The Wuhan National Pulse High Magnetic Field Science Center has been struck by electromagnetic storms one after another, causing the substance to be forced to reveal unknown secrets. The most advanced power supply, magnet and control system have created this world's top-level strong magnetic field device.

"The magnetic field is proportional to the current, and the force and heat the magnet bears are proportional to the square of the magnetic field. So the harder it is to go up. It's as difficult as a 100-meter race from 9.9 seconds to 9.8 seconds." National Pulsed Strong Magnetic Field Li Liang, director of the Science Center, said.

"It took 20 years for the United States to go from 70T (Tesla) to 90T, 15 years for Germany, and 4 years for China." Li Liang said.

The service life of the coil is nearly double that of the international counterparts

In 1820, the Danish Oster accidentally discovered that a small magnetic needle nearby would jump when the wire was energized. Since then, people have been using energized coils to create magnetic fields. A strong magnetic field of a few milliseconds generated by an instantaneous strong current is called a pulsed strong magnetic field, which is higher than the steady-state magnetic field.

Researchers from the National Pulsed High Magnetic Field Science Center told us: They use a small finger-thick wire, wound into a coil, connected to a voltage of 25,000 volts, and an instantaneous current of 40,000 amperes can generate a pulsed magnetic field of tens of T. .

However, the energized coil will be stretched and crushed by the internal stress of the magnet. Li Liang said that the stress on their coils is 50 times the pressure that the "Jiaolong" is facing at 7,000 meters under the sea. Moreover, the magnet will generate huge heat during the discharge process, and the coil is soaked in liquid nitrogen to cool down; the liquid nitrogen at minus 200 degrees Celsius will evaporate at the moment of electrification.

In a few milliseconds, the energy of the magnet, such as the energy of 10 kilograms of TNT, is often broken. Scientists in Russia and Japan have used extreme methods to create a magnetic field of thousands of T, which is currently the strongest magnetic field that mankind has achieved. However, the coil must be sacrificed, which is disposable, and the magnet coil cannot be reused.

The record-breaking coil of the National High Magnetic Field Laboratory of the United States is made of copper-niobium alloy, which is the strongest-the biggest contributor to the 100T magnetic field record. The Chinese team uses a domestically developed copper-niobium alloy whose strength is only 2/3 of the former.

The Chinese took a different approach, tapped the potential from theoretical analysis, and through precise calculations, greatly improved the effect of layer-by-layer winding and reinforcement of polymer fibers, just like an iron ring holding a gun barrel. It is stronger than Kevlar and is the strongest fiber in the world. The fiber impregnated with epoxy resin also acts as an insulating layer for the coil.

Professor Tao Peng takes full-time winding. "If the resin is not soaked sufficiently, the reflection will be different." Peng Tao said. From the lines and colors of the coils, veterans can see the flaws. The flaw caused the coil to collapse earlier.

The American magnet coil has a diameter of one meter, which is much larger than China's, and has lower stress and is not easy to break. However, the American coil will break if it is energized 500 times; the coil made by Peng Tao can be used 800 times.

China's power supply is cheap and powerful

Li Liang has worked in Europe and the United States for many years, and he participated in the design of most of the world's strongest magnets since 1992.

"We are overtaking on corners." Li Liang said, "We started from nothing and summed up the experience of each company, so the design is more integrated and systematic."

The generator power supply of the National Strong Magnetic Field Laboratory of the United States, two stories high, a huge flywheel energy storage, and instantaneous discharge-originally used in nuclear fusion experiments, the second largest contributor to the 100T record.

China's power supply is divided into three parts: generator power supply (power is less than 1/10 of the United States); more than 20 capacitor energy storage power modules; lead-acid battery packs. Through structural optimization, only a few capacitive energy storage power modules are used to superimpose the peak value of the magnetic field pulse of 90.6T. The United States needs 115 megajoules of energy to achieve 100T, while China only uses 10 megajoules to achieve 90T.

Professor Ding Hongfa, who specializes in power supply, said: "The switching time difference of dozens of modules should be limited to microseconds. Components should also be selected to make the circuit delays consistent."

Construction started in 2008, most of the equipment is self-developed and manufactured, and the localization rate is 85%. "People are outsourced to professional companies, we do it ourselves." Li Liang said.

Each group of capacitor energy storage type power supply is 1 million yuan. The investment in the entire power supply system is only a fraction of that of the Americans. The liquid helium recovery system is only 1/4 of the price of similar foreign equipment, and the recovered helium can save 5 to 6 million yuan in experimental consumption each year.

The pulse flat-top magnetic field is very important, but the Americans can only use it 50-60 times a year for the flat-top pulse magnetic field. The Chinese improved the power supply and control system, which is as simple as turning on a microwave oven.

"International experts say that we spent 120 million yuan and did 120 million US dollars of work." Li Liang said.

Measurement needs to be extremely fine

In October 2013, under the witness of experts from all over the world, the Chinese device made its first show successfully. An international authoritative report says: China's magnet and power supply technology is the world's top; the control system is internationally leading.

Although the highest magnetic field record is not as good as that of the United States, the Chinese device has obvious advantages-a central control system realizes the flexible combination of three types of power supplies and eight experimental stations. This is a killer feature of China's magnetic field, and it is more conducive to experimentation. Professor Han Xiaotao, the person in charge of the control system, said: “Everyone generates one waveform with one magnet. Ours can generate multiple waveforms with one magnet.”

The United States achieved 100.75T in 2013, Germany achieved 94T, and China also became a member of the 90T club with 90.6T. At present, only the United States and China are expected to set new records.

The strong magnetic field will test out new material properties and give birth to the next generation of electronic materials and chips. Professor Zengwei Zhu said: "Semi-metals such as bismuth and antimony are suitable for studying extreme situations under strong magnetic fields."

In November 2018, Peking University discovered "pair quantum oscillation", and the experiment was done in Wuhan. Five oscillations were clearly observed under the 58T magnetic field, and the logarithmic law was discovered.

The “eye of the storm” with a strong magnetic field is only 20 millimeters long, and all the samples and sensors are arranged in an almond-sized space, similar to micro-sculpting.

"Eddy currents, thermal effects, vibrations, electromagnetic interference... measurement is always accompanied by noise." said engineer Zuo Huakun.

It is difficult to measure a high-quality signal at a time because there are too many interference factors. The sample rod is immersed in liquid helium and is separated from the outside by a layer of vacuum, but the momentary high temperature of the coil will still cause a thermal disturbance of a few tenths of a degree in the sample. The circuit of the sensor may also vibrate several microns to produce noise under the action of the pulsed magnetic field.

There are many unmeasurable factors, such as the instability of the "zero" voltage of the ground wire and the difference in humidity, which may cause different measurement results.

"If it is not so unpredictable, those physical problems will not be left over to the present." Zuo Huakun said that the Wuhan National Pulsed High Magnetic Field Science Center is no weaker than any peer in terms of electromagnetic measurement accuracy.

"After the center was completed, not only did domestic scientists basically stop going abroad to do experiments, but also attracted many foreign users such as Cambridge and Stanford." Li Liang said.

(Original title The world's leading strong magnetic field was built by the Chinese)