A 12-tonne double beam box has been lifted into position on MAST Upgrade, the spherical tokamak run by the UK Atomic Energy Authority at its Culham campus near Oxford. The component carries two more neutral beam injectors and will double the machine’s heating power to 8.8 MW. UKAEA’s head of MAST Upgrade operations, Andrew Thornton, called the lift a huge step forward for the programme.
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Neutral beam injectors fire high-energy neutral atoms into the plasma, raising its temperature above 30 million degrees Celsius. With two more of them, plasma temperatures are expected to roughly double.
The double decker beam box
UKAEA describes the new hardware as a double decker arrangement, with the two injectors stacked inside a single vessel, according to World Nuclear News reporting on the lift. The vessel has a volume of 20 cubic metres and is built from 316L stainless steel, chosen because it is compatible with the vacuum the machine requires. At 12 tonnes it is a heavy single lift, which is why the move into place was newsworthy enough for the agency to announce.
The previous experimental campaign set a record of 3.8 MW of heating injected into the plasma, a figure UKAEA gave when it announced the machine’s fifth experimental campaign in January 2026. The agency said at the time that the two added neutral beams, due between 2026 and 2027, are expected to double the heating power, and that together with a new Electron Bernstein Wave system they would be the first heating systems of this kind to operate together in a spherical tokamak. Heating power is the limiting resource for the physics MAST Upgrade wants to test. More injected power raises both temperature and pressure in the plasma, and pressure is what a power plant needs, because fusion output climbs steeply with it. The UKAEA campaign page frames the upcoming work around four themes: understanding high-pressure plasmas, optimising energy control and stability, improving divertor design, and testing the computer models that predict heating, stability and exhaust behaviour for STEP and ITER.
Higher pressure and a hotter exhaust test
Thornton explained that the extra power will let researchers study higher-pressure plasmas and test the exhaust system under hotter conditions. The exhaust, called the divertor, is the core focus of MAST Upgrade because it must handle the particles and heat leaving the plasma, a problem any future power plant has to solve in a compact footprint.
In August, the machine finished its fifth round of experiments, reaching the highest plasma pressure it has achieved while overcoming plasma instability issues, as detailed in the report on those experiments. The new injectors are intended to push that frontier further. Alongside them, an Electron Bernstein Wave system is being installed to add 1.6 MW of heating, a technology planned for STEP.
James Harrison, head of MAST Upgrade science, is the other named UKAEA voice on the programme. The goals he and Thornton have described converge on one design question: whether a compact spherical tokamak can control its exhaust at power-plant conditions. The Super-X divertor, one of the exhaust configurations the campaign is testing, is part of that answer, and the added beam heating is what lets the machine reach the conditions under which it matters.
Reusing JET’s power gear
Not everything is new. Power supply equipment is being transferred and repurposed from the Joint European Torus, which retired in 2023 after 40 years, as the account of JET’s retirement recalls. Giving that gear a second life on MAST Upgrade keeps expensive infrastructure working and saves the cost of buying equivalents.
The machine is funded by the Department for Energy Security and Net Zero and forms part of the EUROfusion European Medium-Sized Tokamak Programme. Its three stated aims are to investigate novel exhaust concepts, to reduce the risk of the spherical tokamak design for a future power plant, and to widen physics knowledge for the wider fusion programme.
Timeline to 2028 and the road to STEP
All major hardware is due to be fitted inside the vessel by the end of autumn 2027, with scientific experiments using it set to begin in 2028. The results are meant to feed the design of STEP, the Spherical Tokamak for Energy Production, the UK’s prototype fusion power plant planned for West Burton in Nottinghamshire, with operations due to begin by 2040. The World Nuclear Association’s fusion overview and the UKAEA’s government page give the wider programme context.
The hardware schedule leaves a gap of roughly a year between the lift and the finished installation. The stages in between, from fitting the remaining major components to commissioning the injectors, are where schedules on first-of-a-kind equipment tend to slip, and UKAEA has not reported contingency time in the 2028 date. Neither the beam box lift nor the Electron Bernstein Wave installation changes the target, but both must finish before the doubled power can be used.
What remains open is whether the doubled heating delivers the pressure gains researchers expect. That test cannot start until the hardware is complete and the 2028 campaign begins.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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