Miniature magnets break field strength record

Physicists at ETH Zurich in Switzerland have produced magnetic fields as high as 40 T in a superconducting coil that has a bore diameter of just 3.1 mm. Until now, creating such intense fields required large and expensive facilities and tens of megawatts of power. The new miniaturized structure requires a few thousand times less power than larger magnets and it could help bring ultrastrong benchtop magnets closer to reality.

“All previous 40 T class magnets have been metres in size, weigh more than six tons, and require about 20 MW of power to operate,” says Alexander Barnes, who led the research effort. “Our miniature magnet can also generate a 40 T magnetic field, but it is small enough to fit in the palm of your hand and requires a few watts or less to operate.”

Such a device could be extremely useful for scientists who use strong magnets in their research, he adds. “Rather than having to travel to the few locations in the world that have the resources and space to house a strong magnetic field, with this technology scientists in the future could have access to these magnets in their own laboratory.”

Making the magnet tiny

Barnes and his colleagues, who are nuclear magnetic resonance (NMR) spectroscopists, came up with the idea for their new magnet by asking themselves a simple question: “what do we need to put inside it in our experiments?” The answer was: only the sample and an NMR detection coil.

“So, instead of making magnets expensive and big enough to house all different kinds of equipment, we decided to make the magnet tiny – and just big enough to be able to fit inside it what we need to fit inside it,” says Barnes. In this way, any bulky components can be placed outside the magnet and only the essential elements within the high-field region inside it.

“Think about the right-hand rule and the Biot-Savart law we all learn in first year physics,” he explains. “This law tells us the more electrons moving in a circle, the higher the magnetic field. And the more electrons moving in a circle in a smaller volume close to the sample also means a higher magnetic field. This is all we did – we tried to maximize the electrons moving in a circle near our sample.”

High-temperature superconducting tapes

Strong magnets are needed in a host of research and technology areas, from magnetic resonance imaging (MRI) and particle accelerators to NMR spectroscopy. Magnetic fields greater than 40 T can be produced using high-temperature superconducting (HTS) tapes. These structures can also be wound together to increase their already very high critical current even further, something that allows the resulting coils to reach higher magnetic fields. A famous example, Barnes reminds us, is the world-record 45.5 T steady-state magnet, which uses a HTS coil as an insert within a resistive background magnet. The problem, however, is that these high-field hybrid magnets are huge and require a lot of power.

Barnes’ team says it might now have overcome this issue with its two compact HTS magnets wound with a conducting tape coated with the superconducting ceramic REBCO. The first magnet, composed of two pancake coils, produces a magnetic field of 38 T and the second, composed of four (quad) pancake coils, a field of 42 T. The researchers say they used a specialized winding technique combined with soldering to make sure there was a jointless connection between the pancake coils at a winding diameter of 3.5 mm.

The strong magnetic fields of the coils stem from the high current-carrying ability of REBCO and the extremely small magnet bore diameter of 3.1 mm. “These magnets reach current densities of 2257 and 1880 Amm⁻² at peak currents of 1246 and 1038 A, respectively,” says Barnes, “and despite the much higher current density, they consume a few thousand times less power and require a coil volume over 1000 times smaller than that of the 45.5 T hybrid magnet.”

“Amazing” materials

He says he imagines a “bright future” where there are hundreds and thousands of benchtop magnets capable of 50 T and more, all over the world in academia and industry.  These magnets can be used for NMR and electron paramagnetic resonance (EPR) spectroscopy, but also quantum computers and other applications. For instance, the ETH Zurich team is working on a project that uses these magnets to build miniature gyrotrons, which are microwave generators. “We have plans to use such devices for spectroscopy, but also for nuclear fusion heating and even vaporizing holes deep in the Earth to extract geothermal energy,” Barnes tells Physics World.

It will not all be plain sailing, however, say the researchers. One of the main challenges in this work, which is detailed in Science Advances, is to avoid damaging the REBCO-coated tapes. These tapes are “amazing” materials, says Barnes. They are a single crystal of rare-earth barium copper oxide and are more than 100 m long, but the problem is that they are subject to mechanical strain. If this strain exceeds a certain, critical threshold, then the superconducting layer can crack, leading to reduced current-carrying capacity as the structure’s resistance increases.

The researchers say they are now busy working on increasing the magnetic fields – they are targeting 50 T soon – and performing NMR inside their existing coils. “ResonX, the commercial partner on this study, is also actively commercializing these magnets,” reveals Barnes.

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