UKAEA has completed the fifth experimental campaign on MAST Upgrade at Culham Campus in Oxfordshire, reporting the highest plasma pressure yet achieved on the machine without loss of stability. The campaign ran across 2025 and 2026 and produced more than 1,100 plasmas, which UKAEA has presented as a flagship delivery within its 2026-2030 strategy. For policymakers, the result matters because it moves the discussion from laboratory performance to plant design. A higher-pressure plasma is closer to the operating conditions a commercial fusion power station would need, so the test is no longer only whether plasma can be created, but whether it can be sustained at useful performance without unacceptable damage or downtime.
That distinction is important in the UK programme. Government policy has linked fusion research to long-term net-zero and energy security goals through STEP, the prototype plant planned for West Burton in Nottinghamshire, and through the wider case that public R&D spending can produce a low-carbon power source over the longer term. Hydrogen isotopes must be heated, compressed and confined at extreme temperature and pressure. As density and temperature rise, fusion reactions rise quickly as well, which is why plasma pressure is such a practical measure. In simple terms, more stable pressure means more power from a smaller volume and better evidence on whether future stations can be built at workable scale.
According to the government announcement, the main technical barrier in this campaign was the suppression of edge localised modes, or ELMs. These are bursts at the outer edge of the plasma that can eject as much as a tenth of stored energy in a single event, reduce plasma pressure and, over time, damage wall and exhaust components. That makes ELM control a commercial issue as much as a scientific one. If a plant suffers repeated surface damage, maintenance intervals tighten, component life shortens and operating costs rise. For ministers and programme managers, that affects the credibility of claims that fusion can become dependable grid infrastructure rather than a high-cost test facility.
MAST Upgrade addressed that problem through several stable operating regimes. UKAEA said the machine used Quasi-Continuous Exhaust mode and Resonant Magnetic Perturbation control, applying three-dimensional magnetic fields to reduce pressure at the plasma edge and keep the boundary stable. The team also accessed Quiescent H-mode and I-mode, both of which offer stronger energy confinement while limiting the large edge bursts associated with ELMs. Access to four high-performance regimes on MAST Upgrade, under conditions that differ from other machines, gives engineers a broader evidence base for setting operating windows in STEP and other future devices.
A separate result concerns control systems. UKAEA said the team developed a new method for tracking the plasma position by measuring visible light from deuterium at the upper and lower outer divertors, allowing very small imbalances to be detected in real time. That matters because a commercial plant cannot rely on continuous manual adjustment. Real-time control is tied to automation, machine protection and routine operation, all of which feed into plant availability, staffing models and the eventual regulatory case for a station that has to run repeatedly rather than only during short research campaigns.
Heat exhaust was the other major focus. MAST Upgrade used its Super-X divertor to spread intense heat and particle exhaust more effectively, and the team found that injecting small amounts of nitrogen at the plasma edge caused a large share of exhaust power to be emitted as light before it struck internal surfaces. In practice, that lowers peak heat loads on the machine wall and divertor and reduces wear. UKAEA said this impurity-assisted approach is expected to be needed in a power plant because geometry on its own would still leave heat loads too high. The campaign was also the first detailed study of this interaction in a tightly baffled Super-X, double-null arrangement on a spherical tokamak.
The campaign also examined negative triangularity plasma shapes, an approach closely watched across the international research community because it may allow high-power operation without large ELMs. Together with the other operating modes, that gives the UK programme more than one route for managing the plasma boundary, rather than depending on a single answer. James Harrison, UKAEA's Head of MAST Upgrade Science, said the results are informing future plant design. For policy purposes, the significance is straightforward: these are not only performance records. They are engineering data on stability, exhaust management and control systems, which are the issues that tend to determine capital cost, maintenance schedules and whether a prototype can be scaled.
The results were presented at the European Physical Society Plasma Physics Conference 2026 in Edinburgh and are being shared to inform STEP and ITER. UKAEA will now move into a further enhancement phase, including two additional neutral beam injectors that will double neutral beam heating capacity and an Electron Bernstein Wave system adding 1.6 MW of heating power. That programme is expected to conclude in 2027, with a sixth experimental campaign focused on STEP-relevant research planned for 2028. The sequence matters for UK energy strategy because it links public research spending at Culham to the engineering choices now being shaped for West Burton. If later campaigns confirm that high-pressure plasmas can be controlled with lower wall damage and better automation, the case for prototype construction becomes materially stronger.