Overview of recent results and future plans of high-field spherical tokamak ST40

Parameters achieved by ST40 are: R₀≈0.4−0.5m, I_p≈0.25−0.8MA, B_t(R=0.4m)≈0.7−2.1T, κ<1.9, A≈1.6−1.9, P_NBI~1.8MW, and t_flattop≤200ms. Recently, diverted H-mode scenarios with I_p>750kA and t_flattop~50ms were developed. At I_p=450kA, early transition to diverted configuration enabled I_p flattops with up to 150ms of H-mode, with sustained ELM-free phases. L-H-transition often happens via an intermediate state that has been identified as I-phase, indicating proximity to the L-H power threshold. Early results from experiments utilising an impurity powder dropper to inject lithium into ST40 indicate that, for high performance pulses, lithium had no measurable effect on core parameters such as W_p, T_e, or n_e, whereas for low performance pulses lithium advanced H-mode entry. Transport analysis of ST40 plasmas with B_t=1.0–1.5T suggests a weaker dependence of τ_E on B_t compared to scalings obtained on other STs at B_t<1.0. Scrape-off-layer power fall-off width (λ_q) has been analysed in a variety of scenarios using an infrared thermography analysis chain. Upcoming upgrades on ST40 will enable studying the effect of lithium-coated all metal PFCs and low wall recycling in a plasma with pellet fuelling and 1MW of RF (f=137GHz/104GHz) heating.