Standards for Superconductor and Magnetic Measurements
This project develops standard measurement techniques for critical current, residual resistivity ratio, and magnetic hysteresis loss, and provides quality assurance and reference data for commercial hightemperature and low-temperature superconductors. Applications supported include magnetic-resonance imaging, research magnets, magnets for fusion confinement, motors, generators, transformers, high-quality-factor resonant cavities for particle accelerators, and superconducting bearings. Superconductor applications specific to the electrical power industry include transmission lines, synchronous condensers, magnetic energy storage, and fault-current limiters. Project members assist in the creation and management of international standards through the International Electrotechnical Commission for superconductor characterization covering all commercial applications, including electronics. The project is currently focusing on measurements of variable-temperature critical current, residual resistivity ratio, magnetic hysteresis loss, critical current of marginally stable superconductors, and the irreversible effects of changes in magnetic field and temperature on critical current.
This project serves the U.S. superconductor industry, which consists of many small companies, in the development of new metrology and standards, and in providing difficult and unique measurements. We participate in projects sponsored by other government agencies that involve industry, universities, and national laboratories.
The potential impact of superconductivity on electric power systems, alternative energy sources, and research magnets makes this technology especially important. We focus on: (1) developing new metrology needed for evolving, large-scale superconductors, (2) providing unique databases of superconductor properties, (3) participating in interlaboratory comparisons needed to verify techniques and systems used by U.S. industry, and (4) developing international standards for superconductivity needed for fair and open competition and improved communication.
Electric power grid stability, power quality, and urban power needs are pressing national problems. Superconductive applications can address many of them in ways and with efficiencies that conventional materials cannot. “Second-generation” Y-Ba- Cu-O (YBCO) superconductors are approaching the targets established by the U.S. Department of Energy. The demonstration of a superconductor synchronous condenser for reactive power support was very successful and has drawn attention to the promise of this technology. Previous demonstration projects had involved first-generation materials, Bi-Sr-Ca-Cu-O (BSCCO). Variable-temperature measurements of critical current and magnetic hysteresis loss will become more important with these AC applications, and methods for reducing losses are expected to evolve as second-generation materials become commercial.
Fusion energy is a potential, virtually inexhaustible energy source for the future. It does not produce CO2 and is environmentally cleaner than fission energy. Superconductors are used to generate the ultrahigh magnetic fields that confine the plasma in fusion energy research. We measure the magnetic hysteresis loss and critical current of marginally stable, high-current Nb3Sn superconductors for fusion and other research magnets. Because of the way superconductors are used in magnets, variable- temperature critical-current measurements are needed for more complete characterization.
The focus of high-energy research is to probe and understand nature at the most basic level, including dark matter and dark energy. The particle accelerator and detector magnets needed for this fundamental science continue to push the limits of superconductor technology. The next generation of Nb3Sn and Nb-Ti wires is pushing towards higher current density, less stabilizer, larger wire diameter, and higher magnetic fields. The resulting higherectronics and Electrical Engineering Laboratory current required for critical-current measurements turns many minor measurement problems into significant engineering challenges. For example, heating of the specimen, from many sources, during the measurement can cause a wire to appear to be thermally unstable. Newer MgB2 wires may be used for specialty magnets that can safely operate at the higher temperatures caused by high heat loads. We need to make sure that our measurements and the measurements of other laboratories keep up with these challenges and provide accurate results for conductor development, evaluation, and application.
Possible spin-off applications of particle accelerators are efficient, powerful light sources and freeelectron lasers for biomedicine and nanoscale materials production. The heart of these applications is a linear accelerator that uses high-efficiency, pure Nb resonant cavities. We conduct research on a key materials property measurement for this application, the residual resistivity ratio (RRR) of the pure Nb. This measurement is difficult because it is performed on samples that have dimensions similar to those of the application. Precise variabletemperature measurements are needed for accurate extrapolations.
International Standards — 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).
Critical Current Measurements — 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. The critical current is determined from a measurements of voltage versus current. Typical criteria are electric-field strength of 10 microvolts per meter and resistivity of 10–14 ohm-meters.
Critical-current measurements at variable temperatures are needed to determine the temperature margin for magnet applications. The temperature margin is defined as the difference between the operating temperature and the temperature at which critical current Ic is equal to the operating current. When a magnet is operating, transient excursions in magnetic field H or current I are not expected; however, many events can cause transient excursions to higher temperatures T, such as wire motion, AC losses, and radiation. Hence the temperature margin of a wire is a key specification in the design of superconducting magnets. Variable-temperature critical-current measurements require data acquisition with the sample in a fl owing gas environment rather than immersed in a liquid cryogen. Accurate high-current (above 100 amperes) measurements in a fl owing gas environment are very difficult to perform.
