In 2012 EFDA published “Fusion Electricity – A roadmap to the realisation of fusion energy ” [https://www.euro-fusion.org/wpcms/wp-content/uploads/2013/01/JG12.356-web.pdf] that set out a strategic vision to demonstrate the generation of electrical power by a Demonstration Fusion Power Plant (DEMO) by 2050.

 

The roadmap elaborates 8 strategic missions to tackle the main challenges in achieving this overall goal. The need for the WPDTT2 (Definition and Design of the Divertor Tokamak Test Facility) Project is derived from Roadmap Mission 2: “Heat-exhaust systems”:

 

Heat-exhaust systems must be capable of withstanding the large heat and particle fluxes of a fusion power plant. The baseline strategy for the accomplishment of Mission 2 consists of reducing the heat load on the divertor targets by radiating a sufficient amount of power from the plasma and by producing “detached” divertor conditions. Such an approach will be tested by ITER, thus providing an assessment of its adequacy for DEMO. However, the risk exists that high-confinement regimes of operation are incompatible with the larger core radiation fraction required in DEMO when compared with ITER. If ITER shows that the baseline strategy cannot be extrapolated to DEMO, the lack of an alternative solution would delay the realisation of fusion by 10-20 years. Hence, in parallel with the necessary programme to optimise and understand the operation with a conventional divertor, e.g. by developing control methods for detached conditions, in view of the test on ITER, an aggressive programme to extend the performance of water-cooled targets and to develop alternative solutions for the divertor is necessary as risk mitigation for DEMO. Some concepts are already being tested at proof-of-principle level in ≤1MA devices (examples are super-X, snowflake, liquid metals). These concepts will need not only to pass the proof-of-principle test but also an assessment of their technical feasibility and integration in DEMO, perhaps by adjusting the overall DEMO system design to the concept, in order to be explored any further. The goal is to bring at least one of the alternative strategies (or a combination of baseline and some alternative strategy) to a sufficient level of maturity by 2030 to allow a positive decision on DEMO even if the baseline divertor strategy does not work. As the extrapolation from proof-of-principle devices to ITER/DEMO based on divertor/edge modelling alone is considered too large, a gap exists in this mission. Depending on the details of the most promising chosen concept, a dedicated test on specifically upgraded existing facilities or on a dedicated Divertor Tokamak Test (DTT) facility will be necessary. In either case, it will need sufficient experimental flexibility to achieve the overall target. The facility needs to be ready in the early 2020’s and is a good opportunity for joint programming among the EURATOM member states and for international collaboration. As the extrapolation to DEMO will have to rely on validated codes, theory and modelling effort is crucial for the success of this Mission and the simulation tools should provide reliable predictions on the behaviour of plasma edge and heat-exhaust systems in the DTT regimes.

 

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