Magnetic equilibrium optimisation and divertor integration in spherical tokamak reactors

Abstract

Managing the heat exhaust presents one of the main challenges in the design of a spherical tokamak reactor. The development of a robust exhaust solution is intrinsically linked to the optimisation of the plasma equilibrium, together with a compatible poloidal field (PF) system. A multidisciplinary design framework should balance a host of often conflicting physics and engineering requirements and identify a set of self-consistent constraints. This work employs an iterative approach which integrates the core plasma scenario and divertor magnetic topology, ensuring compatibility with the available technology. The aim of this paper is to develop and assess a number of optimal magnetic configurations consistent with the UKAEA STEP Prototype Plant designs. Firstly, we use the Fiesta free-boundary equilibrium code to optimise the global configuration for a chosen double-null scenario, employing both standard and alternative exhaust-handling schemes. We then analyse the local scrape-off layer (SOL) magnetic topology in order to preliminarily assess the anticipated divertor performance. We discuss the advantages and disadvantages of each configuration in terms of the selected divertor metrics, the effects on the core plasma scenario, engineering feasibility, and the implications for whole reactor design.The inner divertor proves to be the most challenging issue in a compact device such as STEP. The small target area and reduced SOL width at the small strike-point radius lead to excessive heat loads beyond the power-handling capacity of standard concepts. We propose an alternative inner X-divertor configuration and compare it with the standard divertor optimised for maximal connection length and poloidal flux expansion. The objective is to achieve the optimal global configuration with a realistic coil set, taking into account space constraints and material limitations. We discuss the feasibility of each considered exhaust solution in STEP, where the PF shaping coils can be placed either in unfavourable locations outside of the toroidal field (TF) coil or mounted inside the TF and operating near their engineering limits.