Towards a compact spherical tokamak fusion pilot plant

The question of size of a tokamak fusion reactor is central to current fusion research especially with the large device, ITER, under construction and even larger DEMO reactors under initial engineering design. In this paper, the question of size is addressed initially from a physics perspective. It is shown that in addition to size, field and plasma shape are important too, and shape can be a significant factor. For a spherical tokamak (ST), the elongated shape leads to significant reductions in major radius and/or field for comparable fusion performance. Further, it is shown that when the density limit is taken into account, the relationship between fusion power and fusion gain is almost independent of size, implying that relatively small, high performance reactors should be possible. In order to realize a small, high performance fusion module based on the ST, feasible solutions to several key technical challenges must be developed. These are identified and possible design solutions outlined. The results of the physics, technical and engineering studies are integrated using the Tokamak Energy system code, and the results of a scoping study are reviewed. The results indicate that a relatively small ST using high temperature superconductor magnets should be feasible and may provide an alternative, possibly faster, ‘small modular’ route to fusion power. This article is part of a discussion meeting issue ‘Fusion energy using tokamaks: can development be accelerated?’.