The world’s largest operational spherical tokamak, built at Culham in the UK, which demonstrated a tenfold reduction in divertor heat loads using the revolutionary Super-X divertor configuration.
MAST Upgrade (MAST-U) is a spherical tokamak operated by the UK Atomic Energy Authority (UKAEA) at the Culham Campus near Abingdon, Oxfordshire. It achieved first plasma on 29 October 2020 after a seven-year, £55 million upgrade that replaced approximately 90% of the original MAST components.[1]
The centrepiece innovation is the Super-X divertor, which routes plasma exhaust along extended “legs” before it reaches the target plates. This geometry spreads heat over a larger area and provides more volume for radiative cooling.[2]
Results demonstrated approximately a tenfold reduction in heat load on divertor targets — from ~50 MW/m² to ~5 MW/m². UKAEA described the reduction as going “from blowtorch level down to more like that you’d find in a car engine.” Critically, H-mode core confinement was maintained (H98 ~ 1.0) with no discernible impact on plasma performance.[3]
MAST-U is actively operating as of 2026, with five scientific campaigns completed involving 200+ researchers from 40+ institutions. In 2025, the device achieved complete ELM suppression using resonant magnetic perturbation coils — a first for spherical tokamaks.[4]
MAST-U serves as the primary experimental testbed for the UK’s STEP (Spherical Tokamak for Energy Production) programme, a prototype fusion power plant targeting ~100 MWe and tritium self-sufficiency by around 2040.