World-first use of 3D magnetic coils to stabilise fusion plasma in spherical tokamak

Scientists at the Culham Centre for Fusion Energy in Oxfordshire, UK, have for the first time used magnetic coils to apply a 3D magnetic field to ‘completely suppress’ instabilities in a spherical tokamak. The achievement at the MAST (Mega Amp Spherical Tokamak) Upgrade project could help overcome one of fusion’s toughest challenges – maintaining stable plasma conditions long enough to generate power, according to UKAEA.  

Fusion, the same process that powers the sun and stars, involves combining light atomic nuclei at temperatures exceeding 100mn °C to release energy without producing greenhouse gases or long-lived radioactive waste. However, confining such an energetic plasma in a magnetic field has long proved difficult. If the plasma current, pressure or density becomes too high, instabilities known as edge localised modes (ELMs) can arise, threatening to damage reactor components.

The MAST Upgrade reactor is designed in the shape of a cored apple, in contrast to other ring-shaped tokamaks, and is the largest spherical tokamak currently operating in the world. Using resonant magnetic perturbation (RMP) coils, the scientists successfully suppressed ELMs entirely in the tokamak, demonstrating that control techniques pioneered in conventional, ring-shaped tokamaks can be successfully adapted to the compact geometry of spherical machines, explains James Harrison, Head of MAST Upgrade Science at UKAEA.  

The demonstration forms part of MAST Upgrade’s fourth scientific campaign, focused on plasma properties and exhaust management. UKAEA says that the next campaign will build on these results, helping to inform ELM control system design for the UK’s STEP (Spherical Tokamak for Energy Production) fusion programme.  

In another first, the MAST Upgrade scientists also report they achieved independent control of plasma exhaust in the upper and lower divertors – a critical step toward managing the extreme heat and particle fluxes in future power plants. By using nitrogen injection at the plasma edge, the researchers were also able to distribute heat more evenly across plasma-facing components, reducing localised wear and improving operational flexibility.

The machine also achieved a new record for injected power, reaching 3.8 MW using neutral beam heating, enabling exploration of higher-performance plasma regimes. ‘This milestone supports higher performance plasma scenarios and contributes to the development of power plant-relevant conditions,’ says UKAEA. (Neutral beams are energetic beams of particles with no electric charge, typically deuterium, the rare isotope of hydrogen with one neutron as well as a proton and an electron, injected into plasma to heat it by ionising and colliding with plasma particles, according to ScienceDirect.)

Tokamak Energy ‘captures a star’ in living colour

Just a few miles away from the MAST Upgrade project, Tokamak Energy – a UKAEA spin-out company based at Milton Park, Oxfordshire – has captured the inner life of a fusion plasma in striking colour for the first time. Using a high-speed camera capable of 16,000 frames per second, the company recorded visible light emitted from plasma inside its ST40 tokamak – an image described as ‘capturing a star on Earth’.

In the footage, lithium is dropped into the plasma, initially glowing red before turning green as it ionises and traces the magnetic field lines around the tokamak’s core. The core itself remains invisible, radiating at temperatures hotter than the surface of the sun.

The visualisation forms part of Tokamak Energy’s $52mn ST40 upgrade programme, conducted in partnership with the US Department of Energy (DOE) and the UK Department for Energy Security and Net Zero (DESNZ). The project focuses on applying lithium coatings to plasma-facing components (PFCs) – a technique shown in US laboratories to dramatically enhance plasma performance by improving confinement and reducing impurities.

Research at Tokamak Energy is also exploring X-point radiator (XPR) regimes, which aim to cool plasma at the exhaust before it reaches PFCs, thereby extending component lifetimes without sacrificing performance.  

‘The coloured camera is especially helpful for experiments like these,’ explains Dr Laura Zhang, a Tokamak Energy physicist. ‘It helps us immediately identify whether the gaseous impurities we’re introducing are radiating at the expected place, and whether lithium powders are penetrating to the plasma core.’

First colour camera images of plasma glowing inside Tokamak Energy’s ST40 fusion machine

Photo: Tokamak Energy 

US DOE roadmap sets fusion on a commercial trajectory

Across the Atlantic Ocean, the US Department of Energy (DOE) has published its Fusion Science and Technology (FS&T) Roadmap, setting out a national strategy to accelerate the delivery of commercial fusion power. The roadmap outlines DOE’s ‘Build–Innovate–Grow’ strategy, aligning public research with private investment in a bid to achieve grid-connected fusion in the US by the mid-2030s.

‘By accelerating progress toward commercial fusion power, DOE is strengthening America’s grid, rebuilding critical supply chains, and securing a new era of abundant, reliable, American-made energy,’ the Agency says.

The document identifies key technical gaps across six core areas: structural materials, PFCs, confinement systems, fuel cycles, breeding blankets and plant integration.

‘Fusion is real, near, and ready for coordinated action,’ comments Jean Paul Allain, DOE Associate Director for Fusion Energy Sciences. ‘This roadmap provides the strategic foundation for building the scientific, technical and industrial base needed to ensure American leadership in commercial fusion.’

The US has over $9bn of private capital already invested in prototype reactors and burning plasma experiments. DOE’s plan seeks to unify these efforts into a coordinated national programme capable of bridging the remaining technology gaps.