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“It could offer a limitless source of clean energy, if only scientists could make it work

ET interior with superimposed plasma. CREDIT: UKAEA

Culham, in Oxfordshire, home to just 500 people. Yet the English village houses some of the world’s most ambitious science projects. On February 9th researchers at Culham’s Joint European Torus (JET) facility announced that they had released 59 megajoules of energy—enough to run a 60-watt light bulb for 11 days—from nuclear fusion, the reaction that powers the sun. That is a new record, breaking JET’s achievement of 22 megajoules in 1997, and the latest in a series of breakthroughs which suggest that fusion, long anticipated as a source of limitless, clean energy, is inching closer to viability. But what is nuclear fusion, and how does it work?

The process involves encouraging atomic nuclei, usually hydrogen isotopes, to overcome their natural repulsion and combine. Doing so creates a helium nucleus, a spare neutron and large amounts of energy. If done correctly, fusion can release almost 4m times more energy than burning the equivalent mass of oil, and four times as much energy as a nuclear fission reaction, the atom-splitting process used in nuclear power plants today. It is also a clean process, producing no greenhouse gases or long-term toxic nuclear waste.

Given those stellar characteristics, scientists have long wanted to develop power plants that use nuclear fusion. But although individual fusion reactions have been achieved for many decades, power-generating plants remain elusive, owing to the large amounts of energy required to kick-start and sustain fusion reactions on Earth. Whereas the sun’s gravity forces atoms of hydrogen together and allows fusion to occur at a mere 15m°C, on Earth scientists have found that their reactions need temperatures closer to 100m°C. At such high temperatures solids and gases cannot exist and scientists must instead manipulate a fourth state of matter, plasma, which is a hot, ionised gas. They have so far struggled to find an energy-efficient way of doing so. The method with the longest pedigree, and the one used at JET, traps the plasma within powerful, doughnut-shaped magnetic fields. At its heart is a Soviet-designed system called a tokamak reactor. Another popular approach, inertial-confinement fusion, uses powerful lasers to implode pellets containing hydrogen atoms, and compressing that fuel to the point of fusion. Startups, attracted by the potential financial returns that could come from successful fusion, are developing new methods. One is trying “magnetised target fusion”, which would use electrical pulses to create plasma and steam-powered pistons to compress it.

So far every fusion reaction on Earth has consumed more energy than it has released, making the process useless as an alternative to fossil fuels. But recent developments have made some people optimistic that “net energy gain” reactions—the holy grail where a nuclear fusion reaction becomes self-sustaining—could soon be achieved. In May 2021 a Chinese reactor sustained a fusion reaction at 120m°C for 101 seconds, a new record. In August, America’s National Ignition Facility using lasers came the closest yet to achieving net energy gain. And in September Commonwealth Fusion Systems (CFS), a startup that counts Jeff Bezos and Bill Gates among its backers, generated a powerful magnetic field that, in theory, could be used in a net-positive tokamak. Put together, the breakthroughs suggest that scientists and engineers are on the brink of something important—certainly the pace of development has been moving faster than ever before. CFS claims, for example, that it can achieve net positive fusion by 2025. Investors, who have recently handed it $1.8bn, will be hoping it will.”