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Within months researchers will attempt to create a ball of plasma hotter than the sun inside a doughnut-shaped machine in south-east England. It will be the country鈥檚 first nuclear fusion operation since the last century.

The attempt in November to fuse two forms of hydrogen at the Joint European Torus (JET) in Culham, Oxfordshire, will be the first since the facility broke the then record for nuclear fusion power production for less than a second in 1997.

鈥淗umans don鈥檛 do this very often,鈥� says Howard Wilson at the University of York. He says the fusion reaction in the ring-shaped tokamak at JET is 鈥渆xtremely important鈥� for informing the first plasma created at a much bigger fusion project being built in France called ITER.

Commercial nuclear fusion power holds the promise of clean, limitless energy but is still considered many decades away. So far test projects have consumed more power creating the reaction than they produce. The UK is keen to be a leader in the field, with the government last year committing 拢200m for a plan to build a commercial power station in the future.

JET will import the fuel for the November reaction, a few grams each of the hydrogen isotopes deuterium and tritium, from Canada within the next few months. When fused, they will produce a plasma with a temperature of 100 million掳C, which will be held in place by magnets.

There are two key differences between this year鈥檚 reaction and the one 23 years ago. The big one is that the materials used inside the reactor have been changed, with carbon-based materials such as graphite swapped for tungsten and beryllium. Carbon acts as a sponge for hydrogen, so the change should mean more of the hydrogen fuses in the plasma, rather than ending up in the wall.

The second difference is how long the plasma should last. In 1997, the peak ouptut of 16 megawatts lasted just milliseconds before the plasma terminated. 鈥淚鈥檓 not going to get into whether we beat that number, but you鈥檇 be looking to do something equivalent to that but for a much longer time,鈥� says a spokesperson for the UK Atomic Energy Authority, which runs JET. The group hopes this time the plasma could be held for as long as 5 seconds.

Whatever the outcome, Wilson says the resulting data will be vital for helping ITER when it makes its first plasma, which is currently pencilled in for 2025. ITER is designed to generate 10 times the power it consumes, and researchers hope it will be the stepping stone towards commercial nuclear fusion power decades later. Two UK companies, Tokamak Energy and First Light Fusion, .

Learning more about fusion with tritium should reveal insights for future fusion projects, says Juan Matthews at the University of Manchester. 鈥淵ou need to make sure you contain tritium as there are strict limits on its emissions into the environment, but also because it is expensive at around $30,000 a gram. So you need good systems for its recovery and recycling,鈥� he says.