Reports of critical current suppression during cryogenic ion irradiation of rare earth barium copper oxide (REBCO) tapes have raised concerns for the operational margins of fusion power plant (FPP) magnets.
However, the data regarding beam heating remains inconclusive due to the difficulty of measuring local temperatures with contact probes. This leaves a critical knowledge gap concerning the mechanism behind suppression and whether the so-called beam-on effect is to be expected under neutron irradiation during FPP operation.
Now, a series of experiments have demonstrated that this instantaneous effect of neutron bombardment, known as the “beam on effect,” should not be an issue during reactor operation, thus clearing the path for projects such as the ARC fusion system being developed by MIT spinoff company Commonwealth Fusion Systems.
MIT professor Michael Short said, “Nobody knew if it would be a concern. Our group thought, man, somebody should look into this. But now, luckily, the paper’s result is: It’s conclusively not a concern.”
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The issue was first noticed during initial tests of the REBCO tapes for the ARC system. Devitre recalls a night when they were in the accelerator lab, and the critical current measurement suddenly dropped by 30 percent under radiation conditions compared to after irradiation.
Previously, researchers had tested the REBCO tapes after irradiation. Short mentions that they decided to measure during irradiation, as it would be in a working reactor. This showed a significant difference, which is crucial for reactor design.
After running many carefully calibrated tests, researchers found that the drop in critical current was not caused by irradiation but by temperature changes due to the proton beam used in the experiments. This wouldn’t be an issue in an actual fusion plant.
MIT graduate student Alexis Devitre says they repeated the experiments many times and collected about a thousand data points. Statistical analysis showed that the effects were the same when the material was just heated and when it was both heated and irradiated. This ruled out that the suppression of critical current was caused by the “beam on effect,” at least within the sensitivity of their tests. Short mentions that while they can’t say there’s no effect, they can confirm no significant effect.
Only a few such facilities exist worldwide, and building a special facility was necessary for these tests. Short notes that they wouldn’t have found the answer without this custom-built facility.
The finding that this issue doesn’t affect fusion plant design shows the importance of negative results. Proving something doesn’t happen can save scientists time and effort. In this case, fusion companies can ignore this effect in their designs, reducing one risk.
This is a relief for Commonwealth Fusion Systems and other companies working on fusion plants. There are still concerns about the long-term degradation of REBCO over years or decades, which researchers are investigating. Others using these magnets for satellite thrusters and particle accelerators can also worry less about this issue now.
Journal Reference:
- A R Devitre, D X Fischer, N Riva, M Rae, L D Kortman, K B Woller, Z L Fisher, M P Short, D G Whyte and Z S Hartwig. Beam heating explains critical current suppression measured during ion irradiation of REBCO tapes. Superconducting Science and Technology. DOI 10.1088/1361-6668/ad95c2
