The UK Atomic Energy Authority (UKAEA) has completed the fifth experimental campaign on its flagship MAST Upgrade fusion machine, producing more than 1,100 plasma discharges during experiments carried out across 2025 and 2026.
The campaign focused on tackling key stability and heat-management challenges that must be solved before fusion power can become commercially viable.
Researchers achieved the highest plasma pressure yet recorded on MAST Upgrade without triggering destabilising events.
The result provides new evidence that advanced techniques can maintain high-performance plasmas while controlling the potentially damaging instabilities that occur at their edges.
The campaign also demonstrated new approaches to plasma control and heat exhaust, including access to several stable operating regimes and a new real-time method for detecting small shifts in plasma position.
The findings will contribute to the development of future fusion facilities, including the UK’s STEP prototype power plant and the international ITER project.
Tackling a major fusion stability problem
Fusion requires hydrogen isotopes to be heated to extreme temperatures and confined as plasma.
Increasing the plasma’s density and temperature can raise the amount of fusion power produced, but operating at higher pressure also makes maintaining stability more difficult.
One of the most persistent problems is the occurrence of Edge Localised Modes (ELMs). These sudden bursts of energy at the edge of a plasma can rapidly reduce pressure and eject a significant proportion of its stored energy.
Repeated events can also place severe loads on the materials lining a fusion machine. For a commercial reactor, frequent ELMs could increase component degradation and maintenance requirements.
During the latest MAST Upgrade campaign, researchers used techniques including Quasi-Continuous Exhaust (QCE) and Resonant Magnetic Perturbations (RMPs) to control these instabilities.
Magnetic coils generate three-dimensional fields that modify conditions at the plasma boundary, helping prevent large bursts from developing.
The team also accessed Quiescent H-mode (QH-mode) and I-mode, two operating regimes that can provide strong plasma confinement while reducing the problems associated with conventional ELM activity.
MAST Upgrade is particularly valuable because these techniques are being tested in a spherical tokamak configuration and under conditions that differ from those found on conventional machines. UKAEA describes the facility as the world’s largest operational spherical tokamak.
New approach to automated plasma control
The campaign produced another significant advance in controlling the position of the plasma.
Researchers developed a technique that uses visible light generated by deuterium in the upper and lower outer divertors to identify very small differences in the plasma’s position in real time.
Such measurements could eventually support automated control systems capable of responding to changes without relying on continuous manual intervention.
That capability will become increasingly important as fusion machines move towards longer and more demanding operating periods. Reliable automated control will be needed to keep the plasma within its intended operating envelope while protecting components inside the reactor.
Super-X divertor helps manage heat
Controlling plasma stability is only part of the challenge. A future fusion power plant must also deal with enormous heat and particle loads escaping from the plasma.
MAST Upgrade’s Super-X divertor is designed to spread these loads over a larger surface area, reducing the intensity of heat reaching plasma-facing components. Previous experiments have demonstrated a substantial reduction in divertor heat loads using the system.
During the latest campaign, researchers found that injecting small quantities of nitrogen near the plasma edge can cause a significant proportion of exhaust power to be emitted as light.
This dissipates some of the energy before it reaches the machine’s walls and divertor, potentially reducing peak heat flux and component wear.
The work represents the first detailed examination of this interaction within a tightly baffled Super-X, double-null configuration on a spherical tokamak.
Researchers also investigated negative triangularity, a plasma shape that could enable high-power operation while avoiding conventional ELMs.
James Harrison, Head of MAST Upgrade Science at UKAEA, added: “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.”
MAST Upgrade prepares for next phase
The results form part of UKAEA’s wider programme to establish the scientific foundations for deployable, low-carbon fusion energy.
The organisation’s 2026-2030 strategy places MAST Upgrade among its key programmes for improving understanding of the plasma conditions required by future spherical tokamak power plants.
Further upgrades are now planned for MAST Upgrade. Two additional neutral beam injectors are being installed, which will double its neutral-beam heating capacity, while an Electron Bernstein Wave (EBW) system will add 1.6MW of heating power.
The EBW technology is also intended for STEP, the UK’s planned prototype fusion power plant.
The enhancement programme is expected to run through 2027, with a sixth experimental campaign planned for 2028. That work will focus more directly on research relevant to STEP and the operating scenarios needed for future spherical tokamak power plants.
The latest MAST Upgrade results therefore represent more than another experimental milestone. By demonstrating higher-pressure operation while managing plasma instability, exhaust heat and automated control, UKAEA is addressing several of the practical problems that stand between experimental fusion devices and functioning power plants.
