STEP – the Spherical Tokamak for Energy Production – seeks a steady state pathway to net power delivery in its fusion pilot plant: the STEP Prototype Plant (SPP). The ST has a number of beneficial properties for fusion power production, one of which is the compatibility with high fusion performance, steady state operation. Focussing on the plasma scenario requirements, we will explore how we achieve this on the SPP, consistent with a primary objective of delivering ~100MW of electrical power to the grid. We show how SPP is pushed into a novel fusion plasma regime. For example, the plasma turbulence that governs confinement will be dominated by electromagnetic fluctuations that display a complex relationship with plasma pressure gradient and flows, in part due to a proximity to “second-ballooning access”, which we will discuss. The compact nature of the ST poses challenges to managing the exhaust heat load but, benefitting from exciting results from MAST-U, we have a viable solution for SPP based around the Super-X divertor configuration. Demonstration of su icient plasma control remains an open issue, and we illustrate progress with three key experimental results obtained in the past year: control of the magnetic geometry during vertical plasma oscillations on TCV; independent control of the so-called “detachment front” in the upper and lower divertors, and suppression by magnetic perturbations of the plasma eruptions called edge-localised modes (ELMs) on MAST-U – a world-first for a ST.

Acknowledgement: This work has been funded by STEP Fusion, 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.