S L. Bray

High Tc superconductors and critical current measurement

Abstract With the introduction of high Tc superconductors, a number of problems associated with critical current, Ic, measurement have arisen. The existing Ic, measurement practices were developed and proved for low Tc superconductors. There are substantial differences between the two classes of materials. When the Ic concept was casually extended to the high Tc conductors, measurement inconsistency, ambiguity and, in some cases, invalidity followed. A discussion of the underlying philosophy of Ic measurement is presented and a number of measurement variables that can influence the measured Ic are discussed. Many of the problems stem from inadequate reporting practices, and recommendations are given for improving measurement reports.

VAMAS intercomparison of critical current measurement in Nb/sub 3/Sn wires

A technical working party in the area of superconducting and cryogenic structural materials has recently carried out the first worldwide intercomparison of critical-current, I/sub c/, measurement on multifilamentary Nb/sub 3/Sn wires. Three sample wires were supplied from each of the European Community countries, Japan, and the United States. There were few restrictions for the I/sub c/ measurement at participant labs. The standard deviations of the I/sub c/ values reported from these labs varied among test samples, and were 6-21% of averaged I/sub c/s at 12 T. >

Critical-current degradation in multifilamentary Nb/sub 3/Al wires from transverse compressive and axial tensile stress

The effect of transverse compressive stress and axial tensile stress on the critical current of multifilamentary Nb/sub 3/Al superconducting wires has been measured. Compared with commercial Nb/sub 3/Sn, Nb/sub 3/Al exhibits a relatively small sensitivity to both axial and transverse stress. For a given degradation of critical current at 9 T, Nb/sub 3/Al will tolerate about twice as much axial stress as Nb/sub 3/Sn and about five times the transverse stress. The elastic modulus of Nb/sub 3/Al, 169+or-20 GPa, was measured for the first time at cryogenic temperatures and found comparable to that of Nb/sub 3/Sn. A comparison between the effect of axial and transverse stress showed that Nb/sub 3/Al, like Nb/sub 3/Sn, is more sensitive to transverse stress than axial stress. For a given level of critical-current degradation, the transverse stress tolerance of Nb/sub 3/Al is about half the axial stress tolerance. The favorable electromechanical characteristics of Nb/sub 3/Al, compared with Nb/sub 3/Sn, may allow increased operating limits for the next generation of large high-field superconducting magnets.<<ETX>>

Tensile measurements of the modulus of elasticity of Nb/sub 3/Sn at room temperature and 4 K

The critical current of Nb/sub 3/Sn superconductors is highly sensitive to strain. Consequently, accurate mechanical modeling of these conductors is necessary to interpret experimental data and to predict conductor performance in applications such as large magnet systems. A key parameter in these models is the modulus of elasticity (E, Youngs modulus); however, there are large discrepancies the available data, and there are no published tensile-test data on E for Nb/sub 3/Sn. Tensile test specimens were prepared from a starting material of Nb tape with 1.4 wt.% ZrO/sub 2/ precipitates. Tensile measurements of unreacted Nb and partially reacted Nb-Nb/sub 3/Sn tapes were made at room temperature (293 K) and at 4 K. A modulus of elasticity of 65/spl plusmn/15 GPa was extrapolated from these measurements for polycrystalline Nb/sub 3/Sn at 4 K, and 150/spl plusmn/15 GPa at room temperature.

Integrity Tests for High-Tc and Conventional Critical-Current Measurement Systems

Critical-current measurement systems must be extremely sensitive to the small differential voltage that is present across the test specimen as it changes from the zero resistance state to the flux-flow resistance state. Consequently, these measurement systems are also sensitive to interfering voltages. Such voltages can be caused by ground loops and by common mode voltages. Specific methods for testing the sensitivity of critical-current measurement systems and for detecting the presence of interfering voltages are discussed. These include a simple procedure that simulates the zero resistance state and the use of an electronic circuit that simulates the flux-flow resistance state.