Resiudual Resistivity Ratio Measurements — The RRR is defined as the ratio of electrical resistivity at two temperatures: 273 kelvins (0 degrees Celsius) and 4.2 kelvins (the boiling point of liquid helium). The value of RRR indicates the purity and the low-temperature thermal conductivity of a material, and is often used as a materials specification for superconductors. The low temperature resistivity of a sample that contains a superconductor is defined at a temperature just above the transition temperature or is defined as the normal-state value extrapolated to 4.2 kelvins. For a composite superconducting wire, RRR is an indicator of the quality of the stabilizer, which is usually copper or aluminum that provides electrical and thermal conduction during conditions where the local superconductor momentarily enters the normal state. For pure Nb used in radio-frequency cavities of linear accelerators, the low temperature resistivity is defined as the normal-state value extrapolated to 4.2 kelvins. This extrapolation requires precise measurements. We have studied some fundamental questions concerning the best measurement of RRR and the relative differences associated with different measurement methods, model equations for the extrapolation, and magnetic field orientations (when a field is used to drive the superconductor into the normal state).
Magnetic Hysteresis Loss Measurements — As part of our program to characterize superconductors, we measure the magnetic hysteresis loss of marginally stable, high-current Nb3Sn superconductors for fusion and particle-accelerator magnets. We use a magnetometer based on a superconducting quantum interference device (SQUID) to measure the magnetic hysteresis loss of superconductors, which is the area of the magnetization-versus-field loop. In some cases, especially for marginally stable conductors, we use special techniques to obtain accurate results. Measurement techniques developed at NIST have been adopted by other laboratories.
- Superconductor Data Enables U.S. Company to Offer Product to Korean Project — New bismuth-based high-temperature superconductor wires are under active consideration for a 600 kilojoule superconducting magnetic energy storage (SMES) project lead by the Korea Electrotechnology Research Institute. The purpose of the SMES system is to stabilize the electric power grid. The magnet will be wound with 10-kilo-ampere superconducting cables composed of many round wires. It will be cooled to 20 kelvins by cryocoolers. A U.S. company turned to us for critical current measurements at 20 kelvins to determine whether its conductor could meet the project’s specifications for critical current. Critical current, the largest current a superconducting wire can carry, is a key performance and design parameter. Critical current depends on temperature, magnetic field, and, in many cases, the angle of the magnetic field with respect to the conductor.
We made variable-temperature critical-current measurements on three wire specimens in magnetic fi elds up to 8 teslas, at various magnetic- fi eld angles, and at temperatures from 4 to 30 kelvins. NIST has the only such multiparameter, high-current, variable-temperature measurement capability in the U.S. The largest current applied to the 0.81 millimeter diameter wire samples was 775 amperes.
The results showed that the angle dependence of critical current for the wires was less than just 3 percent over the useful range of field and temperature, and that the round wires could be used at higher magnetic fields and temperatures than tape conductors. These data will be used to design the safe operating limits of the SMES magnet system.
- Key Measurements for the International Thermonuclear Experimental Reactor — Superconducting magnets are used in fusion energy projects, such as the International Thermonuclear Experimental Reactor (ITER), to confine and heat the plasma. The superconductors for ITER’s large magnet systems are all “cable-in-conduit conductors” (CICC), which provide both mechanical support for the large magnetic forces and a flow path for the liquid helium required to cool the cable. The superconducting magnet must be operated below the critical current of the cable, which is a function of magnetic field and temperature. Temperature is an important variable, and the local temperature of the conductor depends on the mass-flow rate of the coolant and the distribution of the heat load along the CICC.
We designed and constructed a new variable temperature probe that allows us to make measurements in our 52-millimeter bore, 16-tesla magnet. This probe replaces one that was designed for our 86-millimeter bore, 12-tesla magnet. Fitting everything into the smaller bore was difficult, but the new probe performed as expected and allows us to make measurements at the ITER design field of 13 teslas. We made measurements up to 765 amperes with a Nb3Sn sample in fl owing helium gas. Measurements were made at temperatures from 4 to 17 kelvins and magnetic fields from 0 to 14 teslas. Some measurements were made at 15 and 16 teslas for temperatures from 4 to 5 kelvins; however, these magnetic fields can be generated only when a sample is measured in liquid helium. The results of our unique variable-temperature measurements provide a comprehensive characterization and form a basis for evaluating CICC and magnet performance. We used these data to generate curves of electric field versus temperature at constant current and magnetic field. In turn, these give a direct indication of the temperature safety margin of the conductor.
