Evolutions of EU DEMO reactor Magnet System design along the recent years and lessons learned for the future

The DEMO reactor is expected to be the first application of fusion for electricity generation in the near future. In the DEMO power plant the management of magnet system is of central importance as being driver on many crucial aspects such as nominal power plant performance (toroidal field scales fusion power), overall investment budget (about 1/3 of the total construction cost), production efficiency (full power total availability heavily impacted by magnet off-normal events). Therefore a careful approach is requested for this kind of component to ensure a safe design compatible with a power plant production conditions, keeping a control on the factors prone to degrade the economic model (cost, risk). The derivation of those considerations into practical activities results in a constant attempt to lead in parallel extensive design activities and the mastering of upstream knowledge in magnet behavior. In this purpose design activities on DEMO magnet system were continuously conducted in Europe, particularly evolving since 2012 in structured environments, always backed on the association of several laboratories. Since then, the actors underwent preparatory design phase and then the preconceptual design activity (CDA) phase, that led to evolutions of design in many aspects, from design features bottom lien themselves to evolution of associated tools and methods up to global strategic considerations.