Overview of the physics results of the first campaign of experiments on the MAST upgrade | Events
MAST Upgrade is a new spherical tokamak based on the MAST device , with low aspect ratio (R/a ~ 0.85/0.6 ~ 1.4), increased toroidal field (up to 0.8 T at R = 0.8 m) and plasma current maximum (up to 2 MA) and a pulse duration (up to 5 s) . The two Neutral Positive Ion Injectors (NBI) are capable of injecting up to 2.5 MW to provide on- and off-axis heating and current control. It has a large number of poloidal field coils, providing significant flexibility to modify the shape of the central plasma (elongation κ ~ 2.2 reached, maximum ~ 2.5, triangularity δ ~ 0.5) and magnetic geometry in the entertainers. The divertors are capable of producing and investigating conventional and alternative divertor configurations including Super-X, X-divertor, snowflake, and X-point target with symmetrical up-down and tightly baffled divertor chambers. A comprehensive diagnostics suite enables detailed physical studies of key fusion issues, including thermal and super-alfvenic fast particle transport in the core and basement, MHD stability, and plasma escape.
The first plasma was achieved in October 2020, followed by the 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 the escapement plasma, specifically characterizing and understanding the differences between conventional and Super-X divertor configurations. In spherical tokamaks, the Super-X configuration should have more significant advantages due to the large reduction in the toroidal field with the increase in the main radius of the point of impact and the proportional increase in the wetted area. Initial experiments with 3.2 MW of injected NBI power demonstrated a ~10-fold reduction in external divertor power loads in the Super-X configuration compared to a conventional divertor, in good agreement with modeling predictions . More detailed L-mode experiments confirmed these results and found a ~2-fold reduction in midplane outer separatrix density for detaching the outer divertors.
A combination of increased divertor closure and better plasma shaping improves edge density control, which in turn results in pedestal temperatures reaching 400 eV. ELM attenuation was obtained with n=1 RMP coils. MAST-U pedestals are limited in lift-off mode, which can facilitate access to H-mode scenarios without ELM.
High energy H ELMy type 1 plasmas were held for 1 second using both beams to inject an average power of 3 MW. A population of super-Aflvénic fast particles has been shown to excite a range of instabilities, including putative toroidal Alfvén eigenmodes, fishbones, and compressional or global Alfvén eigenmodes.