Most of the energy that powers life and weather on Earth comes from the Sun. It drives the water cycle, generates winds, enables plants to grow, powers photovoltaic panels and, over geological timescales, led to the formation of fossil fuels. Even when we talk about wind energy, biomass or the water cycle, we are indirectly referring to solar energy.

On average, about 1.4 kilowatts of solar power per square metre reaches the top of Earth's atmosphere. Because the Earth is a rotating sphere, only part of its surface is illuminated at any given time. Averaged across the entire planet, this amounts to around 0.34 kilowatts per square metre. Some of this energy is reflected by clouds, the atmosphere and Earth's surface, some is absorbed by the atmosphere, and the rest warms the land and oceans. Considering the size of our planet, the Sun delivers a total of around 170 million gigawatts of power to Earth. The figure is so enormous that all of humanity's technological energy production is little more than a footnote.

Where does this energy come from? Not from combustion, even though images from NASA or the European Space Agency may make the Sun appear to be burning. Instead, it is produced by nuclear fusion, in which light atomic nuclei fuse into heavier ones and a small fraction of their mass is converted into energy, according to Einstein's famous equation E = mc². In the Sun, the fusion of hydrogen into helium is the dominant process. This raises an obvious question: if almost all of Earth's natural energy ultimately comes from stellar fusion, could even a small part of that process be recreated on Earth?

Fusion experiments on Earth typically focus on a more achievable reaction: the fusion of deuterium and tritium, two heavy isotopes of hydrogen. To make this happen, the fuel must be heated to temperatures of tens of millions of degrees. At these temperatures, atoms lose their electrons, forming plasma—a mixture of free electrons and atomic nuclei often referred to as the fourth state of matter. Plasma is far from a simple, quiescent gas. It conducts electricity, responds to magnetic fields, develops instabilities and can rapidly release energy into its surroundings. A future fusion power plant must therefore not only confine this extremely hot plasma but also keep it away from the reactor walls, maintain its stability and safely remove excess heat.

MAST Upgrade, a British spherical tokamak operated by the UK Atomic Energy Authority at Culham, was designed to tackle precisely these challenges. A tokamak is a device that uses magnetic fields to confine plasma within a closed chamber. MAST Upgrade is not a prototype power plant but a highly specialised research facility where scientists can alter the plasma shape, heating method, magnetic field configuration and energy removal system to determine which approaches improve stability and which lead to particle losses or excessive stress on reactor components.

This work shows that fusion is about much more than achieving extremely high temperatures. Scientists must simultaneously keep energy inside the plasma, limit violent events at its edge, understand how fast particles escape and safely dissipate heat. The results of the MAST Upgrade programme, published in the journal Nuclear Fusion, highlight these challenges and the progress being made towards overcoming them. Scientists from the Institute of Plasma Physics and Laser Microfusion in Warsaw and the University of Opole contributed to the research.

In a future fusion reactor, fast particles produced by fusion reactions should transfer their energy back into the plasma, helping to maintain the temperatures required for sustained fusion. If they escape too easily, the plasma loses heat and the reactor walls are exposed to greater stress. At MAST Upgrade, researchers investigated the behaviour of fast ions generated during plasma heating, including losses caused by plasma waves and interactions with neutral atoms. In other words, they examined how energy that should remain inside the plasma escapes from the system.

Another key challenge lies at the plasma edge. Even when the hot plasma core is successfully confined by magnetic fields, heat and particles must eventually leave the system. In a tokamak, this is the role of the divertor, a component designed to remove heat, particles and impurities. MAST Upgrade allows researchers to test different divertor configurations with the aim of reducing the heat flux before it reaches the reactor walls. For a future fusion power plant, this is not merely an engineering detail but a prerequisite for long-term operation.

The research also showed that it is possible to maintain favourable conditions in the plasma core while simultaneously reducing the heat load on the divertor. Scientists investigated how pumping systems and gas flow control can influence where energy leaves the plasma. This represents an important step towards reactors capable not only of producing fusion plasma but also of controlling how energy flows through the system.

Another challenge involves sudden bursts of energy from the plasma edge. In a commercial fusion reactor, such events could overload the divertor and damage reactor components. At MAST Upgrade, researchers achieved operating conditions in which these bursts were significantly reduced or even eliminated. They also showed that carefully controlled magnetic field perturbations could suppress them and investigated alternative plasma shapes that may promote more stable reactor operation.

MAST Upgrade is not a miniature power plant. Rather, it is a research facility designed to develop the technologies needed for larger fusion projects, including ITER, the international experimental fusion reactor under construction in France; DEMO, the planned demonstration fusion power plant; and the UK's STEP programme, which aims to build a spherical tokamak capable of generating electricity.

The MAST Upgrade programme allows researchers to investigate how to control plasma, limit instabilities, reduce fast particle losses and safely dissipate heat without damaging reactor walls. Solving these challenges will be essential to developing a device capable of generating electricity not from the Sun's rays, but from the same process that powers the Sun itself.

Krzysztof Petelczyc (PAP)

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