Story by JEN A. MILLER
Shojiro Takeyama was braced for an explosion. A blast with the force of five sticks of dynamite was about to rock his laboratory at the Institute for Solid State Physics at the University of Tokyo.
Shojiro Takeyama (right) and engineer Hironobu Sawabe in the "anti-explosion house" before the big blast.
But he wasn't frightened: He was excited. After all, he had been carefully planning the explosion for more than five years.
The point wasn't to make a big bang — although he would. Rather, that big bang was just a tool to generate an extremely high magnetic field using a technique Takeyama had been developing for years, called electromagnetic flux-compression.
On that day in April 2018, he and his team were hunkered down, their experimental device contained within an iron "anti-explosion house" nearby. They were far enough away that they didn’t need earplugs, and could see what happened via a monitor.
When the system did, in fact, explode, the team was safe — and they did hear "a big sound," Takeyama said. They were also overjoyed. They had hoped to create a magnetic field of 700 teslas (the unit of magnetic field strength). Instead, they reached 1200 teslas, about 400 times stronger than a typical MRI machine, and a new world record for a controlled magnetic field.
"I was surprised," said Takeyama of the explosion, which dislodged the doors of the anti-explosion house, "but the machine was designed in such a careful way that I knew this could be achieved."
Such high magnetic fields have been made before, but only with TNT detonated outside and resulting in an uncontrolled explosion.
Takeyama's goal was to create a field that was both ultra-high and manageable, so that it could be used in experiments to study materials in extreme environments. Instead of using explosives, the Takeyama group used a set of nested coils. The first coil created a static magnetic field of about 3.2 teslas (in the range of what an MRI machine generates). In the middle, they added a coil attached to capacitors storing five megajoules of energy (think of a minivan moving at 100 miles per hour). Inside that coil was a lightweight copper ring. When the capacitors released their charge, it created, thanks to electromagnetic induction, a sudden, strong magnetic field that counteracted that static magnetic field. The resulting forces caused the copper ring to implode, which in turn compressed the magnetic field inside the ring, causing it to surge to a whopping 1200 teslas. When it couldn’t compress anymore, it exploded out. In a matter of 40 microseconds, it was all over.
"In general, producing higher magnetic fields comes at the expense of shorter duration," said Ross McDonald, deputy director of the Pulsed Field Facility, a branch of the National High Magnetic Field Laboratory that is located in Los Alamos, New Mexico. The facility has developed and maintained a set of pulsed magnets, including a 100-tesla instrument that creates the highest nondestructive field in the world.
Reaching fields up to 45 teslas, continuous-field research electromagnets can run indefinitely, McDonald said, as long as you keep running current through them and the heat produced is dissipated. The higher-field pulsed magnets can only operate for seconds, or even milliseconds, at a time. Still, that's enough time to learn a lot about the material placed inside that field for the experiment.
Illustration of the magnet infrastructure.
Takeyama's magnet is a kind of pulsed magnet. Although it self-destructs, scientists should still be able to get valuable data out of an experiment during its brief duration.
"Above 100 teslas, there is no current apparatus strong enough to generate even a short-duration field pulse without being destroyed," McDonald said. "The novelty of professor Takeyama's design is that one pulsed magnetic field is used to compress the other, as opposed to chemical explosives. This enables the apparatus to be operated in a laboratory as opposed to outside at a dedicated firing site."
Because the magnetic field created by Takeyama's device is far briefer and more compact than that of the 100-tesla magnet, it also requires less energy — only a few megajoules compared to several hundred.
Takeyama said these ultra-high fields could reveal never-before-seen behaviors in electrons, which could have implications for both material science and fusion power generation. "We can expect to see new physics of electrons in solids," he said.
Many future breakthroughs will require the kind of tools Takeyama is developing.
"Society's ability to take advantage of new materials, in particular for new electronics applications, requires deep, fundamental understanding of how the electrons in a given material behave," said McDonald. "High-field research provided this information for the semiconductors in today's electronics decades prior to their common application. Our ability to design using materials with new functionalities ultimately requires even higher fields to gain comparable understanding."
For more details, read the Takeyama group's journal article, Record indoor magnetic field of 1200 T generated by electromagnetic flux-compression in Review of Scientific Instruments.