Critical Current Metrology

Summary

The main focus of the project is to develop standard techniques for the measurement of critical current of high-temperature and low-temperature superconductors. Some applications for which these types of measurements are crucial include: magnetic-resonance imaging, research magnets, fault-current limiters, magnetic energy storage, motors, generators, transformers, transmission lines, synchronous condensers, high-quality-factor resonant cavities for particle accelerators, and superconducting bearings. One area in which superconductors have the potential for making a significant impact is in fusion energy. 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 ultra-high magnetic fields that confine the plasma in fusion energy research. Physical Measurement Laboratory (PML) staff measure the magnetic hysteresis loss and critical current of marginally stable, high-current Nb₃Sn superconductors for fusion and other research magnets.

Description

PML staff have completed the construction and testing of a variable-temperature and variable-strain, or unified, apparatus for measuring critical current. The apparatus combines world class capabilities in variable-temperature and variable-strain measurements and is expected to be the highest-current apparatus of its type in the world. The new apparatus will help answer fundamental questions about the performance of strain sensitive superconductors. Measurements taken on the new apparatus facilitate the investigation of scaling models.

The top photograph shows the new high-current apparatus constructed at NIST to measure the critical-current dependence on strain, temperature and magnetic field. The worm-wheel that torques the spring can be seen through the small, round window. The lower photograph shows the CuBe spring with a helical sample soldered to the spring. Three pairs of voltage taps cover the three central turns of the spring. The current contacts are made at each end of the spring.
Photo Credit: Loren Goodrich

Scaling models are very complicated, nonlinear functions of magnetic field, temperature, and strain versus pinning force, or critical current. There are many scaling models currently in use, so a long-term objective of this project is to provide some guidance to the superconductor community regarding the best scaling models. SED staff have succeeded in fitting the three types of unified scaling models (temperature, strain, and combined temperature and strain) to critical current data. The data and subsequent model fits will be used to verify or determine the limits of scaling laws. Such information would greatly reduce the amount of data and liquid helium required to measure new samples in the future.

Major Accomplishments

A review paper summarizing our findings, "Unified Scaling Law for Practical Superconductors, Part II: Parameterization Testing, Scaling Constants, and the Extrapolative Scaling Expression," by J. Ekin, N. Cheggour, L. Goodrich, and J. Splett, is in preparation.