Re-design of EU DEMO with a low aspect ratio

The design point that had been chosen for EU DEMO in 2016 is reviewed here and a modification is proposed with a lower aspect ratio. Previously the same aspect ratio, A, was chosen for EU DEMO as in major tokamak experiments including ITER (A = 3.1), and, to rely on mature technology, a peak magnetic field no greater than 13 T was considered. Here we do not consider these limitations recognizing the recent commissioning of JT60-SA with A = 2.5 and the successful recent operation of a model coil at a field of >20 T. EU DEMO must have a burning plasma and meet performance requirements relevant to a fusion power plant - at present, 2 GW fusion power and 2 h pulse length. The better plasma confinement at higher magnetic field allows reaching this condition in a smaller plasma. Thus, increasing the magnetic field appears as an obvious strategy to reduce the machine size. We confirmed though previous observations that the choice of a high magnetic field is associated with a large aspect ratio, mainly to generate space for the larger TF coils. In practice the magnetic field strength on DEMO-size TF coils is limited to ~12 T by the high electromagnetic loads. Also, the extreme heat flux on the divertor increases further with the magnetic field. Hence the magnetic field on the plasma axis is limited in EU DEMO to ~5.4 T, its aspect ratio to approximately 3. The limiting factor to lowering the aspect ratio is the space on the inboard side. This is primarily driven by the requirement to integrate the central solenoid to drive the plasma current inductively. Our literature review suggests that non-inductive plasma scenarios, as considered in most power plant studies in literature, are optimistic and not sufficiently supported by experimental results. Also, the space required for the superconducting toroidal field coils, the tritium breeding blanket, and the neutron shield is substantial. For a DEMO device the space on the inboard side becomes insufficient for aspect ratios below ~2.6. We therefore conclude the aspect ratio of EU DEMO should be chosen within the range ~2.6 - ~3.0 trading-off lower magnetic field and lower divertor heat loads against machine compactness.