Overview of recent ST40 experiment and modelling results

ST40 is a Spherical Tokamak (ST), built by Tokamak Energy Ltd and operating at toroidal magnetic fields more than twice those achieved in other STs. Its main parameters are: R_geo≈0.4-0.5m, A≈1.6-1.9, κ<1.9, I_P≈0.3-0.8MA and B_T≈0.8-2.1T. Two co-current deuterium neutral beams deliver 0.9MW at 55kV and 0.7MW at 24kV. Recent highlights of ST40 operations include the development of double-null diverted, upper single-null diverted, and H-mode plasmas. These new scenarios and significantly improved diagnostics capabilities along with sophisticated integrated modelling and simulation tools have paved the way for more detailed physics studies. An infra-red camera viewing the upper divertor, and Langmuir probe arrays, on both upper and lower divertor, have enabled examining the scrape-off-layer power fall-off length, λ_q. Data from the recently installed Thomson scattering system, soft X-ray camera, and bolometer have been used in ASTRA / TRANSP transport simulations to extend the ST energy confinement-time scaling to higher magnetic fields. A new Tokamak simulator, SOPHIA, has been developed to support the ST40 programme by integrating the ASTRA-SPIDER free boundary evolution and transport code to the real-time Plasma Control System. In 2025, the capabilities of ST40 will be extended further: installation of a 1 MW dual-frequency (104/137 GHz) gyrotron, and a pellet injector, will take ST40 towards more reactor relevant scenarios, and allow exploring of radio frequency start-up in a high-field ST.