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The paper gives an overview of the latest magnet design activities at CEA for the EU-DEMO Magnet Systems. It explains the latest changes in design constraints, inputs and definitions, the updates that have been implemented in the design tools, and shows the latest CEA proposed design for the TF magnet. We will discuss the recent updates in the TF and PF design tools which were used in 2022 and 2023. In particular, the tools have evolved to include additional constraints or operating points like possible sub-cooling of the coils or synchronous discharge of the axisymmetric PF/CS system. The electromagnetic coupling of the TF coils with coil case and radial plates is also discussed during a fast safety discharge, along with energy extraction and deposition sharing. Furthermore, a new 2D TF mechanical model is presented that permits to make a fast iteration with design tools and optimize the radial build. Finally, the paper also includes an update on the CEA TF conductor qualification sample, describing in particular the latest results obtained in the Twente Press.

For the ITER superconducting magnets, evaluation of their AC losses with computer models is essential for the preparation of the ITER magnet system commissioning and operation. A benchmarking effort on the recently tested CS Module 2 and 6 cold test AC loss data, revealed a significant discrepancy between model and experiment. For better understanding, the magnetization is measured on virgin strands and compared with AC loss of full-size CS CICCs in virgin condition and after being subjected to transverse load cycling. Eleven different ITER Nb3Sn strand types are tested in a Vibrating Sample Magnetometer (VSM) for different angles between strand axis and magnetic field orientation. In addition, few longer strand samples with different winding density were tested in the Twente magnetometer for comparison to short VSM sample results. The comparisons serve to evaluate the potential impact of cabling, transverse load strand damage, demagnetization and field angle. It was found that these effects were not large enough to explain the discrepancy between short sample measurements and model at one side and CS module loss at the other side.

For fusion tokamaks, such as International Thermonuclear Experimental Reactor (ITER), the magnet systems are based on superconducting cable-in-conduit conductors (CICCs). Due to the limited production length of the CICCs and the necessary electrical and thermal connections, joints are widely used to connect the CICC units within the magnet coils, as well as the electrical and thermal interfaces outside the magnet. The joints dominate the current redistribution in the coils and, thus, are critical for the magnet stability. In this study, an ITER poloidal field coil joint operating in pulsed mode is chosen to study the current nonuniformity and relevant stability issues. An optimized joint design by applying high resistive polyimide layers, called “masks” to actively block the large induced coupling currents, is tested with the numerical code JackPot-AC/DC. The current nonuniformity, power dissipation, and thermal evolution of the joints with different configurations are quantitatively analyzed.

State of the art MgB2 is reviewed as a potential material for the poloidal field (PF) coils of the future fusion reactor due to its high critical temperature and low material cost. The heat treatment process is a crucial step in the development of MgB2 magnets. The temperature lag in heat treatment of large magnets can lead to insufficient thermal reaction time. It may be infeasible to control the temperature of a magnet according to the heat treatment scheme recommended for the MgB2 wire. Hence, the heat treatment process of a large magnet needs to be evaluated. Therefore, the dynamic temperature distribution of a MgB2 PF coil is obtained by simulating the heat transfer in heat treatment process. A suitable heat treatment schedule for a large magnet is proposed and the experimental results of a sub-size Cable-In-Conduit Conductor manufactured with MgB2 strand confirmed the feasibility of the newly proposed heat treatment process. The results provide a reference for the heat treatment method of a future larger MgB2 coil.