Initial Operation of the ST40 Spherical Tokamak

Advances in High Temperature Superconductor magnet technology allow a significant increase in the toroidal field (TF), which has been found to improve confinement in STs. The combination of the high b that has been demonstrated in STs and the high TF that can be produced by HTS TF magnets opens a path to lower-volume fusion reactors, since fusion power is proportional to b² B_t⁴ V (V is the plasma volume). Tokamak Energy is aiming to exploit this concept as a route to fusion energy production. A high field ST, ST40 (R=0.4-0.6m, R/a=1.6-1.8, I_p=2MA, B_t=3T, κ=2.5, t_pulse~1-10sec, 2MW NBI), was partially assembled and an experimental campaign conducted between January and June 2018. The main aim was to test Merging/Compression (MC) start-up. Plasma currents up to 350kA at 0.8T were obtained. Following MC, plasma was sustained for up to 15ms, even without a solenoid. Experiments on MAST and TS3 show efficient ion heating due to reconnection and this has been confirmed on ST40, with T_i > 1keV measured using Doppler broadening spectrometry. Plasmas with H-mode signatures were observed. ST40 is being moved to a larger facility, able to accommodate NBI and neutron shielding. The experimental results, an outline of the parallel HTS R&D and future plans will be presented.