Influence of a high magnetic field to the design of EU DEMO

The limitation considered in the definition of DEMO in 2016 to not exceed the magnetic field strength foreseen in the ITER machine is reviewed here. At the time, a Nb₃Sn superconductor operated in a magnetic field, B_max, no greater than 13T was considered to rely on existing and mature technology. This limitation is no longer seen as a boundary here, recognizing that the first model coil with Rare Earth Yttrium Barium Copper Oxid (REBCO) superconductor was recently operated at a field of >20T. 

DEMO must have a burning plasma which, at a higher magnetic field, is reached in a significantly smaller plasma volume. Accordingly, a system code study was carried out to size the DEMO machine by varying the magnetic field. The outcome confirmed previous observations that the magnetic field strength is linearly dependent on the plasma aspect ratio, A, when we target a minimum machine size. It was notable that in a machine with a high A, the increased B_max raises the plasma performance but does not allow for reducing its size. The reduced plasma elongation of plasmas with high aspect ratio is a caveat as it reduces the gain in plasma performance achieved in the high field. Furthermore, the higher magnetic field at high A challenges the engineering of the magnets and associated structures, and substantially increases the heat loads on the divertor. This article assesses the engineering feasibility assessment of large DEMO coils operated at high field, also considering advanced i.e., non-ITER like, mechanical concepts. No advanced coil concepts appeared suitable for a large device like DEMO. Consequently, DEMO design points with lower B_max and lower A were investigated. This resulted in the identification of a maximum magnetic field of approximately 11–12 T to ensure the engineering feasibility of the DEMO TF coils.