Sustaining superconductors

The National High Magnetic Field Laboratory (NHMFL) spans three locations: the University of Florida in Gainesville, Florida; Florida State University in Tallahassee, Florida; and Los Alamos National Laboratory in Los Alamos, New Mexico.

Each site has a magnetic field specialty, and Los Alamos is home to the Pulsed Field Facility (locally called “the MagLab”), where strong magnetic fields can be generated up to a few seconds following a pulse of electric current. This differs from the Tallahassee and Gainesville facilities, which generate weaker magnetic fields continuously. The magnets at Los Alamos are some of the most powerful in the world, and the strongest magnet at Los Alamos is capable of reaching 100 tesla (for comparison, a refrigerator magnet is about 0.005 tesla, and the Earth’s magnetic field is about 0.00005 tesla).

As part of the NHMFL, the MagLab is a user facility that is open to researchers from across the world. Hundreds of scientists use the facility annually, and included in that group are many Los Alamos scientists. Their experiments cover a range of programmatic and exploratory areas, including research with potential implications for the energy sector.

For the past two decades, the MagLab has been particularly useful for the study of superconductors, which are materials that typically expel magnetic fields and have no electrical resistance. High-temperature superconductors—superconductors that are capable of operating at just above -200 degrees Celsius—are key to creating commercial fusion energy because their exceptionally powerful magnetic fields can quickly and efficiently confine plasmas.

The Achilles heel of superconductors are magnetic vortices, which appear inside type II superconductors that are subjected to forces from electrical currents. These electrical forces cause the vortices to move, and the movement of these vortices generates dissipation preventing the superconductor from doing its job. 

However,  the vortices can be anchored at “pinning centers,” which are essentially material defects inside the superconductor. This anchoring process prevents energy dissipation, allowing the superconductor to stay in a superconducting state at higher currents. 

“Any superconductor that is useful for power applications is a type II superconductor,” says Los Alamos scientist Boris Maiorov, who has worked at Los Alamos for more than 20 years. “Back in the early 2000s, we found that columns were created when we added barium zirconate [to superconductors]; these columns are really good pinning centers that allow the superconductor to generate and withstand much higher magnetic fields.” 

While introducing defects through the addition of materials is now standard in superconductor production, adding too many defects can kill a superconductor. In a fusion reactor, for example, a superconductor is bombarded with neutrons, which produces defects. Determining how many defects a superconductor can sustain and how they react at high magnetic fields is now a focus of research at the MagLab.

“In the beginning, those defects are going to help you,” Maiorov says. “Then the question becomes: how many is too many?” To answer this question, Maiorov and other researchers are leveraging the Lab’s extensive plutonium expertise. Maiorov and his colleagues can identify properties and behaviors of plutonium that are applicable to superconductors, and then use that information to predict how a superconductor might behave. “For example,” he says, “we’re studying the self-irradiation of plutonium as a function of time and temperature, and we can use what we learn to try and model how superconductors might respond to the radiation of a fusion reactor”—an attractive outcome in more ways than one. ★