Standards for Superconductor Characterization
Performing a demonstration with a levitated model train at the NIST-Boulder Centennial Open House.
This project develops standard measurement techniques for critical current and provides quality-assurance and reference data for commercial high-temperature and low-temperature superconductors. Applications supported include magnetic-resonance imaging, research magnets, fault-current limiters, magnetic energy storage, magnets for fusion confinement, motors, generators, transformers, transmission lines, magnets for crystal growth, and superconducting bearings. Project members assist in the creation and management of international standards for superconductor characterization covering all commercial applications, including electronics. The project is currently focusing on critical-current measurements of marginally stable superconductors, on temperature-variable critical-current measurements, and on measuring the irreversible effects of changes in magnetic field and temperature on critical current.
We serve the U.S. superconductor industry, which consists of many small companies with limited resources for committing to the development of new metrology and standards. We participate in projects sponsored by other government agencies that involve U.S. industry, universities, and national laboratories.
The potential impact of superconductivity on electric-power systems makes this technology very important. We focus on (1) developing new metrology needed for evolving, large-scale superconductors, (2) participating in interlaboratory comparisons needed to verify techniques and systems used by U.S. industry, and (3) developing international standards for superconductivity needed for fair and open competition and improved communication.
One of the most important performance parameters for large-scale superconductor applications is the critical current. Critical current is difficult to measure correctly and accurately; thus, these measurements are often subject to scrutiny and debate. Another activity is the measurement of the magnetic hysteresis loss in superconductors. With each significant advance in superconductor technology, new procedures, interlaboratory comparisons, and standards are needed. International standards for superconductivity are created through the International Electrotechnical Commission (IEC), Technical Committee 90 (TC 90).
The next generation of Nb3Sn and Nb3Al wires is pushing towards higher current density, less stabilizer, larger wire diameter, and higher magnetic fields. The latest Nb-Ti conductors are also pushing these limits. The resulting higher current required for critical-current measurements turns many minor problems into significant engineering challenges. For example, specimen heating, from many sources during the measurement, can cause a wire to appear to be thermally unstable.
During FY 2002, we will continue to solicit and incorporate comments from U.S. participants in IEC/TC 90 standards development. We will review, edit, and comment on draft standards in the 11 Working Groups in IEC/TC 90.
During FY 2002, Loren Goodrich will continue to serve as Chairman of IEC/TC 90 and manage and coordinate the development of standards for superconductivity.
Characterization of Superconductors
Preparing to measure the electrical transport properties of a superconducting wire.
During FY 2002-2004, we will further develop our variable-temperature critical-current measurement capability and provide a critical-current database to the U.S. Department of Energy, Office of Fusion Energy Sciences program. The primary focus will be on Nb3Sn wires.
During FY 2002-2004, we will develop routine high-current testing of marginally stable Nb3Sn conductors for the U.S. Department of Energy, Office of High Energy Physics program. The commonly used techniques have been shown to be inadequate for many of the latest conductors. More precise measurements are needed to evaluate conductor performance and provide reliable feedback to the development process. The development of testing will involve input from the other U.S. testing laboratories and will likely include interlaboratory comparisons.
During 2002, we will continue to provide measurements of critical current, residual resistivity ratio, and hysteresis loss for U.S. companies and national laboratories.
IEC Technical Committee, Led by NIST, Publishes Five New Superconductivity Standards - Five new international standards on superconductivity were recently published by the IEC/TC 90. The documents are:
IEC 61788-3 Superconductivity - Part 3: Critical current measurement - DC critical current of Ag-sheathed Bi-2212 and Bi-2223 oxide superconductors
IEC 61788-4 Superconductivity - Part 4: Residual resistivity ratio measurement - Residual resistivity ratio of Cu/Nb-Ti composite superconductors
IEC 61788-5 Superconductivity - Part 5: Matrix to superconductor volume ratio measurement - Copper to superconductor volume ratio of Cu/Nb-Ti composite superconductors
IEC 61788-6 Superconductivity - Part 6: Mechanical properties measurement - Room temperature tensile test of Cu/Nb-Ti composite superconductors
IEC 60050-815 International Electrotechnical Vocabulary - Part 815: Superconductivity
We have worked extensively on these documents and helped resolve many difficulties encountered during the development process. Loren Goodrich serves as Chairman of TC 90 and manages the international work. Thirteen countries participate in TC 90. The vocabulary was created under TC 90, but all vocabulary publications are listed under TC 1. This vocabulary contains 301 terms and their definitions. The standard on the critical current of oxide superconductors is the first IEC standard on the newer high-temperature superconductors. This brings the number of IEC TC 90 published standards to seven. Currently, seven more documents are at various stages of development within TC 90.
