MAST Upgrade is the 📝spherical tokamak operated by the UK Atomic Energy Authority at its Culham campus in Oxfordshire, which took its first 📝plasma on 29 October 2020 and is the first machine anywhere to run a Super-X 📝divertor.
MAST stands for Mega Ampere Spherical Tokamak; the original ran from 2000 to 2013, and the seven-year upgrade that followed replaced enough of it to constitute a new device. Published design figures give a major radius of about 0.85 metres against a minor radius of about 0.65, an 📝aspect ratio near 1.3 that is the defining feature of the spherical class, a 📝plasma current up to 2 megaamperes, a 📝toroidal field below 1 📝tesla, and pulses up to five seconds. Low field is the trade the geometry makes: a spherical tokamak buys high 📝plasma beta and compactness at the cost of room for a central magnet stack.
The Super-X is what the machine was rebuilt for. It routes exhaust plasma along a much longer field line to a target at large radius, spreading and cooling the heat before it lands; the 2021 campaign measured at least a tenfold reduction in target heat load against the conventional configuration, the first experimental demonstration of the concept. That result feeds STEP, the UK's Spherical Tokamak for Energy Production programme for a prototype 📝fusion pilot plant, and it makes MAST Upgrade the natural place to develop the radiated-power diagnostics an exhaust-dominated machine needs — 📝bolometer systems, including an infrared video bolometer prototype, were commissioned and first measured there in 2023.