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Experiments on ST40 at high magnetic field

The combination of the high beta values, which have been achieved in spherical tokamaks, and high toroidal field (TF) that can be produced by high temperature superconductor (HTS) TF magnets, opens a path to lower-volume fusion reactors, in accordance with the fusion power scaling ∼β t 2 B t 4 V. High field spherical tokamak ST40 is the first prototype on this path and is now operating and results of first experiments are presented. Increase in T e, T i and the thermal energy with the TF has been observed for TF up to ∼2 T in OH operations with a sharper increase at TF above 1 T, which suggests transition to a better confinement above this threshold. First results of experiments with the neutral beam injection show expected increase in the T i, plasma density and the thermal energy. Results of the merging-compression plasma formation (m/c) studies show agreement with the previous studies, however at the high TF, formation of run-away electrons has been observed for the first time during m/c start-up. Advances in the HTS development are also presented.