Standards for Superconductor and Magnetic Measurements

Goals

Loren Goodrich positioning probe with superconductor wire into the bore of a high-field superconducting magnet.

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 high-temperature and low-temperature superconductors. Applications supported include magneticresonance imaging, research magnets, fault-current limiters, magnetic energy storage, magnets for fusion confinement, motors, generators, transformers, transmission lines, synchronous condensers, high-quality-factor resonant cavities for particle accelerators, and superconducting bearings. 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.

Customer Needs

We serve the U.S. superconductor industry, which consists of many small companies, in the development of new metrology and standards. 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 makes this technology especially 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.

Technical Strategy

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).

Illustration of a superconductor's voltage-current characteristic with two common criteria applied.

Critical Current Measurements — One of the most important performance parameters for largescale 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 figure below is an illustration of the voltage-current characteristic and two criteria for critical current. Typical criteria are electric-field strength of 10 microvolts per meter and resistivity of 10^–14 ohm-meters.

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, heating of the specimen, from many sources, during the measurement can cause a wire to appear to be thermally unstable.

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. A few years ago we demonstrated that flux jumps could be suppressed during the measurement of hysteresis loss by immersing marginally stable Nb3Sn conductors in liquid He. The increased thermal conduction affords dynamic stability against flux jumps, which allows AC losses to be estimated from the area of the magnetization-versus-field loop. Many measurements we do for superconductor wire manufacturers require special techniques to obtain accurate results.

Magnetic Thin-Film Standard Reference Material — The Magnetics Group is developing a magnetic-moment, thin-film, standard reference material for the calibration of magnetometers used in the magnetic recording industry. The properties of the films will be traceable to fundamental quantities.

Accomplishments

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.

Earlier magnet systems that used CICC experienced unexpected degradation of their superconducting properties. To help determine the source of such degradation, we measured variable-temperature critical current of a "witness" superconductor strand that was thermally processed along with the superconducting cables used to make the latest two ITER test conductors.

The results of our unique variable-temperature measurements provide a comprehensive mapping of critical current as a function of magnetic field (0 to 12 teslas) and temperature (4 to 17 kelvins), and form a basis for evaluating CICC and magnet performance. We used the data to generate curves of electric field vs. temperature at constant current and magnetic field. These give in turn a direct indication of the temperature safety margin of the conductor.

Our results will be used by Lawrence Livermore National Laboratory, which will test CICC samples at the Plasma Physics Research Center in Villigen, Switzerland, with current up to 100 000 amperes and magnetic fields up to 12 teslas, while controlling the mass-flow rate of the coolant.

Diagram of the ITER fusion magnet systems. The central solenoid is along the vertical axis. The Dshaped structure is the toroidal field coil. At its periphery are six poloidal field coils. A human-scale figure is at the bottom. See http://www.iter.org/.

Electric field vs. temperature of the superconductor witness strand at a magnetic field of 11 teslas. These curves can be used to determine the temperature safety margin at different operating currents.

Superconductor Measurements for New Type of Magnetic Resonance Imaging — The critical current of a superconductor — the largest amount of current it can carry without reverting to the normal state — is arguably its most important parameter but the most difficult to measure accurately. This applies to new "dip-coated" bismuthbased high-temperature superconductor tapes on metal substrates.

Such conductors are to be used in a Superconductivity Partnership Initiative sponsored by the Department of Energy to make the first magnetic-resonance-imaging (MRI) magnet to operate near 25 kelvins without the use of a liquid cryogen. The magnet will have an open geometry for greater patient comfort.

We made variable-temperature critical-current measurements on five dip-coated tape specimens for a company making conductors for this magnet. Critical current depends on temperature, magnetic field, and the angle of the magnetic field with respect to the conductor. We made measurements in magnetic fields up to 3 teslas, at various magnetic-field angles, and temperatures from 4 to 30 kelvins. Ours is the only laboratory in the U.S. that has such a multi-parameter measurement capability. The largest transport current applied to the tape samples was 900 amperes.

A multi-parameter characterization of critical current may be used to optimize MRI magnet design and conductor performance. For a given target magnetic field, the total length of conductor needed can be calculated. Magnetic-field angle can be manipulated by the addition of magnetic flux guides, which can increase the critical current by a factor of two or more. System operating cost and complexity can be reduced by operating at higher temperature.

A prototype high-temperature superconductor MRI coil and cryostat system. The coil is conductively cooled, which allows for more flexibility in the system geometry.

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.

Standards Committees

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.

Standards

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