In a significant stride towards harnessing sustainable fusion power, scientists at the UK Atomic Energy Authority (UKAEA) have announced a world-first breakthrough: successfully stabilizing fusion plasma instabilities within a spherical tokamak. The achievement, made on the Mega Amp Spherical Tokamak (MAST Upgrade) at the Culham Science Centre, marks a critical advance in overcoming one of the major hurdles to viable fusion energy. The breakthrough was announced on Monday, October 20, 2025, following the fourth scientific campaign of MAST Upgrade.
A Breakthrough in Fusion Stability
The primary accomplishment involves the complete suppression of Edge Localised Modes (ELMs) in a spherical tokamak for the first time. ELMs are violent eruptions that occur at the edge of a superheated fusion plasma, capable of damaging reactor components and disrupting the fusion process. The UKAEA research team tackled this challenge by applying a subtle, three-dimensional magnetic field using Resonant Magnetic Perturbation (RMP) coils at the plasma edge. This innovative use of 3D magnetic fields effectively tames these turbulent outbursts, preventing them from harming the tokamak’s internal walls and ensuring the stability needed for sustained fusion reactions.
James Harrison, Head of MAST Upgrade Science at UKAEA, described the suppression of ELMs in a spherical tokamak as a “landmark achievement.” He emphasized its importance in developing the scientific basis for future power plants like the Spherical Tokamak for Energy Production (STEP), demonstrating that advanced control techniques can be adapted to compact fusion configurations.
The Unique Advantage of Spherical Tokamaks
The MAST Upgrade facility, currently the world’s largest spherical tokamak, is designed with a distinctive “cored apple” shape, a departure from the traditional doughnut-shaped tokamaks. This compact, high-performance design is championed by the UK for its potential to offer improved plasma stability and efficiency, paving the way for more compact and potentially cheaper fusion power plants. Operating at temperatures soaring to 35 million degrees Celsius—more than twice the heat at the Sun’s core—MAST Upgrade is a vital testbed for demonstrating that high plasma performance can be scaled to a commercial fusion reactor.
Beyond Plasma Stability: Additional World-Firsts
The stabilization of plasma is not the only success to emerge from MAST Upgrade’s latest campaign. UKAEA researchers also achieved two other world-firsts:
- Independent Divertor Control: The team demonstrated the ability to independently control the plasma exhaust in both the upper and lower divertors of the MAST Upgrade without impacting the plasma’s performance or density in the main chamber. Divertors are crucial exhaust systems that manage the immense heat and particles ejected from the plasma. This breakthrough significantly enhances the robustness and flexibility of future power plant operations by offering new ways to manage energy flow.
- Record Power Injection: The research also set a new record for the amount of power injected into the plasma, reaching 3.8 megawatts using neutral beam heating. This demonstrates enhanced capabilities for heating and maintaining the fusion fuel at the extreme temperatures required.
Paving the Way for Commercial Fusion Energy with STEP
These groundbreaking findings are expected to have a direct impact on the UK’s ambitious Spherical Tokamak for Energy Production (STEP) program. STEP aims to design and build a prototype fusion power plant by the 2040s, with the goal of putting fusion energy onto the UK grid. The data and lessons learned from MAST Upgrade’s advanced control techniques, particularly the ELM suppression system, will be directly integrated into STEP’s design.
Fulvio Militello, Executive Director of Plasma Science and Fusion Operations at UKAEA, expressed his delight with the team’s achievements, noting that they “reinforce the UK’s leadership in fusion research and bring us closer to realising fusion as a clean, safe, and abundant energy source for the future.”
The Path to Limitless Clean Energy
Fusion energy, which powers the sun and stars by fusing lighter atomic nuclei to release vast amounts of energy, offers the promise of a virtually limitless, safe, and low-carbon electricity supply. While the scientific and engineering challenges remain complex, these recent achievements by UK engineers in stabilizing and controlling fusion plasma in spherical tokamaks represent a monumental step forward. They underscore the UK’s commitment to delivering sustainable, clean energy and solidify its position at the forefront of global fusion research. Further research campaigns are planned to build upon these world-first findings, moving humanity closer to a future powered by the stars.
