In a groundbreaking scientific breakthrough, the UK’s national fusion experiment, MAST Upgrade, has made multiple world-first accomplishments and solved key fusion challenges during its fourth scientific campaign, which took place on Monday 20 October, 2025.
For the first time in a spherical tokamak, small magnetic coils have been used to stabilize instabilities in fusion plasmas. This achievement was made possible by the UK Atomic Energy Authority (UKAEA), who used magnetic coils to apply a 3D magnetic field and stabilize the instabilities in a spherical tokamak plasma. This marks a significant step forward in the development of sustainable fusion energy within spherical tokamaks.
James Harrison, Head of MAST Upgrade Science at UKAEA, commented on this achievement saying, “Suppressing ELMs in a spherical tokamak is a landmark achievement. It is an important demonstration that advanced control techniques developed for conventional tokamaks can be successfully adapted to compact configurations to develop the scientific basis for future power plants like STEP, the Spherical Tokamak for Energy Production.”
In addition to this breakthrough, researchers at UKAEA have also demonstrated independent control of upper and lower divertors, which opens new capabilities for future fusion power plants. By controlling the plasma exhaust in the upper and lower divertors without impacting the performance or density of the plasma in the main chamber of the tokamak, researchers have shown the potential for more robust and flexible operations in future fusion power plants.
Fulvio Militello, Executive Director of Plasma Science and Fusion Operations at UKAEA, expressed his excitement about these accomplishments, stating, “I’m delighted with the ground-breaking findings from our team at UKAEA. These achievements 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.”
In addition to these breakthroughs, MAST Upgrade also set a record for power injected into its 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.
Furthermore, the team achieved the best plasma shape ever recorded on the machine, with an elongation of 2.5 – meaning the plasma height is 2.5 times its width. This shaping of the plasma has a stabilizing effect and enables higher-performance plasmas, making it a key target for future fusion power plants.
The results from these experiments will directly inform the design of ELM control systems for the UK’s STEP Fusion program, as well as help eliminate ELMs as a barrier to commercial fusion viability. The findings also bring MAST Upgrade, the largest spherical tokamak operating in the world, one step closer to realizing fusion as a clean and abundant source of energy for the future.
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