Key factors controlling the radiation tolerance of REBCO tapes in a nuclear fusion environment

Abstract

Superconducting magnets based on Rare-Earth Barium Copper Oxide (REBCO) enable energy break-even in compact fusion power plants (FPPs). Given the critical role of the magnet system, the radiation tolerance of REBCO informs the thickness of the neutron shield, which has direct consequences on the ultimate FPP size and cost. The performance of fusion magnets is often gauged by the maximum current that these can carry without resistance, known as the critical current, I_c. However, predicting the radiation response of REBCO remains challenging because defect formation, evolution, and their net effect on I_c depend on the initial microstructure and operating conditions. An empirical approach is needed to identify the factors controlling I_c degradation in a fusion environment and to efficiently reassess the radiation tolerance of new REBCO tape formulations.

To address this, we designed and commissioned a cryogenic ion-irradiation platform capable of exposing REBCO tape to controlled, homogeneous light and heavy ion beams to emulate the consequences of fusion neutron bombardment. The system controls temperature (20-300 K) and assesses the superconducting properties of REBCO tapes with transport current measurements (100 nA-100 A, δV ≈ 0.1 μV) before, during and after irradiation — capturing for the first time all aspects of fusion magnet operating conditions, with the recent addition of a 0.33~T permanent magnet embedded in the target holder.

The results reveal that I_c degrades 1.6x faster at cryogenic temperatures than at 300 K—the latter is typical of in-core neutron irradiations previously used for lifetime dose assessments. Real-time measurements during irradiation also show a reversible suppression of I_c due to ion beam heating, enabling a direct determination of the beam spot temperature. Careful experimental design and statistical analysis were used to isolate and exclude the effect of ballistic displacements relevant to neutron irradiation. Preliminary results further suggest an acceleration of I_c degradation in the presence of a background magnetic field.

The main conclusion of this thesis is that radiation tolerance of REBCO tapes may have been overestimated in prior assessments. Our results provide the first dataset capturing all major operational factors and will inform future "design rules" for optimizing the radiation shielding of superconducting magnets in fusion power plants.