Sizing the STEP Prototype Powerplant
The Spherical Tokamak for Energy Production (STEP) programme aims to deliver a UK prototype fusion energy plant, targeting 2040, and a path to commercial viability of fusion. The STEP Prototype Powerplant (SPP) has the objectives to generate 100 MWe of net electrical power, be tritium self-sufficient, demonstrate high-grade heat, and demonstrate a route to commercial levels of plant availability. STEP has adopted a spherical tokamak geometry, to allow it to operate at high β (ratio of the plasma to magnetic pressure), high elongation and high bootstrap fraction, with the goal of producing a reduced radial size in the design. To minimise the radial build, STEP has been designed with a non-inductive flat-top plasma scenario, to reduce the central solenoid’s size by restricting its use to just start-up, and dispensing with inboard tritium breeding.
While it has been shown that the primary size constraint in a conventional DEMO-like tokamak is the divertor, for the STEP device with a double-null advanced divertor configuration the inboard build becomes the primary size driver. Within the inboard build there are two key components that drive its size: the inboard Toroidal Field (TF) coil and the shielding it requires. The TF coil layout is driven by the current required within it to deliver a field on the plasma, the structure required to handle the electromagnetic forces and the manufacturability of the coil. This is not a simple relationship, as increasing the magnet size will increase the major radius of the machine that a minimum TF field is required, thus increasing the current in the inner TF coil that then increases its electromagnetic load and size. This cycle needs to be balanced to find an overall size that meets the objectives. The STEP device is also being designed with remountable joints, meaning that the centre column is not a lifetime component. Shielding must be added that balances cost between making a larger device and not replacing the centre column too often leading to low availability.
In this presentation we will explore the size of the STEP design. We will discuss the limitations of the existing published size and the workflow developed to explore alternative sizes. We will show the impact changing the TF size and shielding thickness has on the plasma before discussing how we have used uncertainty quantification to drive the size selection. We will conclude by presenting the updated STEP design.