Reviewed by Lexie CornerMar 3 2025
According to a study published in Superconducting Science and Technology, a series of experiments has shown that the immediate effect of neutron bombardment, known as the “beam on effect,” should not be an issue during reactor operation. This finding clears the way for projects like the ARC fusion system, which is being developed by MIT spinoff Commonwealth Fusion Systems.
High-temperature superconducting magnets made from REBCO, which stands for rare earth barium copper oxide, generate intense magnetic fields capable of confining the extremely hot plasma needed for fusion reactions. These reactions combine two hydrogen atoms to form a helium atom, releasing a neutron in the process.
However, some early tests suggested that neutron irradiation inside a fusion power plant might reduce the ability of superconducting magnets to transport current without resistance (known as critical current), potentially lowering fusion power output.
The findings were shared by Alexis Devitre, an MIT graduate student, along with professors Michael Short, Dennis Whyte, and Zachary Hartwig, among others.
Nobody really knew if it would be a concern. Our group thought, "Man, somebody should really look into this." But now, luckily, the result of the paper is: It’s conclusively not a concern.
Michael Short, Professor, Massachusetts Institute of Technology
The potential issue first emerged during preliminary tests of the REBCO tapes intended for use in the ARC system.
I can remember the night when we first tried the experiment. We were all down in the accelerator lab, in the basement. It was a big shocker because suddenly, the measurement we were looking at, the critical current, just went down by 30 percent.
Alexis Devitre, Graduate Student, Massachusetts Institute of Technology
It was detected under radiation conditions similar to those in the fusion system, rather than after the irradiation process.
Previously, researchers had irradiated the REBCO tapes and then evaluated them, as Short explained.
“We had the idea to measure while irradiating, the way it would be when the reactor’s really on. And then we observed this giant difference, and we thought, oh, this is a big deal. It’s a margin you’d want to know about if you are designing a reactor,” Short added.
After a series of precisely controlled tests, the researchers determined that the decline in critical current was caused by temperature variations from the proton beam used in the irradiation studies, rather than the irradiation itself. Short noted that this would not be a factor in an actual fusion plant.
“We repeated experiments ‘oh so many times’ and collected about a thousand data points,” Devitre stated.
They then conducted a detailed statistical analysis, demonstrating that the effects were identical whether the material was simply heated or both heated and irradiated.
This ruled out the idea that the instantaneous suppression of critical current was linked to the “beam on effect,” at least within the scope of their tests.
Short added, “Our experiments are quite sensitive. We can never say there’s no effect, but we can say that there’s no important effect.”
A specialized facility was required to perform these tests. Only a few similar facilities exist worldwide.
“They are all custom builds, and without this, we wouldn’t have been able to find out the answer,” he added.
The fact that this particular issue is not a worry for the design of fusion plants “illustrates the power of negative results. If you can conclusively prove that something doesn’t happen, you can stop scientists from wasting their time hunting for something that doesn’t exist.”
Short continued, “You can tell the fusion companies: ‘You might have thought this effect would be real, but we’ve proven that it’s not, and you can ignore it in your designs.’ So that’s one more risk retired.”
Devitre noted that this finding may provide reassurance not only for Commonwealth Fusion Systems but also for other companies working on fusion plant designs.
He added, “There’s a bunch. And it’s not just fusion companies.”
The team is now studying the long-term degradation of REBCO, which could occur over years or decades. Other researchers are exploring the use of these magnets in satellite thrusters and particle accelerators for subatomic physics research, where the effect may still be relevant.
For all these uses, “this is now one less thing to be concerned about,” Devitre concluded.
The research team also includes David Fischer, Kevin Woller, Maxwell Rae, Lauryn Kortman, and Zoe Fisher from MIT, as well as N. Riva from Proxima Fusion in Germany. This study was funded by Eni S.p.A. through the MIT Energy Initiative.
Journal Reference:
Devitre, A. R. et. al. (2025) Beam heating explains critical current suppression measured during ion irradiation of REBCO tapes. Superconducting Science and Technology. doi.org/10.1088/1361-6668/ad95c2