Critical current measurements of Nb3Sn superconductors: NBS contribution to the VAMAS interlaboratory comparison

Abstract Critical current measurements on several Nb 3 Sn superconductors were made as part of an interlaboratory comparison (round robin). These measurements were made in conjunction with twenty-four laboratories from the European Economic Community, Japan and the USA as part of the Versailles Agreement on Advanced Materials and Standards (VAMAS). The results of the NBS measurements, including the effect of sample mounting techniques on the measured critical current, are given. A systematic study of the effect of measurement mandrel (tubular sample-holder made from G10 fibreglass-epoxy composite) geometry revealed that a seemingly small change in that geometry can result in a 40% change in the measured critical current at a magnetic field of 12 T. Specifically, the radial thermal contraction of the measurement mandrel depends on its wall thickness and, thus, so does the conductor prestrain (at 4 K) and, ultimately, the measured critical current. Techniques for reducing variation in the measured critical current are suggested.

A flexible high-current lead for use in high-magnetic-field cryogenic environments

A flexible high-current lead for use at cryogenic temperatures and in high-magnetic fields has been developed using high purity aluminum. Readily available high purity aluminum has distinct advantages over copper, namely lower resistivity at liquid-helium temperature, lower magnetoresistance, lower yield stress, lower density, lower cost for material with comparable conductivity, and room temperature annealing. Aluminum may also be used in high magnetic fields, where flexible superconductors cease to function. Practical issues encountered in the design and fabrication of such flexible leads are discussed, such as geometrical considerations where sample loading and heating are important issues.

Current capacity degradation in superconducting cable strands

The electromagnetic properties of NbTi strands extracted from Rutherford cables were studied to clarify the effect of mechanical deformation, caused by the cabling process, on the current capacity of the strands. Three different cables were studied, all of which are prototypes for the Superconducting Super Colliders dipole magnets. The extracted cable strands were instrumented to allow measurement of the voltage across several key regions of mechanical deformation as a function of current and the orientation of the applied magnetic field. The resulting data are presented in terms of the strands voltage profile as well as its critical current to more thoroughly characterize the conductors electromagnetic properties. The cable strands show very localized reductions in current capacity that are well correlated with the regions of high mechanical deformation. >

Current ripple effect on superconductive d.c. critical current measurements

Abstract The effect of current ripple or noise on d.c. critical current measurements was systematically studied. Measurements were made on multifilamentary NbTi superconductor. A low-noise, battery-powered current supply was required in this study in order to make the pure d.c. critical current measurements. Also, an electronic circuit that stimulates a superconductors general current voltage characteristic was developed and used as an analysis tool. In order to make critical current measurements in which current ripple was present, the battery supply was modified to allow the introduction of controlled amounts of a.c. ripple. In general, ripple in a current supply becomes more significant in current supplies rated above 500 A because effective filtering is difficult. The effect of current ripple is a reduction in the measured d.c. critical current; however, ripple of sufficient amplitude can result in arbitrary measurement results. The results of this work are general and quantitatively applicable to the evaluation of critical current data and measurement systems. A theoretical model was developed to further support and explain the ripple effect. An unexpected benefit of this work was a more precise method for general critical current data acquisition. Problems common to all large conductor critical current measurements are discussed.

High Compressive Axial Strain Effect on the Critical Current and Field of Nb3Sn Superconductor Wire

The axial strain dependence of the critical current Ic and effective upper critical field Bc2* have been measured on a series of Nb3Sn wire superconductors having initial compressive strain as large as -0.95% arising from thermal contraction of the conductor matrix. Results include data for binary Nb3Sn and ternary Nb3Sn with Ti additions. The effective upper critical field Bc2* is obtained using a general form of the pinning expression, since the data show that the Kramer method is not generally applicable to ternary Nb3Sn superconductors. The Ic and Bc2* data fit the strain scaling law well. The results are also consistent with earlier-published Tc vs. strain data for Nb3Sn at compressive strain as large as -0.85% and with Ic vs. strain data for stainless-steel reinforced Nb3Sn superconductors at compressive strain as large as -0.65%. The data contradict, however, recently reported Bc2* data obtained on multifilamentary Nb3Sn wires where high compressive strain was applied by soldering the wires to a bending beam and then flexing the beam.