field dependence of the reversible axial-strain effect in Y-Ba-Cu-O coated conductors

The critical-current density J/sub c/ of an yttrium-barium-copper-oxide (YBCO) coated conductor deposited on a biaxially-textured Ni-5at.%W substrate was measured at 76.5 K as a function of axial tensile strain /spl epsiv/ and magnetic field B applied parallel to the YBCO (a,b) plane. Reversibility of J/sub c/ with strain was observed up to /spl epsiv//spl sime/0.6% over the entire field range studied (from 0.05 to 16.5 T), which confirms the existence of an intrinsic strain effect in YBCO coated conductors. J/sub c/ vs. /spl epsiv/ depends strongly on magnetic field. The decrease of J/sub c/(/spl epsiv/) grows systematically with magnetic field above 2-3 T, and, unexpectedly, the reverse happens below 2 T as this decrease shrinks with increasing field. The pinning force density F/sub p/=J/sub c//spl times/B scaled with field for all values of strain applied, which shows that F/sub p/ can be written as K(T,/spl epsiv/)b/sup p/(1-b)/sup q/, where p and q are constants, K is a function of temperature and strain, b=B/B/sub c2//sup */ is the reduced magnetic field, and B/sub c2//sup */ is the effective upper critical field at which F/sub p/(B) extrapolates to zero.

Effect of conduit material on CICC performance under high cycling loads

Recent International Thermonuclear Experimental Reactor (ITER) Model Coils and tests on Nb/sub 3/Sn Cable in Conduit Conductors (CICC) showed a significant and unexpected increase in the broadness of the transition to the normal state, resulting in degradation of superconducting properties. To investigate these phenomena, two CICC samples were built with identical 144 strand cables but different conduit materials. One sample had titanium conduit with low coefficient of thermal expansion, the other had stainless steel conduit. The purpose of this experiment was to study changes in strand properties in the cable (critical current, current sharing temperature, n-value), the effects of cycling and high electromagnetic load, and the effect of the conduit on the CICC performance.

Compressive pre-strain in high-niobium-fraction Nb/sub 3/Sn superconductors

Multifilamentary Nb/sub 3/Sn superconducting strands fabricated with high niobium fractions have exceptionally high critical-current densities but are sometimes marginally stable during testing. We report a technique for determining the pre-strain in such conductors, in which additional stabilizing copper is electroplated onto the conductor and the pre-strain is determined by extrapolation to the as-fabricated niobium fraction. This technique is used to measure the pre-strain in conductors with high niobium fractions of 20% to 30%. Values of the pre-strain /spl epsiv//sub max/ in these conductors are reduced to the range 0.1% to 0.2%, which is significantly less than the /spl epsiv//sub max/ values of 0.2% to 0.4% in traditional bronze-process Nb/sub 3/Sn conductors (where niobium fractions are typically about 10% to 15%). However, including about 20% dispersion-strengthened copper into the conductor matrix restores /spl epsiv//sub max/ to the range 0.25% to 0.35%, thus providing practical levels of /spl epsiv//sub max/ for magnet design in high-niobium-fraction strands.

Effect of Fatigue Under Transverse Compressive Stress on Slit Y-Ba-Cu-O Coated Conductors

The slitting of wide Y-Ba-Cu-O coated-conductor tapes to a width desirable for applications allows for considerable reduction in conductor manufacturing cost. Localized damage induced at the slit edges may be tolerated provided that mechanical cracks formed in the ceramic layers do not propagate deeper inside the conductor due to mechanical forces and thermal cycling to which the strand will be subjected in actual applications. In order to evaluate the effect of slitting, we used fatigue cycling under transverse compressive stress. These tests simulate conditions in applications such as rotating machinery and industrial magnets. Conductors measured had a rolling-assisted biaxially textured Ni-W substrate (RABiTS), or a Hastelloy-C substrate with an ion-beam assisted deposition (IBAD) buffer template. Samples were fabricated with or without a Cu protection layer, added either before or after slitting. For all these geometries, the critical current exhibited no significant degradation during fatigue testing up to 150 MPa transverse compressive stress and 20,000 cycles. Nevertheless, these results do not imply that slitting is not deleterious to the conductor performance under other experimental conditions.

Critical-Current Measurements on an ITER Nb

The dependence of transport critical current I_c on axial tensile strain was measured for a developmental Nb₃Sn multifilamentary strand as a function of magnetic field B between 12 T and 16 T, at the temperature of 4 K. This conductor was from the first stage of strand pre-production for the central solenoid of the International Thermonuclear Experimental Reactor (ITER) project. Straight samples were measured with a stress-free-cooling strain apparatus. The compressive pre-strain and the irreversible strain limit epsiv_irr were 0.19% and 0.8%, respectively; and the ultimate strain where the wire physically broke was about 0.95%. The pinning force F_p ( = I_c x B ) was proportional (B*_c2)^sb^p (1 - b )^q to , where b = B / B*_c2 is the reduced magnetic field, and the scaling constants had values p = 0.58, q = 1.86, and s = 0.7. The strain dependence of the effective upper critical field (the field at which F_p extrapolates to zero) was well described within the measured strain range by B*_c2max [1 - alpha |epsiv - epsiv_max |^u _], where B*_c2 is the maximum value of B*_c2 as a function of strain, u = 1.7, and alpha was about 1230 for the compressive strains and 1670 for the tensile strains. Ekins strain scaling law was applied to calculate the strain sensitivity of I_c at various intrinsic strains between -0.5% and 0.5%, and magnetic fields from 12 T to 16 T.

