High-temperature superconducting large-current conductor with simple stacking

Overview

In a magnetic confinement fusion reactor, a strong magnetic field generated by a superconducting magnet confines ultra-high-temperature plasmas of more than 100 million degrees Celsius. The superconducting magnets used in current experimental devices are cooled to minus 269 degrees Celsius using liquid helium. In order to develop future commercial fusion reactors, it is expected that High-Temperature Superconducting (HTS) magnets that can operate at elevated temperatures will be realized. This will enable operation without consuming large amounts of helium, the supply of which continues to be insecure. It also has the advantage of generating a stronger magnetic field and reducing the size of the reactor, which is expected to accelerate the realization of fusion reactors. A research group led by Professor Nagato Yanagi at the National Institute for Fusion Science has succeeded in developing a stable and strong HTS large-current conductor, named STARS, that can be practically applied to the magnets of fusion reactors and the next-generation fusion experimental devices. A paper summarizing the results of this research was published in the Journal of Physics Conference Series on August 9, 2023.

Background

Superconducting magnets are used in MRI devices for diagnostic imaging in hospitals, which presently employ thin, round, Low-Temperature Superconducting (LTS) wires. In fusion research, they are used widely, such as in the Large Helical Device (LHD) at the National Institute for Fusion Science (NIFS) and the ITER which is under construction in France as an international project of 35 countries. They are cooled by liquid helium and must be kept at a cryogenic temperature of minus 269 degrees Celsius. The LTS wires have been developed since the 1960s and are now technologically mature. High-temperature superconducting (HTS) materials, on the other hand, were discovered in 1986, and more than 30 years of research and development have led to the commercial production of long wires. Compared to LTS, HTS wires can be used in magnets at "high temperatures" around -253 degrees Celsius. Since helium is currently in short supply and will continue to be so in the future, there are high expectations for the use of HTS magnets that can be cooled by helium gas or liquid hydrogen so that overall consumption of helium is reduced.

In 2005, NIFS began the development of HTS large-current conductors applicable to fusion reactor magnets, ahead of other institutions in the world. The conductor employs the second-generation HTS wire, which is presently called Rare-Earth Barium Copper Oxide or REBCO. The wire has a tape form with a typical width of 4-12 millimeters and a thickness of 0.1 millimeter. In large magnets for fusion reactors, dozens of these tapes must be bundled together to form a "conductor" and the iron rule for conventional LTS conductors is to twist and transpose thin, round wires together. Otherwise, the conductor becomes unstable when the current changes quickly. Presently, this iron rule is followed by most of the HTS large-current conductors, and various complex structures have been proposed. However, REBCO-based tapes are difficult to twist and transpose, and there are concerns about having deformation and mechanical weaknesses. In contrast, the STARS conductor developed at NIFS breaks this iron rule by simply stacking tapes, thereby achieving a mechanically strong structure. In an HTS conductor, even if a tape carries a relatively larger current than others and exceeds the critical current, there is still enough room to pass the excess current to other tapes, thus keeping the temperature of the entire conductor from rising. The prototype conductor actually achieved 100,000 amperes in 2014 (still the world’s current record for HTS conductors), which was a verification of the principle, but it took another eight years to make it into a full-scale conductor that could be put to practical use.

Development of HTS large-current conductors applicable to fusion reactors is underway worldwide. Various types of conductors and coils have been developed by many research institutes and private start-up companies, but none of them have been fully perfected, and the competition in the world is intensifying.

Research Results

NIFS has now developed a 20,000 ampere-class HTS STARS conductor that can be applied to next-generation fusion experimental devices. It is characterized by its high current density (current amplitude divided by the cross-sectional area of the conductor). The target current density is 80 amperes per square millimeter, which is about twice that of an LTS conductor of the same size. The STARS conductor consists of 15 layers of REBCO tapes, encased in a stabilized copper jacket and strengthened by an outer stainless steel jacket (IMAGE). A coil-shaped sample having three turns of a diameter of 60 cm and 6-meter-long conductor was fabricated by Metal Technology Co. Ltd. using the state-of-the-art EuBCO tapes manufactured by Fujikura Ltd. It was tested in the NIFS large-diameter high-field conductor testing facility and stably energized up to its rated current of 18,000 amperes at a temperature of minus 253 degrees Celsius and a magnetic field strength of 8 Tesla. This means that the target current density of 80 amperes per square millimeter was achieved. In addition, it was confirmed that the coil was energized stably even when the current was raised and lowered at a high speed of 1,000 amperes per second and repeated a total of more than 200 times with no change, indicating that the STARS conductor is stable and robust. In order to wind a large coil, it is necessary to extend multiple conductors (each ten to several hundred meters long) by connecting them together, and the STARS conductor employs the "mechanical lap joint" technique developed by Dr. Satoshi Ito of the Department of Quantum Science and Energy Engineering, Graduate School of Engineering, Tohoku University. This method enables low-resistance joints of HTS tape inside the conductor. The same technique was applied to the current feeders in the present conductor sample, which was helpful in obtaining good results.

Significance of Research and Future Perspectives

Based on the results of the present conductor test, it was concluded that the 20,000-ampere-class HTS STARS conductor is suitable for the magnets of next-generation fusion experimental devices. If this conductor is further enlarged to a current of 40,000 amperes or more, it can be used in large magnets for future fusion reactors. In the future, it is planned to fabricate a large test coil using the STARS conductor to achieve 20 Tesla or more for further demonstration.

About magnetic confinement fusion

Magnetic confinement is one of the methods to achieve nuclear fusion on Earth. Confinement of plasmas at 100 million degrees Celsius has been demonstrated in tokamaks and stellarator-heliotron devices with cages made of magnetic field lines in the shape of doughnuts.

About High-Temperature Superconductivity (HTS)

It was discovered in 1986 that a special ceramic based on copper oxides exhibited superconductivity when cooled by liquid nitrogen, which has a higher temperature than conventional materials. Since then, more than 30 years of research and development have led to the commercial production of practical wires. In addition to its application in nuclear fusion reactors, HTS technology is being developed for use in power transmission cables, hospital MRIs, transformers, generators, motors, and others.

About STARS conductor
The STARS conductor was named after the acronym “Stacked Tapes Assembled in Rigid Structure”, with a meaning that fusion is to realize the power of stars. The STARS conductor has incorporated three innovations that were unprecedented elsewhere. The first is the simple stacking of REBCO tapes (see the main text for details). The second is the employment of an electrical insulation layer inside the conductor, which facilitates coil fabrication rather than with the usual electrical insulation on the outside of the conductor. The third is the capability of installing a mechanical joint of REBCO tapes inside the conductor, which enables high-speed coil winding using an industrial robot to connect the STARS conductors of about ten meters in piece length. NIFS is also developing other HTS large-current conductors called FAIR and WISE, in parallel.