In the framework of the DEMOnstration fusion power plant (DEMO) design coordinated by the EUROfusion consortium, a pre-conceptual design of the superconducting magnet system has been developed. For the toroidal field coils (TFCs), three winding pack (WP) options have been proposed; exploring different winding approaches (pancakes vs. layers), and manufacturing techniques (react & wind vs. wind & react Nb 3 Sn). Thermal-hydraulic and mechanical analyses on the three WPs have produced encouraging results, with some critical issues to be solved in future studies and optimizations. The experimental tests on TF prototype short sample conductors have demonstrated a limited performance degradation with electromagnetic cycles and significantly lower effective strains than most of the large-size Nb 3 Sn conductors reported in literature. The toroidal field quench protection circuit has been studied, starting from different topologies and focusing on the most promising one. Two designs are also presented for the central solenoid magnet, with preliminary evaluations on the AC losses during the plasma breakdown. Finally, the design of a TF winding pack based on HTS conductors and the experimental tests on "fusion-relevant" HTS cables are illustrated.

The European DEMO, i.e. the demonstration fusion power plant designed in the framework of the Roadmap to Fusion Electricity by the EUROfusion Consortium, is approaching the end of the pre-conceptual design phase, to be accomplished with a Gate Review in 2020, in which all DEMO subsystems will be reviewed by panels of independent experts. The latest 2018

State-of-the-art high field solenoids make use of hybrid designs exploiting the superior high field performance of High Temperature Superconductors (HTS) in the innermost region. The benefits of a hybrid Central Solenoid in a pulsed tokamak like the EU DEMO can be two-fold: either to reduce its outer radius (which would result in a reduced overall size and cost of the tokamak), or to increase the generated magnetic flux (which could extend the plasma burn time and possibly increase the power plant efficiency). In the framework of the pre-conceptual design studies for DEMO coordinated by EUROfusion, a hybrid Central Solenoid is proposed based on ten layer-wound sub-coils using HTS, Nb3Sn, and Nb-Ti conductors respectively for the high, medium, and low field sections. The design exploits the flexibility of layer winding by grading both the superconductor and the stainless steel cross sections in each sub-coil, which has the potential for space and cost savings. Mechanical analyses have identified fatigue as the main design driver for the EU DEMO Central Solenoid. Possible alternatives to reduce the sensitivity of the proposed design to fatigue are currently under investigation.

The largest modular stellarator, the Wendelstein 7-X (W7-X), has completed its second phase of operation, OP1.2, in Greifswald, Germany. The inertially cooled divertor installed between mid-2016 and mid-2017 has allowed a wider range of plasma configurations in comparison with the first operation phase, OP1. The sophisticated W7-X superconducting magnet system is further loaded up to 70% of its maximum design loads for all main components. The extensive set of mechanical sensors clearly shows a highly nonlinear system response, which is in rather good correspondence with the predictions from the available advanced numerical models. However, there are also significant deviations observed in several areas. Therefore, modeling improvements and/or parameter variation analyses are necessary to clarify the issues in preparation for the upcoming, more demanding phase OP2 (2021+) with the actively cooled divertor and longer plasma pulses to guarantee safe and reliable W7-X operation. The updated strategy to release multiple new plasma configurations being compatible with W7-X component design values is described briefly. In this approach, the numerical model linearization in the vicinity of an accurately analyzed point is a key method to accelerate the process and to highlight areas for vacuum field parameters not allowed for plasma operation due to structural criticality. A brief overview of the W7-X measurement results, the observed deviations with numerical models, and the implemented improvements, as well as the lessons learned so far, are presented.

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