IEC technical Commitee 90
TC 90 Working Groups and Status
1. Terms and definitions (301 terms)
2. Ic measurement Cu/Nb-Ti
3. Ic measurement of Bi-based superconductors
4. Residual resistivity ratio measurement
5. Room Temperature tensile test
6. Matrix composite ratio measurement
7. Ic measurement of Nb3Sn
8. Electronic characteristic measurements
9. AC loss measurement
10. Trapped flux desity measurements of oxides
11. Critical temperature measurement
IS & CDV
IS & CDV
Document Stages: Working Draft (WD), Committee Draft (CD), Committee Draft for Voting (CDV), Final Draft International Standard (FDIS), International Standard (IS).
• Old Standards Withdrawn - The two American Society for Testing and Materials (ASTM) standards for superconductors were withdrawn, and Subcommittee B01.08 on superconductors was dissolved by Loren Goodrich, the former Chairman of B01.08. These two ASTM standards (B 713-82 and B 714-82) were used to draft parts of three IEC standards under TC 90. These two ASTM standards are now superseded by three IEC standards (IEC 61788-1, IEC 61788-2, and IEC 60050-815). The members of ASTM B01.08 had agreed that when these new IEC standards were published, then the above actions should be taken. These ASTM standards have served the superconductor industry well during the last 18 years, and they live on in the new IEC standards.
Characterization of Superconductors
Lowering a superconductor test fixture into liquid helium. The cloud at the top of the cryostat results from condensed moisture in air cooled by cold helium gas.
• Verified Performance of Conductor for Large Hadron Collider - A national laboratory involved in the U.S. program for the international Large Hadron Collider program asked NIST to conduct critical-current verification on several Cu/Nb-Ti strands. Loren Goodrich was a co-author on a paper that detailed the characterization of 2000 kilometers (about 18 tonnes) of superconducting strand for this program. The paper included an interlaboratory comparison among three laboratories on measurements of critical current, n-value, and residual resistivity ratio. This was the highest current comparison ever on a single strand (as opposed to a cable), with an average critical current of about 2000 amperes at 5 teslas and 4.2 kelvins.
In the conductor design, the amount of Cu stabilizer in the strand was kept to a minimum and additional high-purity Al and alloy Al were added to the cable to improve the stability and mechanical strength of the final product. However, the individual strands need to be tested before they are cabled in order to avoid introducing inferior strand into the cable, which would waste even more strand. This resulted in the need to develop new measurement procedures on strands that are designed to be marginally stable.
This type of work gives us the experience needed to develop future measurement standards and keeps us up to date on the latest conductors and measurements challenges. There is no good substitute for the experience and insight gained by performing routine measurements on the latest conductors in the advancing technology of superconductors. The unexpected scientific and practical discoveries continue to be reviewed by this work. The next accomplishment below, on a new source of misinterpretation in superconductor measurements, is one example.
• New Source of Misinterpretation in Superconductor Measurements - As part of our program to develop standard measurement techniques for superconductors, we have identified and studied a new source of misinterpretation in critical-current measurements of superconductors. The critical current is the maximum current a conductor can carry before a quench, when it reverts to the normal, resistive state. Researchers can tell when the critical current is reached by measuring the resistive voltage on pairs of voltage taps soldered to the superconductor wires. However, we discovered that anomalous inductive voltages can be induced in the loop formed by the voltage taps. The inductive voltages vary systematically with current, current sweep direction (increasing or decreasing), applied magnetic field, and whether the specimen was driven into the normal state in an immediately previous measurement. Furthermore, the decay time of the inductive voltage signal, after ending the current ramp, is longer near the onset of the resistive transition. These decay times are even longer during a superconductor's first current sweep after a quench.
I think the new procedures developed and disseminated by Goodrich and Goldfarb have already allowed IGC-AS and the rest of the superconductor community to greatly improve their superconductor characterization.
Dr Eric Gregory
Manager, Research and Development
Many superconductor applications now require higher current densities, larger wire diameters, and less copper stabilizer, all of which results in marginally stable conductors with high critical currents above 1000 amperes. Variable induced voltages and long decay times become a concern when currents or current-ramp rates are high, or when voltage curves need to be extrapolated for measurements on marginally stable conductors. The resulting data can be mistakenly attributed to (1) a bad conductor, (2) a damaged specimen, (3) an electrical ground loop, (4) a low critical current, or (5) specimen motion in the background magnetic field.
To avoid anomalous induced voltages, we recommend cycling the current before acquiring data after a quench, avoiding data acquisition while the current is being ramped, and allowing 3 seconds of settling time after current levels are changed before measurements are made near the critical current.