“The results genuinely shape the design of future fusion power plants,” James Harrison, Head of MAST Upgrade Science at UKAEA, said after the latest MAST Upgrade campaign at Culham. That may sound like lab language, but the significance reaches beyond Oxfordshire. With the UK’s prototype fusion plant, STEP, planned for West Burton in Nottinghamshire, this is the sort of result that starts to matter to places watching for long-term technical jobs, contracts and supply-chain work. UKAEA says the machine has now completed its most ambitious run so far, producing more than 1,100 fusion plasmas across experiments carried out through 2025 and 2026. In that run, the team reached the highest plasma pressure yet seen on MAST Upgrade while overcoming control problems that have to be solved before fusion can move from research promise to commercial power.

That pressure point matters because fusion only becomes useful when hydrogen isotopes are heated to extreme temperatures, squeezed together and held steady for long enough. Higher-pressure plasmas can produce more fusion power in less space, which makes them a much better stand-in for what a real power station would need to do. What UKAEA is claiming here is not simply a record for record’s sake. The machine achieved that higher pressure without the plasma tipping into unstable behaviour. For engineers thinking ahead to STEP at West Burton, that is the difference between an eye-catching number and something that can guide design choices.

The hardest problem in this campaign was taming Edge Localised Modes, or ELMs, the sharp bursts that appear at the plasma’s outer edge. When they hit, they can dump up to a tenth of the plasma’s stored energy in a single event, cutting pressure and battering the machine’s inner wall and exhaust components over time. That is one of the blunt commercial questions hanging over fusion. A future plant cannot spend its life damaging its own surfaces and then stopping for repairs. If ELMs are not controlled, maintenance costs rise and confidence falls. That is why this part of the work carries weight well beyond the lab.

The Culham team built on earlier ELM-suppression work by using Quasi-Continuous Exhaust mode and Resonant Magnetic Perturbations, with 3D magnetic coils used to steady the plasma edge. It also reached two further stable operating states, known as Quiescent H-mode and I-mode, both valued because they improve confinement without bringing back the large, damaging bursts operators are trying to avoid. Taken together, that gave MAST Upgrade access to four stable high-performance plasma regimes, including QCE, QH-mode and I-mode, under conditions that differ from those on other machines. For a national programme that wants STEP to be more than a well-funded concept, that matters. It suggests the UK is producing the sort of evidence designers can actually use.

One of the more striking advances was a new way of controlling the plasma’s position in real time. By measuring visible light from deuterium released at the machine’s upper and lower outer divertors, the team could spot tiny positional imbalances as they happened and correct them. UKAEA describes that as a world first, and it points towards the kind of automated control a working fusion plant would need day in, day out. The campaign also pushed on heat exhaust, which is another problem that does not go away just because the plasma behaves itself. Researchers found that adding small amounts of nitrogen at the plasma edge caused a large share of the exhaust power to be radiated away as light, spreading the heat before it hit inner walls and divertor surfaces. That sat alongside work on MAST Upgrade’s Super-X divertor, designed to handle and spread intense heat more effectively.

Researchers also explored negative triangularity plasma shapes, an approach being watched closely across the international fusion community because it may allow high-power operation without ELMs. In a tightly baffled Super-X, double-null geometry on a spherical tokamak, UKAEA says this is the first detailed study of how those heat-management and impurity-control methods work together. The results were featured at the European Physical Society’s Plasma Physics Conference 2026 in Edinburgh and are now being shared more widely to inform both STEP and ITER. Harrison said MAST Upgrade is working at the leading edge of what is possible, and UKAEA is plainly making the case that Britain remains one of the serious players in fusion research.

The next stage is practical. Later in 2026, MAST Upgrade is due to receive two new neutral beam injectors, doubling its neutral beam heating capacity, alongside an Electron Bernstein Wave system that will add a further 1.6 megawatts of heating power. That EBW technology is planned for use in STEP, which is why West Burton and the firms hoping to work around it will be watching these upgrades closely. UKAEA expects the enhancement programme to run through 2027, with a sixth experimental campaign focused on STEP-relevant research planned for 2028. There is still a fair distance between a record campaign in Culham and a working fusion plant in Nottinghamshire. Even so, this latest run gives the West Burton project a firmer technical footing, and that is worth noting well beyond the lab.