Sizing the Spherical Tokamak of the STEP Prototype Plant

The Spherical Tokamak for Energy Production (STEP) program’s mission is to deliver a U.K. prototype fusion energy plant, targeting 2040, and a path to commercial viability of fusion. An initial concept for the prototype plant was presented to achieve the objectives of producing at least 100-MW${}_{e}$net electrical output power, be tritium self-sufficient, demonstrate high-grade heat, and demonstrate a route to commercial levels of plant availability. This design had an inboard build of$1.6{\,}$m, a major radius of$3.6{\,}$m, and a toroidal field (TF) on the major radius of$3.2{\,}$T. The primary size driver of this design was the inboard build and it was found that the drive to minimize this space had led to the design presenting challenging requirements on the systems. The most notable of this was the shielding of inboard TF coils, resulting in inadequate lifetime. In this article, we present a resizing study for the spherical tokamak of the STEP Prototype Plant (SPP), focused around the magnets, shielding, and plasma. We apply Monte Carlo techniques to the magnet size and shield thickness to account for uncertainty and revise the size of these components. This larger inboard build, and larger major radius at fixed aspect ratio, allows us to lower the toroidal magnetic field on the plasma. This results in a new size for the STEP prototype with an inboard build of$1.9{\,}$m and a major radius of$4.275{\,}$m, with a reduced TF on the major radius of$3.0{\,}$T.