The Exhaust design for the Spherical Tokamak for Energy Production (STEP)

STEP is a UK programme that aims to deliver a prototype fusion power plant based on the spherical tokamak concept targeting 2040. A key challenge is designing a divertor that ensures robust, fully detached operation resilient to transients, while maintaining acceptable core-edge conditions. This requires high power and particle handling, effective helium exhaust (c_He<10%) and low impurity levels at the core plasma separatrix (c_Ar<2%). STEP adopts a double-null configuration to shield the inner divertor legs, with an inner X-divertor and extended outer leg configuration. An optimised first wall structure, developed with engineers, is shown to provide adequate exhaust performance (T_i < 5 eV, q_⊥ ≤ 10 MW/m²) for powers crossing the separatrix up to ~150 MW. The design is validated using 2D SOL fluid codes with kinetic neutrals (SOLPS-ITER, EDGE2D), both standalone and integrated with the core (JINTRAC/COCONUT). For rapid scenario and control integration, DART (Detachment Analysis with Reduced modelling Tools) has been developed and verified against 2D SOL modelling and measurements on AUG, JET, and MAST-U. An overview of the design will be given.