The United Kingdom’s largest operating fusion experiment has pushed its plasma to the highest stable pressure yet achieved in the machine—while suppressing the violent edge bursts that can damage a reactor.
The result comes from the fifth experimental campaign of MAST Upgrade, a compact “spherical tokamak” at the UK Atomic Energy Authority’s Culham Campus. The campaign ran through 2025 and 2026 and produced more than 1,100 experimental plasmas, according to the UKAEA’s 6 August 2026 announcement.
This is meaningful engineering progress, but it is not a fusion-power breakthrough in the sense of producing net electricity. MAST Upgrade is an experimental machine. Its job is to solve the difficult control and heat-exhaust problems that a future power plant would face.
What changed on MAST Upgrade?
Fusion machines try to hold an extremely hot, electrically charged gas called plasma inside magnetic fields. Pressure matters because fusion reactions become more likely as the plasma becomes hotter and denser. Higher pressure can therefore mean more fusion power from a given volume—but it also makes the plasma harder to control.
During its latest campaign, MAST Upgrade reached the highest plasma pressure in its own operating history without the plasma becoming unstable. The team also accessed four high-performance operating regimes designed to reduce damaging bursts at the plasma edge. ITER’s summary of the campaign says the results will inform both the international ITER programme and STEP, the UK’s planned prototype spherical-tokamak power plant.
MAST Upgrade has a more compact, cored-apple shape than a conventional doughnut-like tokamak. Spherical tokamaks are attractive because they may achieve strong performance in a smaller machine, but their compact geometry leaves less room for magnets and other internal systems. The design must combine high pressure, stability and effective heat removal rather than optimise any one of them in isolation.
Why edge-localised modes threaten fusion reactor walls
One of the campaign’s main targets was the edge-localised mode, or ELM. An ELM is a brief instability at the outer edge of a confined plasma. It can release a sharp pulse of heat and particles toward the machine wall. UKAEA says a single burst can eject up to one tenth of the plasma’s stored energy.
A research device can tolerate occasional pulses. A commercial plant expected to run repeatedly for long periods cannot afford constant erosion of its inner wall and exhaust components. Replacing those parts too often would reduce availability and raise costs.
The team used several approaches rather than betting on one universal solution. Quasi-continuous exhaust mode releases energy in smaller, more frequent events. Resonant magnetic perturbation coils apply carefully shaped three-dimensional magnetic fields to adjust conditions at the edge. The researchers also operated in quiescent H-mode and I-mode, two regimes that aim to preserve useful energy confinement without large ELMs.
A new way to keep the plasma centred
The campaign also produced a new real-time control method. Researchers measured visible light from deuterium leaving the upper and lower outer divertors—the exhaust regions of the machine. A difference in brightness revealed a tiny vertical imbalance in the plasma’s position.
That optical signal could then be used by the control system to keep the plasma centred. The basic idea is intuitive: if more plasma exhaust is reaching one side than the other, the light gives the machine an immediate clue that its position needs correction.
This matters because a future plant will need automated control that reacts faster and more consistently than a human operator. The achievement does not mean that autonomous fusion plants are ready. It shows that an additional physical signal can be turned into a practical feedback tool under experimental conditions.
Heat must leave as carefully as it is confined
Stability is only half of the problem. A fusion plant must also remove enormous heat loads without destroying the surfaces that receive the exhaust.
MAST Upgrade uses a highly optimised “Super-X” divertor, which lengthens and spreads the path taken by exhausted plasma. Earlier MAST Upgrade experiments reduced heat loads on divertor components by roughly a factor of ten.
In the latest campaign, researchers also injected small amounts of nitrogen near the plasma edge. The nitrogen encouraged more exhaust energy to leave as light before reaching a solid surface, lowering peak heat flux. The team studied this technique inside a tightly baffled, double-null Super-X configuration—a geometry relevant to future compact reactors.
There is an important balance. Too little radiation leaves excessive heat concentrated on the wall; too much impurity in the wrong place can cool or contaminate the core plasma. A power plant will need to control both the high-performance core and the cooler exhaust region at the same time.
Why instability research remains difficult
Not every plasma disturbance occurs only at the outer edge. Neoclassical tearing modes can form magnetic “islands” deeper inside a high-performance plasma, flattening its temperature profile and allowing energetic ions to escape.
A 2025 peer-reviewed MAST Upgrade study used high-speed diagnostics to map fast-ion losses associated with these tearing modes. It found that the most damaging mode interacted with a broad range of fast-ion trajectories, illustrating why simply adjusting one beam or one orbit may not remove the problem.
The new campaign’s success should therefore be read as progress across a control toolbox—not a declaration that plasma instability has been solved everywhere and under all reactor conditions.
What the record does—and does not—prove
- It does show that MAST Upgrade can combine its highest stable plasma pressure with several edge-control strategies.
- It does show that visible divertor light can provide a useful real-time signal for plasma-position control.
- It does not show net energy gain, continuous operation or electricity delivered to a grid.
- It does not yet establish how these methods will perform in a much hotter, longer-running, reactor-scale plasma.
The detailed results were presented to the plasma-physics community and are being shared for future machine design. The headline findings currently come mainly from official campaign reporting; full peer-reviewed papers for the newest campaign results will be needed to examine the methods, uncertainties and repeatability in detail.
MAST Upgrade’s heating improvements and planned 2028 campaign
MAST Upgrade will now pause for hardware improvements. UKAEA says two new neutral-beam injectors will double its neutral-beam heating capacity, while an electron Bernstein wave system will add 1.6 megawatts of heating. The same type of wave-heating technology is planned for STEP.
A sixth experimental campaign focused on STEP-relevant research is planned for 2028. The crucial question is whether the machine can retain its stability and exhaust advantages as heating power and plasma performance increase.
Fusion’s public milestones often focus on temperature or energy records. MAST Upgrade’s latest result highlights a less dramatic but equally important truth: practical fusion will depend on controlling the plasma edge, keeping the plasma accurately positioned and moving waste heat out of the machine without destroying it.
Reporting note: The record-pressure and control findings are official campaign results announced by UKAEA and summarised by ITER. Detailed peer-reviewed publications covering the newest campaign results were not yet available at the time of writing. MAST Upgrade is an experimental device and did not generate net electricity.
For the wider steps between experiments and electricity, read about the US roadmap toward commercial fusion power.
Sources and further reading
- UK Atomic Energy Authority: Under Pressure—Record plasma for UK flagship machine
- UKAEA Fusion Energy: MAST Upgrade fifth-campaign summary
- ITER News Hub: Record plasma for MAST Upgrade
- UKAEA: MAST Upgrade and the Super-X divertor
- Plasma Physics and Controlled Fusion: Fast-ion losses induced by tearing modes in MAST Upgrade
- UKAEA: Scientific goals for the fifth MAST Upgrade campaign
FutureTechDose covers biotechnology, AI, data-centre and energy-sector research and industry progress for a general audience. This article is informational and does not provide medical or investment advice.
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