- International Standards on Superconductivity — Many of the 14 published IEC/TC 90 standards on superconductivity contain “precision” and “accuracy” statements rather than currently accepted statements of “uncertainty.” NIST has advocated that TC 90 adopt a more modern approach to uncertainty. In collaboration with the Information Technology Laboratory, we have developed a 50-page report on the possibility of changing statements of “accuracy” to statements of “uncertainty” in IEC/TC 90 measurement standards, which was presented at TC 90 meetings in June 2006. They included proposed change sheets for 13 of the 14 TC 90 document standards. Ultimately, all TC 90 delegates voted in favor of changing to uncertainty statements during the maintenance cycle of existing standards and during the development of new standards.
- Current Ripple a Source of Measurement Errors — All high-current power supplies contain some current ripple and spikes. New high-performance conductors have high critical currents that require current supplies over 1000 amperes. Highcurrent power supplies with the lowest level of current ripple and spikes are often more than a factor of ten times more expensive than conventional supplies. In addition, current ripple and spikes are a greater problem for short-sample critical current testing than for magnet operation because of the smaller load inductance. Therefore, we need to understand the effects of ripple and spikes on the measured critical current (Ic) and “n-value,” the index of the shape of the electric field-current curve. We focused on how ripple changes the n-value and showed that, in terms of percentage change, the effect of ripple on n-value was about 7 times that on Ic Interlaboratory comparisons often show variations in n-value much larger than the variations in Ic. We examined models and use the measurements on simulators to attempt to reproduce and understand the effects observed in measurements on superconductors. We believe that current ripple and spikes are sources of differences in n-values measured at different laboratories.
- New Method to Evaluate the Relative Stability of Conductors — We recently started measuring voltage versus magnetic field (V-H) on Nb3Sn wires to assess their relative stability. Voltage versus current (V-I) at constant field is usually measured to determine Ic. Low-noise V-H measurements were made at constant or ramping current with the same electronic instruments, apparatus, and sample mount as used in Ic measurements. High-performance Nb3Sn wires exhibit flux-jump instabilities at low magnetic fields, and low-noise V-H curves on these wires show indications of flux jumps. V-H measurements also reveal that less stable wires will quench (abruptly and irreversibly transition to the normal state) at currents much smaller than Ic at the lower magnetic fields. This new method needs to be further understood and may be standardized to ensure that it provides accurate and reliable data.
- Loren Goodrich is the Chairman of IEC/TC 90, the U.S. Technical Advisor to TC 90, the Convener of Working Group 2 (WG2) in TC 90, the primary U.S. Expert to WG4, WG5, WG6 and WG11, and the secondary U.S. Expert to WG1, WG3, and WG7.
- Ted Stauffer is Administrator of the U.S. Technical Advisory Group to TC 90.
In recent years, we have led in the creation and revision of several IEC standards for superconductor characterization:
- IEC 61788-1 Superconductivity - Part 1: Critical Current Measurement - DC Critical Current of Cu/Nb-Ti Composite Superconductors
- IEC 61788-2 Superconductivity - Part 2: Critical Current Measurement - DC Critical Current of Nb3Sn Composite Superconductors
- IEC 61788-3 Superconductivity - Part 3: Critical Current Measurement - DC Critical Current of Ag-sheathed Bi-2212 and Bi-2223 Oxide Superconductor
- IEC 61788-4 Superconductivity - Part 4: Residual Resistance Ratio Measurement - Residual Resistance Ratio of Nb-Ti Composite Superconductors Critical current vs. temperature of a Bi-2212 tape at a magnetic field of 0.5 tesla and various magnetic field angles. Such curves are used to determine the safe operating current at various temperatures and field angles.
- 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 61788-7 Superconductivity - Part 7: Electronic Characteristic Measurements - Surface Resistance of Superconductors at Microwave Frequencies
- IEC 61788-8 Superconductivity - Part 8: AC Loss Measurements - Total AC loss Measurement of Cu/Nb-Ti Composite Superconducting Wires Exposed to a Transverse Alternating Magnetic Field by a Pickup Coil Method
- IEC 61788-10 Superconductivity - Part 10: Critical Temperature Measurement - Critical Temperature of Nb-Ti, Nb3Sn, and Bi-System Oxide Composite Superconductors by a Resistance Method
- IEC 61788-11 Superconductivity - Part 11: Residual Resistance Ratio Measurement - Residual Resistance Ratio of Nb3Sn Composite Superconductors
- IEC 61788-12 Superconductivity - Part 12: Matrix to Superconductor Volume Ratio Measurement - Copper to Non-Copper Volume Ratio of Nb3Sn Composite Superconducting Wires
- IEC 61788-13 Superconductivity - Part 13: AC Loss Measurements - Magnetometer Methods for Hysteresis Loss in Cu/Nb-Ti Multifilamentary Composites
- IEC 60050-815 International Electrotechnical Vocabulary - Part 815: Superconductivity