High-temperature superconducting magnets constructed from REBCO, which stands for rare earth barium copper oxide, enable the generation of a powerful magnetic field capable of confining the extremely heated plasma essential for fusion reactions, which combine two hydrogen nuclei to produce a helium atom, releasing a neutron during the process.
However, initial assessments indicated that neutron exposure within a fusion energy facility might abruptly diminish the superconducting magnets’ capacity to conduct electricity without resistance (referred to as critical current), potentially leading to a decrease in the fusion power yield.
Currently, a collection of experiments has distinctly shown that this abrupt impact of neutron irradiation, known as the “beam on effect,” is unlikely to pose a problem during reactor operations, thereby facilitating initiatives like the ARC fusion system being developed by the spinoff company from MIT, Commonwealth Fusion Systems.
The results were published in the journal Superconducting Science and Technology, in a study by MIT graduate student Alexis Devitre and professors Michael Short, Dennis Whyte, and Zachary Hartwig, among others.
“No one really knew if it would be an issue,” Short clarifies. He recalls examining these early results: “Our group thought, wow, someone should definitely investigate this. But now, fortunately, the conclusion of the paper is: It is definitively not a concern.”
The potential problem first emerged during preliminary tests of the REBCO tapes intended for use in the ARC system. “I remember the evening when we conducted the experiment for the first time,” Devitre recounts. “We were all in the accelerator lab, down in the basement. It was a huge surprise because suddenly the measurement we were observing, the critical current, just dropped by 30 percent” under radiation conditions (simulating those of the fusion facility), compared to when it was merely measured after irradiation.
Prior to that, researchers had exposed the REBCO tapes to irradiation and then conducted tests afterward, Short notes. “We devised the idea to measure during irradiation, mirroring how it would function when the reactor is fully operational,” he remarks. “Then we noticed this significant difference and thought, oh, this is a serious issue. It’s a variable you would want to consider when designing a reactor.”
After a series of meticulously controlled tests, it became clear that the decrease in critical current was not due to the irradiation itself, but was merely an effect of temperature variations induced by the proton beam utilized for the irradiation studies. This factor would not influence an actual fusion plant, Short indicates.
“We repeated experiments ‘oh so many times’ and gathered approximately a thousand data points,” Devitre states. They conducted comprehensive statistical analyses to demonstrate that the effects were identical, under conditions where the material was heated compared to when it was exposed to both heat and irradiation.
This eliminated the possibility that the abrupt suppression of the critical current was connected to the “beam on effect,” at least within the sensitivity range of their experiments. “Our tests are highly sensitive,” Short comments. “We can’t definitively claim there’s no effect, but we can assert that there’s no significant effect.”
Conducting these tests required the construction of a specialized facility for this purpose. Such facilities are extremely limited globally. “They’re all custom designs, and without this, we would not have been able to uncover the answer,” he states.
The finding that this specific concern does not pose an issue for the development of fusion plants “demonstrates the value of negative outcomes. If you can conclusively establish that something is not occurring, you can prevent scientists from squandering time searching for something nonexistent.” In this instance, Short adds, “You can inform the fusion companies: ‘You might have believed this effect was real, but we have proven that it is not, and you can disregard it in your designs.’ Thus, that’s one additional risk mitigated.”
This may provide reassurance not only to Commonwealth Fusion Systems but also to various other companies pursuing fusion plant designs, Devitre explains. “There are several. And it’s not limited to fusion companies,” he continues. There remains the significant concern of long-term degradation of the REBCO, which could occur over years or decades, and which the team is currently examining. Others are exploring the application of these magnets for satellite propulsion and particle accelerators to analyze subatomic physics, where the impact could have also been an issue. For all these applications, “this is now one less concern,” Devitre concludes.
The research team also included David Fischer, Kevin Woller, Maxwell Rae, Lauryn Kortman, and Zoe Fisher at MIT, as well as N. Riva at Proxima Fusion in Germany. This research was funded by Eni S.p.A. through the MIT Energy Initiative.