Conceptual design workflow for the STEP Prototype Powerplant

The STEP (Spherical Tokamak for Energy Production) Programme aims to deliver a UK prototype fusion energy plant, targeting 2040, and a path to commercial viability of fusion. To deliver on this aim, we have performed initial scoping to identify the design point for a spherical tokamak prototype powerplant producing at least 100MWe of net electric output and that is self-sufficient in tritium production. We have developed a rapid workflow that starts with the systems code process to develop an initial design point that can then be iterated with the plasma codes jetto and fiesta to add fidelity. Promising solutions can then be taken forward for further analysis of the tritium breeding ratio, inboard build, exhaust and power balance. Solutions that are best matched to the requirements are then taken on by a wider team to assess. We have applied this technique using a number of different input technology options and assumptions and arrived at the most promising design point for the STEP Prototype Powerplant (SPP). The unique features of this device are no inboard breeding blanket, the use of remountable magnet joints, high β and bootstrap plasma operation, a Super-X divertor, the use of just microwave heating, and enclosing poloidal field coils inside the toroidal field cage.