Tokamak Energy has set three new performance records ahead of a major upgrade to its ‘punching above its weight’ fusion energy machine ST40.
The company ended 2025 by achieving its highest plasma current, highest stored energy, and highest fusion triple product – all key measures on the path to delivering clean, limitless fusion energy.
The compact spherical tokamak at the company’s Oxfordshire HQ is now preparing for a major upgrade for a new campaign of ground-breaking experiments in partnership with the U.S. Department of Energy (DOE) and the UK Department for Energy Security and Net Zero (DESNZ).
Otto Asunta, Tokamak Energy Experiments Chief for ST40, said: “These superb results are a great way to finish the year and demonstrate how the ST40 team continues to push the limits, improving the already impressive performance of our compact high-field device.
“Breaking into the mega-amp range with our plasma current is a major milestone as we continue to build our knowledge of what will be needed for delivering fusion energy to future grids. We now look forward to working with the U.S. and UK governments on a new wave of experiments to deliver more cutting-edge science with our industry-leading device that has always punched above its weight.”
ST40 set a new record by reaching 1 MA (1,000,000 amperes) of plasma current, surpassing its previous best of 0.85 MA, while stored energy was nearly doubled compared to previous campaigns. This demonstrates that a plasma volume of just one cubic metre can hold immense power – an important insight on the path to energy-producing devices.
Plasma current – the electric current flowing through the plasma inside a tokamak – is crucial for confinement. It generates a poloidal magnetic field that, together with the magnetic fields from coils, keeps the plasma confined and away from material surfaces inside a tokamak. In future devices, a high plasma current is required to also trap the fusion-born alpha particles.
The fusion triple product, a measure combining plasma temperature, density, and energy confinement, is a key indicator of plasma performance.
In the company’s record-breaking 100 million degrees Celsius campaign of 2022, ST40 achieved a triple product of approximately 6 ± 2 × 10¹⁸ m⁻³·keV·s, the highest ever achieved in a privately-owned tokamak. Following an extensive series of experiments at plasma currents >800 kA and toroidal field 2.1 T, the team improved its previous best by a third to ~8 ± 2 × 10¹⁸ m⁻³·keV·s, demonstrating significant progress in plasma performance by operating in H-mode (high-confinement mode).
The latest campaign also saw the implementation of a new software called RT-GSFit that can reconstruct the plasma shape during a pulse at a millisecond time resolution (a thousand times per second). Integrating RT-GSFit into the ST40 Plasma Control System allowed the team to control plasma shape in real-time.
ST40 has now paused operations for the $52 million upgrade programme, known as LEAPS (Lithium Evaporations to Advance PFCs in ST40), in partnership with DOE and DESNZ. The programme will apply lithium coatings to all plasma-facing components (PFCs) using a lithium evaporation technique, building on pioneering work by Princeton Plasma Physics Laboratory and others that has shown lithium PFCs can significantly improve plasma performance.
The ultimate goal is to enable fusion conditions with good confinement that is compatible with sustainment for long durations in a future fusion pilot plant.
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