A new multicomponent, partially-superconducting electromagnet—currently the world’s strongest DC magnet of any kind—is poised to reveal a path to substantially stronger magnets still. The new magnet technology could help scientists study many other phenomena including nuclear fusion, exotic states of matter, “shape-shifting” molecules, and interplanetary rockets, to name a few.
The National High Magnetic Field Laboratory in Tallahassee, Florida is home to four types of advanced, ultra-strong magnets. One supports magnetic resonance studies. Another is configured for mass spectrometry. And a different type produces the strongest magnetic fields in the world. (Sister MagLab campuses at the University of Florida and Los Alamos National Laboratory provide three more high-capacity magnets for other fields of study.)
It’s that last category on the Tallahassee campus—world’s strongest magnet—that the latest research is attempting to complement. The so-called MagLab DC Field Facility, in operation since 1999, is nearing a limit in the strength of magnetic fields it can produce with its current materials and technology.
The MagLab’s DC magnet maintains a steady 45 Tesla of field strength, which until very recently was the strongest continuous magnetic field produced in the world. (Not to be confused with the electric car brand of the same name, Tesla is also a unit of magnetic field strength. The higher its Tesla rating, the stronger the magnet. For comparison, a typical MRI machine is built around a superconducting magnet with approximately 3 Tesla of field strength. The Earth’s magnetic field, felt at the planet’s surface, is 0.00005 T.)
The new research magnet edges out the MagLab DC Field magnet by a hair, maintaining a continuous field of 45.5 T. But it’s not the slight edge in strength that offers such promise, says David Larbalestier, chief materials scientist at the Magnetic Field Laboratory.
“This is a beachhead into the 50 Tesla territory,” Larbalestier says.
The new magnet, described in a recent letter to the journal Nature, uses a high-temperature superconducting material, chilled to liquid helium temperatures that old-school superconductors use. Cooling this particular superconductor below its critical temperature (the temperature below which it loses all electrical resistivity) actually increases its ability to handle higher currents. And higher currents translate, of course, to higher magnetic fields.
Older superconductors like the kinds used in MRI magnets cannot handle magnetic fields that exceed 30 Tesla, Larbalestier says. The Cooper pairs of electrons, key to the material’s quantum superconducting properties, become too unstable, so the superconductor loses its zero-resistance properties, and becomes like an eight-lane highway brought to a standstill.
Avoiding a so-called catastrophic “quench” is essential to operating a superconducting magnet for extended periods of time. (The Large Hadron Collider’s superconducting magnets famously suffered this problem in 2008.)
“We’re running these in liquid helium, because the superconductivity gets stronger, the lower in temperature you go,” Larbalestier says. “And what we want to avoid is the destruction of the superconductivity by the magnetic field.”
The other innovation that helps the magnet avoid or reduce quenching is its lack of insulation. Larbalestier says a typical electromagnet would have electrical insulation between layers of superconducting tape.
But his group discovered that non-insulated tape laid layer upon layer—like multiple Ace bandages wrapped around an athlete’s ankle—behaves a little like a single-layered thick superconductor.
So an obstacle or impurity in the superconducting lattice might, in a single-layer piece of tape made from rare earth barium copper oxide (REBCO), have impeded Cooper pairs and heated that section of the superconductor above the transition temperature. And that’s a quench—which means game over for the magnet’s strong field.
Avoiding the insulation allows for the Cooper pairs to reroute around an impurity in the lattice, avoiding the quench.
The research team has steadily improved the magnet’s ability to handle stronger fields. (They’ve also boosted the field by placing the superconducting magnet inside a larger copper and silver magnet.)
“We’re still interested in pushing the forefronts,” Larbalestier says. “So the inside of the new 32 T user magnet is made of this REBCO tape. And we saw the opportunity to get new variants of the tape… which were very thin—and a new method of constructing a superconducting magnet without insulation, invented by the lead author on our paper, Seungyong Hahn.”
The group thinks it can iterate their technology at least into the 50s of Teslas of field strength. But Larbalestier doesn’t see any clear reason why they’d need to stop there.
“The real significance here is, it’s a validation of these rare earth barium copper oxide [REBCO] superconductors for very high field use at low temperatures,” he says. “And I think it clearly says the road to 60 Tesla… is, in principle, now open.”
This post was updated on 18 June 2019.