On Monday, October 20th, 2025, MAST Upgrade, the UK’s national fusion experiment, announced multiple world-first breakthroughs and successful resolution of key fusion challenges during its fourth scientific campaign.
One of the most significant accomplishments is the successful use of small magnetic coils to stabilize instabilities in fusion plasmas within a spherical tokamak for the first time. The research team at the UK Atomic Energy Authority (UKAEA) achieved this feat by applying a 3D magnetic field using Resonant Magnetic Perturbation (RMP) coils at the plasma edge. This breakthrough marks a major step forward in the development of sustainable fusion energy within spherical tokamaks.
In order to achieve fusion using the Mega Amp Spherical Tokamak (MAST) Upgrade, fusion fuel must be confined at high temperatures within the tokamak to form a plasma. However, if the plasma current, pressure, or density becomes too high, instabilities can occur, reducing performance or causing damage to the tokamak’s components. Therefore, maintaining plasma stability is a crucial challenge in fusion research.
One such instability, known as Edge Localised Modes (ELMs), can occur at the edge of a plasma and pose a serious threat to the internal components of a future fusion power plant. Through the use of RMP coils, the UKAEA team has successfully suppressed ELMs in the MAST Upgrade machine, marking the first time this has been achieved in a spherical tokamak.
James Harrison, Head of MAST Upgrade Science at UKAEA, described this achievement as a “landmark” and highlighted its importance in developing the scientific basis for future power plants, such as the Spherical Tokamak for Energy Production (STEP).
The MAST Upgrade, which is currently the largest spherical tokamak in operation, is shaped like a cored apple, unlike other ring-shaped tokamaks. These breakthroughs were part of the fourth scientific campaign of the MAST Upgrade, which focused on plasma properties and controlling plasma exhaust.
Further research is planned in the next experimental campaign to confirm and expand on these world-first findings. These results will also directly inform the design of ELM control systems for the UK’s STEP Fusion program and help eliminate ELMs as a barrier to commercial fusion viability.
In addition to this breakthrough, the UKAEA researchers also demonstrated the ability to independently control the plasma exhaust in the upper and lower divertors of the MAST Upgrade without impacting the performance or density of the plasma in the main chamber of the tokamak. This achievement could significantly enhance the robustness and flexibility of future power plant operations.
Furthermore, the team also achieved a record for the amount of power injected into the plasma, reaching 3.8 megawatts using neutral beam heating. This milestone supports higher performance plasma scenarios and contributes to the development of power plant-relevant conditions.
The latest round of experiments also saw the team achieving the best plasma shape ever recorded on the machine, with an elongation of 2.5. This means that the plasma height is 2.5 times its width. The shaping of a plasma can have a stabilizing effect, allowing for higher-performance plasmas with higher pressure and better confinement. Greater plasma elongation will be a key target for future fusion power plants like STEP.
Fulvio Militello, Executive Director of Plasma Science and Fusion Operations at UKAEA, expressed his delight with the groundbreaking findings from the team at UKAEA, stating that they reinforce the UK’s leadership in fusion research and bring us closer to realizing fusion as a clean, safe, and abundant energy source for the future.
