Permanent current that has been flowing for two years -what is its meaning?

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The forefront of research December 20, 2021

Nuclear magnetic resonance (NMR) devices are analytical equipment used in scientific research and product development sites.However, the superconducting electromagnet used for this device is large, and the installation location is limited because it is necessary to have a large mechanism to keep it at an extremely low temperature.Therefore, in order to make the device smaller so that it can be used easily, Yoshiki Yanagisawa Unit Leader (UL) is working on the development of electromagnetic stones using high -temperature superconductors.Recently, we have achieved the achievement of continuing the permanent current to electromagnetics for two years, but what does that mean?

Yoshinori Yanagisawa (Yoshinori Yanagi)

Life Function Science Research Center Functional Super High magnetic field Magnet Technical Research Unit Leader Born in 1985 in Chiba Prefecture.He completed the doctoral course of the Graduate School of Engineering, Chiba University.He is a doctor (engineering).Since 2013, he has been a researcher in RIKEN Basic Science, and has been the lab since 2016, and has been incumbent in 2021.

An electromagnet for NMR devices that wait for the introduction of high -temperature superconductors

The NMR device is an analysis device to examine the molecular shape and the nature of the substance in detail.By placing a liquid or solid sample in a strong magnetic field (a space where the effect of the magnet extends), and analyzing the signals obtained when applying electromagnetic waves from the outside, information on the state of atoms that make up the substance can be obtained.。The name of the device may not be used to it, but it is used in a wide range of fields, such as the development of medicines and materials, the research on life science and physics, and we have benefited without knowing it.In Europe, he is also active as a method of finding out the fake of wine.

NMR devices require powerful electromagnet because the stronger the magnetic field (the stronger the power of the magnet) increases the analysis performance.Therefore, a superconducting electromagnet in a coil -shaped wire material (wire) is used.Even with conductors used in elementary school experiments, if the flow of flushing is increased, electromagnetics become stronger, but the size of the current that can be flushed is limited due to electrical resistance.If you use a superconductor with zero electric resistance, you can generate a very strong magnetic field because it can flow very large current.

However, the wiring of the super-power aged (low-temperature superconductor), which is currently widely used, is mainly based on rarely metal niob, and the temperature (superconductible temperature) in which electrical resistance becomes zero is very low, as it is very low, from -250 ° C.。For this reason, it is cooled using a liquid helium at -269 ° C, but the liquid helium is expensive and requires large facilities to handle it, so there are limited research institutions and companies that can introduce NMR devices.

What is attracting attention is the high -temperature superconductor discovered in the late 1980s."The superconducting transition temperature is about -180 ° C, for example, but if it is cooled by a liquid nitrogen (-196 ° C) or a very low hot freezer than a helium, it will be in a superconducting state, so it will be a superconducting state.Yanagisawa UL explains that the device can be more compact and high -performance, and mass production will be possible.This was also the dream of Yanagisawa UL.

I want to connect ceramics perfectly

However, in order to use a high -temperature superconductor for the electromagnet of the NMR device, a "superconducting joint" that connects the wiring of high -temperature superconductor with zero electric resistance must be realized.This was a difficult task.

The wire of a low -temperature superconductor is like a wire, and a technique for creating superconducting joining is established using a superconductor solder.On the other hand, the high -temperature superconductor is a ceramic (pottery), and the wire is attached to the surface of a thin metal tape with a high -temperature superconductor.This wire and solder are not familiar, and the solder cannot be used because electric resistance is generated at the border between the two.What to do then.Many domestic and overseas researchers have narrowed their wisdom and have repeated trial and error.

そうした中で2017年、柳澤ULと住友電気工業株式会社の研究者を含む研究チームは、iGS^Ⓡ接合という斬新な方法を開発した（図1）。「この方法で接合し液体窒素で冷やしたところ、電気抵抗が見事にゼロになりました。高温超電導体の実用的な超電導接合に成功したという報告は、世界で初めてでした」と柳澤ULは胸を張る。

Fig. 1 Superconductor joining between high -temperature superconductor wires

The wire is a multilayer structure in which metal and rare earth -based high -temperature superconductors overlap.A short piece of high -temperature superconducting wires is placed on the arrangement of two of these wires and used as a joint.Between the wire and the short, the fine particles of the high -temperature superconducting material are applied to about 800 ° C.Then, as in the electron micrograph photo on the right, the direction of the crystal of the fine particles is integrated and integrated with the top and bottom.As a result, the electrical resistance of the joint is zero.

High -temperature superconducting electromagnetics for both MRI and linear

Yanagisawa UL and his colleagues later installed high -temperature superconducting electromagnet in the actual NMR device.NMR devices are often operated in "permanent current mode".First, a coil is connected to an external power supply to the external power supply, and a "closed circuit" is created in which the external power supply is disconnected.If this closed circuit is kept cool and the superconducting state is maintained, the current will continue to flow without decreasing.A strong magnetic field that continues to occur is used for measurement.

ただし、閉回路全体を電気抵抗ゼロにするには、永久電流スイッチ（電源との接続を切り替えるスイッチ）とコイルとの接合部分も電気抵抗ゼロでなければならない。そこで、柳澤ULらは、その接合部分をiGS^Ⓡ接合で作成した。そして、高温超電導電磁石をNMR装置内に設置し、永久電流を流した（図2）。2018年のことだ。それから2年間にわたり、発生する磁場を精密に測定することで永久電流がどのくらい減っていくかを調べたところ、減少率はごくわずかで、永久電流がゼロになるには300万年以上かかると計算された。

Fig. 2 NMR device implemented with high -temperature superconducting electromagnetics

レアアース系高温超電導体のコイルと永久電流スイッチとをiGS^Ⓡ接合でつなぎ、低温超電導線材の外層コイルの内側に置いた。超高磁場の高性能NMR装置の実現には、低温超電導体と高温超電導体のコイルの組み合わせが必要となるため、それを想定しての実験。接合部分は磁場の影響を受けやすいため、コイルから離れた位置に設置した。この超電導電磁石をNMR装置の中に収め、‐269℃の液体ヘリウムで冷やして永久電流を流した。右写真の手前に写っているのは、装置の制御や測定操作を行うためのコンソール（制御盤）。

"Not only is the permanent current flowing for two years, but in the meantime, it is actually measured as an NMR device. This success is a major step toward a wide range of practical use of NMR devices using high -temperature superconducting electrometer."And Yanagisawa UL.Of course, superconducting joining is also useful when connecting and longing many wires.If the wire is long, the number of windings of the coil can be increased and the electromagnet is strengthened, so the significance of achieving "usable" high -temperature superconducting joining is very large.

In addition, high -temperature superconductors contribute not only to easy NMR devices, but also to the realization of high -performance NMR devices that use unprecedented ultra -high magnetic fields.Yanagisawa UL and others are developing such high -performance NMR devices in the future social creation project of the National R & D Corporate Science and Technology Agency, and realizes easy -to -develop NMR devices, including high -temperature superconducting joining.I want to make use of it.

One problem for a wide range of practical use is that the wiring of high -temperature superconductors is very expensive because it takes time and effort.However, if a huge demand for wires is born in the energy field, the manufacturing cost is likely to decrease.In that case, not only NMR devices, but also MRI (magnetic resonance images) devices and linear motor cars that use the NMR principle to draw the body, will be used.Yanagisawa UL is steadily conducting research so that the day can come at any time.

(Interview / composition: Seiko Aoyama / Shooting: Masaru Aizawa. / Production Cooperation: Sitech Communications)

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