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The National High Magnetic Field Laboratory (NHMFL), in collaboration with the Francis Bitter National Magnet Laboratory, is constructing a 45-T hybrid magnet system. Here we report the results of studies to characterize candidate Nb/sub 3/Sn superconductor for use in the superconducting outsert coils. We have determined the effects of strain and magnetic field on the critical current, measured ac losses, and measured residual resistivity ratios (RRR). Critical currents in excess of 1000 A mm/sup -2/ non-Cu have been measured in sample wires at 12 T and 4.2 K combined with a hysteresis loss of less than 500 mJ cm/sup -3/ We also present predicted 1.8 K performance, based on empirical models, and present a methodology for calculating sample holder prestrain.<<ETX>>

Sn Strand: Effect of Axial Tensile Strain

A scaling study of several thousand Nb3Sn critical-current (Ic) measurements is used to derive the Extrapolative Scaling Expression (ESE), a relation that can quickly and accurately extrapolate limited datasets to obtain full three-dimensional dependences of Ic on magnetic field (B), temperature (T), and mechanical strain (ε). The relation has the advantage of being easy to implement, and offers significant savings in sample characterization time and a useful tool for magnet design. Thorough data-based analysis of the general parameterization of the Unified Scaling Law (USL) shows the existence of three universal scaling constants for practical Nb3Sn conductors. The study also identifies the scaling parameters that are conductor specific and need to be fitted to each conductor. This investigation includes two new, rare, and very large Ic(B,T,ε) datasets (each with nearly a thousand Ic measurements spanning magnetic fields from 1 to 16 T, temperatures from ∼2.26 to 14 K, and intrinsic strains from –1.1% to +0.3%). The results are summarized in terms of the general USL parameters given in table 3 of Part 1 (Ekin J W 2010 Supercond. Sci. Technol. 23 083001) of this series of articles. The scaling constants determined for practical Nb3Sn conductors are: the upper-critical-field temperature parameter vuf0a0=uf0a01.50uf0a0±uf0a00.04; the cross-link parameter wuf0a0=uf0a03.0uf0a0±uf0a00.3; and the strain curvature parameter uuf0a0=uf0a01.7uf0a0±uf0a00.1 (from equation (29) for bc2(ε) in Part 1). These constants and required fitting parameters result in the ESE relation, given by e e = h m m I B T B C b t t b b , , 1 1 1 s p q c c2 1.5 2 ( ) [ ( )] ( ) ( ) ( – ) with reduced magnetic field buf0a0≡uf0a0B/Bc2 (T,ε) and reduced temperature tuf0a0≡uf0a0T/Tc (ε), where: * * e e = B T B t b , 0,0 1 c2 c2 1.5 c2 ( ) ( )( ) ( ) / * * e e = T T b 0 c c c2 1 3 ( ) ( )[ ( )] Superconductor Science and Technology Supercond. Sci. Technol. 29 (2016) 123002 (38pp) doi:10.1088/0953-2048/29/12/123002 0953-2048/16/123002+38

Characterization of multifilamentary Nb/sub 3/Sn superconducting wires for use in the 45-T hybrid magnet

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submitter : Unified Scaling Law for flux pinning in practical superconductors: II. Parameter testing, scaling constants, and the Extrapolative Scaling Expression

Scaling analysis of several thousand Nb₃Sn critical-current (<italic>I</italic>_c) measurements is used to derive the extrapolative scaling expression (ESE), a fitting equation that can quickly and accurately extrapolate (or interpolate) limited datasets to obtain full three-dimensional dependences of <italic>I</italic>_c on magnetic field (<italic>B</italic>), temperature (<italic>T</italic>), and mechanical strain (<italic>ϵ </italic>). Unlike nonextrapolative fitting equations, the ESE relation is determined completely by fundamental raw scaling data from master pinning-force curves. The results show that extrapolation errors with ESE approach typical <italic>I</italic>_c measurement errors. The scaling expression is simple and robust, providing straightforward extrapolation capability for conductor characterization and magnet design.

submitter : Extrapolative Scaling Expression: A Fitting Equation for Extrapolating Full

This introduction to measurement cryostat design includes illustrations of several practical cryostats for testing low-current thin-films and high-current bulk superconductors. Heat transfer is the heart of most cryostat designs and illustrative calculations are given. Other topics that will be discussed include optimal heat sinking of instrumentation leads, wire materials selection, practical vacuum-tight electrical fixtures, vapor-cooled current leads, high-temperature-superconductor current leads, and variable temperature apparatus. A few properties of technical materials for constructing cryostats will be summarized.