High Temperature Superconductors (HTS) such as Rare-Earth-Barium-Copper Oxide (REBCO) tapes, named coated conductors, are expected to play a technological key role in the road map towards the development of commercially affordable nuclear fusion power plants providing also new alternative approaches. With this perspective, in the last decade, a lot of efforts was spent on the development of HTS based conductors for fusion magnets. Based on its strong expertise and skills in both LTS conductors and HTS materials, the ENEA Superconductivity Laboratory started more than 10 years ago an internal program of development of HTS conductors that now merged in recent years with the activities carried out in the framework of international and national projects on nuclear fusion, such as EUROfusion for DEMO and the Italian DTT Divertor Tokamak Test facility Project [1].
Following the twisted-stacked-tape approach, proposed by MIT [2], ENEA first conceived and investigated, together with its industrial partner TRATOS Cavi, the aluminum slotted-core concept, characterized by stacks of HTS tapes incorporated into the slots created in an extruded Al stabilizer [3]. So far, the cable layout and manufacturing process have been successfully assessed by investigating the electric performances and mechanical behaviour either experimentally or by implementing dedicated codes. More recently, the thermal behavior was also investigated through dedicated experiments carried out at the SULTAN facility on a cable designed for operation in fusion-relevant conditions, at 4.2 K forced-flow cooled by supercritical He under a background magnetic field of 11 T with a critical current in excess of 15 kA. Samples showed good thermal stability and strong tolerance to successive quenches with minimal performance degradation only after a hot-spot temperature of more than 200 K.
In the framework of the EUROfusion work program for DEMO, ENEA has been involved in the development of a HTS conductor for the hybrid Central Solenoid (CS) coil system operating at 18 T, 4.2 K with a rated current of 60 kA. With this aim, a new SECtor ASsembled (SECAS) cable was recently conceived. This cable is based on a multistage manufacturing process in which the first stage is represented by a stack of tapes braided within a copper braid [4]. The main features of such a BRAided-STack (BRAST) are the flexibility and compactness, that could make it more tolerant to typical mechanical stresses occurring upon manufacturing. In the successive step, BRASTs are inserted into triangle-shaped slotted copper cores ultimately assembled in the SECAS conductor. In view of the final qualification of the cable performances to be carried out at the SULTAN facility within the next year, several sub-size prototypes have been manufactured and tested in the liquid nitrogen temperature to investigate the BRAST tolerance to and feasibility of the manufacturing steps for the final conductor assembly.
Since quench detection on HTS coils represents a crucial issue, due to the challenges of conventional voltage-based systems, ENEA has recently started a dedicated R&D activity aimed at exploiting the fiber optic thermometry [5]. An overlook of the activities, describing experimental results, design and modeling predictions together with the first concept and solutions for the integration of fiber optics into BRAST units will be presented and discussed.
Finally, the perspectives of the ENEA activities will be discussed.
[1] https://www.dtt-project.it/
[2] M. Takayasu, et al., Supercond. Sci. Technol., 2012, 25, 014011
[3] G. Celentano, et al., IEEE Transactions on Applied Superconductivity, 2014, 24(3), 6670053
[4] L. Muzzi, et al., IEEE Transactions on Applied Superconductivity, 2023, 33, 4200106
[5] G. Colombo, et al., IEEE Transactions on Applied Superconductivity, 2024, 34, 4702305
This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.
Topic: Magnets (for accelerators and physics research; for fusion; for medical, biological, and analytical applications; others)
Keywords: High Temperature Superconductors, REBCO, HTS conductors for fusion magnet