Under Pressure: Record plasma for UK flagship machine
UKAEA has completed its fifth series of experiments on its MAST Upgrade fusion machine, overcoming plasma instability issues and pioneering novel techniques.
A MAST Upgrade plasma with nitrogen added to spread exhaust heat - Image credit: United Kingdom Atomic Energy Authority
- MAST Upgrade’s fifth series of experiments overcome plasma instability issues considered essential prerequisites before commercial fusion energy power plants can be built.
- A novel technique was pioneered by the team using measurements of light to detect and control tiny plasma imbalances.
The United Kingdom Atomic Energy Authority (UKAEA) has completed its most ambitious series of experiments to date on its flagship fusion machine.
The highest plasma pressure attained by the machine was achieved while overcoming control problems that are considered essential prerequisites for commercial fusion power.
The fifth series of experiments conducted on the MAST Upgrade machine, at UKAEA’s Culham Campus in Oxfordshire, ran through 2025 and 2026 and produced more than 1,100 fusion plasmas.
The scientific results were one of the flagship deliveries of UKAEA’s recently published 2026-2030 Strategy: building the scientific foundations needed to make fusion a deployable, low-carbon energy source.
Under Pressure
To create a plasma in a fusion facility, hydrogen isotopes must be heated, squeezed together and confined at extreme temperatures and pressures.
Fusion reactions increase rapidly as density and temperatures rise. A higher-pressure plasma can produce more fusion power per unit volume, making it more representative of the conditions needed in a commercial power plant.
The experiments conducted by the team demonstrated the highest pressure ever achieved in the MAST Upgrade machine without the plasma destabilising.
Drop It Like It’s Hot
The central challenge of MAST Upgrade’s experiments was to suppress instabilities known as “Edge Localised Modes” or “ELMs”. These are sudden bursts at the plasma’s outer edge that can cause a loss of plasma pressure and eject up to a tenth of its stored energy in a single event and, over time, damage a machine’s inner wall and exhaust components.
Left unaddressed, ELMs are seen as a serious obstacle to fusion’s commercial viability, since frequent wall damage would drive up maintenance costs.
Depeche Mode
Building on ELM-suppression results from previous plasma experiments, the MAST Upgrade team adopted the Quasi-Continuous Exhaust mode (QCE-mode) and Resonant Magnetic Perturbations (RMP) ELM suppression modes using coils that apply 3D magnetic fields to reduce the plasma pressure at the edge to keep it stable.
The team also accessed two additional stable operating regimes known as Quiescent H-mode (QH-mode) and I-mode.
QH-mode and I-mode are improved plasma confinement regimes that deliver better energy confinement while mitigating problems associated with large ELMs. They help manage pressure at the plasma edge without triggering these damaging bursts.
Accessing these modes on MAST Upgrade under conditions significantly different from those on other machines marks an important step forward. It enables plasmas to operate with a more stable boundary, giving genuine confidence that fusion power plants can operate with fewer damaging energy surges.
The MAST Upgrade team also developed a world-first technique for controlling the plasma’s position. By measuring visible light created by deuterium emitted from the machine’s upper and lower outer divertors, minute positional imbalances can be detected in real-time.
This method advances fusion towards using automated, real-time control systems that future power plants will need to operate without constant manual intervention.
These breakthroughs were achieved by exploring advanced techniques to suppress ELMs with innovative plasma control and heat-exhaust methods, such as MAST Upgrade’s Super-X divertor, which manages and spreads intense heat and particle exhaust loads more effectively.
Heat of the Moment
The MAST Upgrade team found that injecting small amounts of nitrogen into the plasma edge causes the plasma to emit a large fraction of the exhaust power as light. This dissipates excess heat volumetrically before it reaches the machine’s inner walls and divertor, lowering the peak heat flux and reducing wear and tear on the inner surfaces.
This impurity-assisted method is expected to be essential in a fusion power plant, where even Super-X geometry alone would leave heat loads too high.
MAST Upgrade’s work is the first detailed study of this interaction inside a tightly baffled Super-X, double-null geometry on a spherical tokamak.
The experiments also explored “negative triangularity” plasma shapes that allow high-power operations without ELMs, an approach being closely watched by the international fusion community.
James Harrison, Head of MAST Upgrade Science at UKAEA, said:
The results genuinely shape the design of future fusion power plants. Accessing four stable high-performance plasma regimes, including QH-mode, QCE and I-mode and our world-first plasma position control technique, demonstrates that MAST Upgrade is producing science at the leading edge of what is possible. The level of international interest in our data reflects the UK’s central role in global fusion research, and these findings take us another step closer to practical fusion energy.”
The Only Way Is Up
The results of the experiments featured at the European Physical Society’s Plasma Physics Conference 2026, hosted by UKAEA in Edinburgh. They are now being shared with the international fusion community to inform the design and operation of STEP and ITER.
MAST Upgrade will undergo further enhancements this year, including the addition of two new neutral beam injectors, doubling the machine’s neutral beam heating capacity, and the installation of an Electron Bernstein Wave (EBW) system that will provide an additional 1.6 MW of heating power.
EBW technology is planned for use in STEP, the UK’s prototype fusion power plant to be based in West Burton in Nottinghamshire.
The enhancement programme is expected to conclude in 2027 and a sixth series of experiments focused on STEP-relevant research planned for 2028.
Watch the video announcing the results of MAST-U’s fifth campaign on YouTube: MAST Upgrade’s fifth campaign achieves record plasma