Description
The Spherical Tokamak for Energy Production (STEP) programme aims to deliver a prototype fusion reactor that demonstrates net electricity production. Impurity seeding will be essential in fusion power plants to ensure a homogeneous distribution of heat fluxes in the form of radiation to the vessel walls and adequate divertor detachment. The amount of seeding must also remain compatible with good core performance and fusion yield, highlighting the importance of predicting impurity screening across the H-mode pedestal. In this study, we present integrated simulations of the new STEP SPP-002 baseline H-mode plasmas using the JINTRAC code. We focus on STEP scenarios heated by electron cyclotron waves and displaying high plasma core radiation fractions of 50% to 70%. Turbulent dynamics are predicted with the Bohm/Gyro–Bohm model and the TGLF code, while the NCLASS and NEO codes provide neoclassical transport coefficients. A parameter-space scan of electron and impurity separatrix densities and pedestal transport coefficients is carried out to determine the optimal impurity seeding level consistent with pedestal physics, divertor detachment, and adequate core fusion performance. Scenarios are selected if they are stable and consistent with the required fusion power of Pfus=1.68 GW. We furthermore present a comparison between reduced and computationally intensive transport models to assess the accuracy of lightweight approaches in predicting impurity transport for STEP H-mode plasmas. Notably, NEO and NCLASS differ in the computed neoclassical coefficients in the pedestal region, which leads to different Ar screening factors and radiation profiles. This in turn reveals different limits for Ar concentration at the separatrix predicted by the two models.
This work has been funded by STEP, a major technology and infrastructure programme led by UK Industrial Fusion Solutions Ltd (UKIFS), which aims to deliver the UK’s prototype fusion powerplant and a path to the commercial viability of fusion.