Loren F. Goodrich

Critical current measurements: A compendium of experimental results

Abstract The results of a programme to evaluate the measurement of the critical current of relatively small (

Reversible effect of strain on transport critical current in Bi2Sr2CaCu2O8?+?x superconducting wires: a modified descriptive strain model

A reversible strain effect on transport critical current Ic was found in Bi2Sr2CaCu2O8 + x (Bi-2212) high-temperature superconducting round wires. Ic showed unambiguous reversibility at 4 K and 16 T up to an irreversible strain limit of about 0.3 % in longitudinal tension, prompting hope that the Bi-2212 conductor has the potential to sustain mechanical strains generated in high-field magnets. However, Ic was not reversible under longitudinal compression and buckling of Bi-2212 grain colonies was identified as the main reason. A two-component model was proposed, which suggests the presence of mechanically weak and strong Bi-2212 components within the wire filaments. Porosity embedded in the weak component renders it structurally unsupported and, therefore, makes it prone to cracking under strain e. Ic(e) is irreversible in tension if the weak component contributes to the transport critical current but becomes reversible once connectivity of the weak component is broken through strain increase or cycling. A modified descriptive strain model was also developed, which illustrates the effect of strain in the Bi-2212 conductor and supersedes the existing descriptive model. Unlike the latter, the new model suggests that higher pre-compressive strains should improve Ic if buckling of Bi-2212 grains does not occur, and should result in a wider Ic(e) plateau in the applied tensile regime without degradation of the initial Ic. The new model postulates that a reversible strain effect should exist even in the applied compressive strain regime if buckling of Bi-2212 grains could be prevented through elimination of porosity and mechanical reinforcement of the wire.

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.

Evidence that the reversible strain effect on critical current density and flux pinning in Bi2Sr2Ca2Cu3Ox tapes is caused entirely by the pressure dependence of the critical temperature

It is well known that the critical temperature of cuprate- and iron-based high-temperature superconductors changes with pressure. YBa2Cu3O7 − δ coated conductors, as well as Bi2Sr2CaCu2Ox and Bi2Sr2Ca2Cu3Ox tapes and wires, show a clear reversible effect of strain on their current-carrying capability, but no clear understanding about the origin of this effect has been obtained. For the first time, we present evidence that the pressure dependence of the critical temperature is entirely responsible for a reversible change in critical current and magnetic flux pinning in Bi2Sr2Ca2Cu3Ox tapes with strain.

Influence of Ti and Ta doping on the irreversible strain limit of ternary Nb3Sn superconducting wires made by the restacked-rod process

Nb3Sn superconducting wires made by the restacked-rod process (RRP®) were found to have a dramatically improved resilience to axial tensile strain when alloyed with Ti as compared to Ta. Whereas Ta-alloyed Nb3Sn in RRP wires showed permanent damage to its current-carrying capacity (Ic) when tensioned beyond an intrinsic strain as small as 0.04%, Ti-doped Nb3Sn in RRP strands exhibits a remarkable reversibility up to a tensile strain of about 0.25%, conceivably making Ti-doped RRP wires more suitable for the high field magnets used in particle accelerators and nuclear magnetic resonance applications where mechanical forces are intense. A strain cycling experiment at room temperature caused a significant drop of Ic in Ta-alloyed wires, but induced an increase of Ic in the case of Ti-doped strands. Whereas either Ti or Ta doping yield a similar enhancement of the upper critical field of Nb3Sn, the much improved mechanical behavior of Ti-alloyed wires possibly makes Ti a better choice over Ta, at least for the RRP wire processing technique.

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.

Suppression of flux jumps in marginally stable niobium-tin superconductors

Niobium-tin superconductor wires with coalesced filaments may have reduced adiabatic stability. Magnetization measurements on such marginally stable conductors exhibit flux jumps, which appear as a sudden decrease in magnetization as the applied field is changed, caused by the unpinning of flux vortices and resistive heat generation. Flux jumps preclude estimation of the hysteresis loss from the area of the magnetization-versus-field loop. Here, we show that flux jumps can be minimized or suppressed during the measurement of hysteresis loss by immersing the specimen in helium liquid instead of helium gas. The better thermal conductivity of the liquid affords additional dynamic stability against flux jumps. This allows one to determine the loss upon field cycling and to calculate an effective filament diameter, often used to gauge losses and the extent of metallurgical interfilament coupling.

Electromechanical Characterization of Bi-2212 Strands

The uniaxial strain dependence of critical current was measured both in tension and compression in Bi2Sr2CaCu2O8+x (Bi-2212) high-temperature superconducting round wires. Permanent damage to the critical current easily occurred due to strain. To improve the electromechanical properties of Bi-2212 wires, development of stronger sheathing materials is needed. Ideal materials would be not only mechanically strong, but also chemically compatible with Bi-2212 during the final heat treatment. To identify such materials, we measured stress-strain properties of some new Ag alloys and extracted their respective Youngs modulus values and yield strength. The database may be useful for development of new Bi-2212 strands for fabricating high-field superconducting magnets above 20 T.

Test Results of the First US ITER TF Conductor in SULTAN

The US Domestic Agency is one of six parties supplying TF cable-in-conduit conductors (CICCs) for ITER. Previous tests have shown that measured performance of the TF CICCs can be much lower than expected from the strand properties at the projected uniaxial strain and that the cabling pattern may also be an important factor. Worst of all, voltage signals well below the expected critical surface could not be reliably interpreted or canceled, making test results very suspect. The TFUS1 sample was prepared to achieve multiple goals: 1) to ensure uniform current distribution and to eliminate parasitic voltage signals by improving joints, 2) to explore the potential benefits of a different cabling pattern for better support of strain-sensitive strands, and 3) to explore the source of voltage development in the cable through the use of innovative penetrating diagnostics. Test results of the first US-made samples are presented and discussed.

Method for determining the irreversible strain limit of Nb3Sn wires

We define a rigorous and reliable method for determining the irreversible strain limit of Nb3Sn wires. The critical current (Ic) is measured as a function of applied longitudinal strain (e), Ic(e), at one magnetic field and a temperature of 4.0 K. The sample is loaded and partially unloaded at progressively higher strain levels to determine the irreversible strain limit, eirr, which is defined as the maximum loaded strain where Ic is still reversible. Our method uses a polynomial fit of the loaded Ic(e) to derive the Ic residuals for the loaded and unloaded points that are analyzed to determine the limit of irreversibility. The effect of varying the amount of strain unloading is also studied. The possibility and problems of using the strain dependent n-value (which indicates the steepness of the electric field–current, E–I, curve) to determine eirr are discussed. The method presented here to determine eirr has proven to be repeatable for many types of commercial Nb3Sn wires. This method can also be more generally used to determine eirr for any brittle low-temperature or high-temperature superconducting material.