For decades, the technology to develop clean, safe fusion power has remained tantalisingly out of reach. Now, though, a new breed of start-ups could have cracked it at last. Will we finally be able to wave goodbye to fossil fuels? 

Flick through any collection of popular science magazines from the last 50 years and the chances are that you will encounter a feature about nuclear fusion. Nuclear fusion is the process of joining lightweight atoms together to release energy; it is the reason the Sun and the rest of the stars shine, and recreating that process on Earth promises an abundant form of low-carbon energy.

As you look through the archival material, keep an eye open for when the pundits think fusion might arrive. It’s an old joke that whatever the decade of the publication, fusion has always been “about 20 years away”. Yet despite five decades of effort, no fusion plant can yet produce more energy than it takes to start the reaction. But don’t let that bare fact make you think that fusion is as far away as ever. There has been a sea change recently that has shrunk the 20-year horizon to a mere decade away. For a start, there has been an increase in our understanding of the science of fusion. Second, there has been an undoubted set of technological breakthroughs. But it is the third reason that may be the deciding factor: a change in mindset. 

“The most important single thing that’s changed in the world of fusion, in my opinion, over the last decade, is the very clear realisation that we need fusion,” says Tim Bestwick. He is the chief technology officer and director of strategy, communications and business development at the UK Atomic Energy Association (UKAEA).

He explains that the two principal factors in this realisation are climate change and energy security. In regard to climate change, fusion offers an abundant source of low-carbon energy that can be used in combination with renewable energy sources such as solar and wind power. This has been accepted for years now, and while energy security has also been talked about for a long time, the sharp need for it has only just recently come into focus, particularly with Russia’s invasion of Ukraine causing many countries to rethink how they buy in energy and fuel from foreign powers. This all loads the dice in favour of fusion.

The UK’s fusion effort is headquartered at the Culham Centre for Fusion Energy in Oxfordshire. This is the home of a long-running experimental fusion reactor called JET, the Joint European Torus. JET is a tokamak, a five-metre-wide doughnut-shaped vessel. The name derives from a Russian word meaning ‘toroidal chamber with magnetic field’.

Since starting operations in 1983, it has made major advances in understanding both the science of nuclear fusion and the engineering required to make it happen. And for inspiration, the researchers need only look up into the sky The Sun is the nearest natural fusion reactor to Earth. Deep below its glowing surface, the temperature soars to 15 million degrees Celsius, and the pressure and density is similarly gigantic. Under these conditions, fusion naturally occurs. It starts with hydrogen and proceeds through a series of interactions that force the hydrogen nuclei together. First the reactions build isotopes of hydrogen and helium, and then ordinary helium itself. 

For an artificial fusion reactor, it is impossible to recreate the kinds of pressures and densities found inside the Sun. Inside JET, for example, the gas density rarely rises above that of the ordinary air outside, and so to compensate, the temperature must be boosted to more than 100 million degrees Celsius.

At such temperatures, the gas becomes electrically charged. This state of matter is known as a plasma, and because of its electrical charge, it can be controlled by magnetic fields. The magnetic field is essential because no material can contain a gas at more than 100 million degrees Celsius. The magnetic field accelerates and controls the flow of the plasma within the reactor, allowing the particles to fuse and release energy.