Overview of ST40 results and future: Expanding the physics basis of high-field spherical tokamaks

Abstract The goal of the ST40 programme is to explore the physics of high-field spherical tokamaks (STs), to validate empirical and theoretical models and, hence, to build confidence in predictions required to support the design of future generations of STs. ST40 is a compact high-field ST that has achieved the following parameters: R0 = 0.4–0.55 m, Ip = 0.20–0.85 MA, Bt(R = 0.4 m) = 0.7–2.1 T, κ ≤ 1.9, and A = 1.6–1.9. Highlights of recent experimental results include (i) H-mode and confinement studies at Bt ≤ 2.1 T, (ii) observation of bifurcation of the scrape-off-layer power fall-off width, λq, into a ‘wide’ branch that follows existing H-mode scalings and a ‘narrow’ branch that exhibits λq values that are up to 10 times lower than the predictions of established scalings, (iii) development of high-performance scenarios with plasma current, Ip, up to 0.85 MA, (iv) development of highly non-inductive scenarios with high βp , and (v) the first ST40 experiments utilising the newly commissioned impurity powder dropper. The work on all these topics has been supported by a number of advancements in ST40 hardware and software, from plasma control to data analysis and interpretation. At the end of 2025, ST40 embarked on a major upgrade to further expand its capabilities by introducing, among other improvements, all-metal plasma-facing components, 1MW of electron cyclotron heating, a pellet injector, and a pair of lithium evaporators for wall conditioning.