An overview of the first physics results from MAST Upgrade

MAST Upgrade is a major enhancement of the MAST spherical tokamak, featuring 19 new poloidal field coils for improved plasma shaping, a new central solenoid with increased inductive flux, tightly baffled upper and lower divertor chambers and an improved diagnostic suite. First plasma was achieved in October 2020. By the end of the first physics campaign 12 months later real time feedback control of the plasma current, vertical position, outer radius and divertor strike points were all achieved and operating routinely.

MAST-U was designed to explore the Super-X divertor (SXD) concept, where the outer divertor leg is at large major radius with significant poloidal flux expansion near the target. Both conventional (CD) and Super-X divertor scenarios were established for plasma currents up to 750 kA; comparisons between the two showed the SXD configuration achieved a 10-fold reduction in target heat flux as measured by IR thermography in plasmas with 3.2 MW of neutral beam injection (NBI) auxiliary heating. Divertor detachment is reached at half the upstream density in SXD configuration compared with CD.

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.

Two NBIs inject at the midplane and 75 cm above the midplane, providing both on-axis and off-axis injection for improved current density control. High energy, type-1 ELMy H-mode plasmas were sustained at 750 kA 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.