MAST Upgrade is a new spherical tokamak based on the MAST device [1], with low aspect ratio (R/a ~ 0.85/0.6 ~ 1.4), increased toroidal field (up to 0.8T at R = 0.8m) and maximum plasma current (up to 2 MA) and pulse length (up to 5 s). The two positive ion neutral injectors (NBI) are capable of injecting up to 2.5 MW to provide on and off-axis heating and current drive. It has a large number of poloidal field coils, providing significant flexibility to modify the shape of the core plasma (elongation κ ~ 2.2 achieved, maximum ~2.5, triangularity δ ~ 0.5) and magnetic geometry in the divertors. The divertors are capable of producing and studying conventional and alternative divertor configurations, including Super-X, X-divertor, snowflake and X-point target with up-down symmetric, tightly baffled divertor chambers. An extensive suite of diagnostics enables detailed physics studies of key issues for fusion, including transport of thermal and super-Alfvenic fast particles in the core and pedestal, MHD stability and plasma exhaust.
First plasma was achieved in October 2020, followed by commissioning of real-time control systems and the first physics campaign took place from March to October 2021. A major theme of the physics campaign was plasma exhaust, specifically characterizing and understanding the differences between conventional and Super-X divertor configurations. In spherical tokamaks, the Super-X configuration is predicted to have more significant benefits due to the strong reduction of the toroidal field with increasing strike point major radius and commensurate increase in the wetted area. Initial experiments with 3.2 MW of injected NBI power demonstrated a factor ~10 reduction in outer divertor power loads in the Super-X configuration compared with a conventional divertor, in good agreement with modelling predictions. More detailed experiments in L-mode confirmed these findings and found a factor ~2 reduction in the outer mid-plane separatrix density to detach the outer divertors.
A combination of increased divertor closure and better plasma shaping improves control of the edge density which in turn results in pedestal temperatures reaching 400 eV. ELM mitigation was achieved with n=1 RMP coils. MAST-U pedestals are peeling mode limited which may aid access to ELM-free H mode scenarios.
High energy, type-1 ELMy H-mode plasmas were sustained for 1 second using both beams to inject an average of 3 MW of power. A super-Aflvénic fast particle population was found to excite a range of instabilities, including suspected toroidal Alfvén eigenmodes, fishbones and compressional or global Alfvén eigenmodes.