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 approach 30 to 34 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.)
But newer so-called high-temperature superconductors like rare earth barium copper oxide (REBCO) avoid quenching in the design, in two ways. First, the researchers found that the system becomes more robust to high magnetic fields, the further it’s cooled below its superconducting transition temperature.
“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 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 REBCO tape, 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.
But a thicker layer of superconductor 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 magnet of 32 T 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.”