In a major scientific achievement, European researchers at the Joint European Torus (JET) facility have set a new world energy record of 69 megajoules released in sustained and controlled fusion energy.

"During the last series or experiment, we were able to perform the tests in a repeated pattern and a new energy record was also set. So we are getting better and better at fusion," said Indrek Jõgi, an associate professor and plasma physicist at the institute of physics at the University of Tartu, which is part of the EUROFusion consortium.

Fusion, the process that powers stars like our sun, promises a clean source of heat and electricity for the long term, using small amounts of fuel that can be sourced worldwide from inexpensive materials. It generates energy from fusing atomic nuclei, producing orders of magnitude less radioactive waste than today's nuclear power plants. Moreover, it would not emit carbon dioxide like fossil-fuel power plants.

Technical complexity still prevents nuclear fusion reactors from being used commercially today, but the joint European consortium is working towards this goal.

Last year's experiments released 69.26 megajoules (MJ) of energy in 5.2 seconds. This is about 10 megajoules more than two years ago. The energy released could have boiled about 210 liters of water with only 0.21 milligrams of fuel used. "In 2021, setting a record was a goal in itself. This time it was not a goal, but because the process worked so well, we were also able to improve the record," Jõgi said.

Specifically, the physicists used a tokamak-type magnetic confinement device for the experiments. To induce nuclear fusion, they pressed a gas mixture of hydrogen isotopes – deuterium and tritium, variants of ordinary hydrogen – tightly together in a doughnut-shaped chamber using magnetic fields. The temperature inside the reactor rose to 150 million degrees Celsius, more than ten times the temperature of the sun's inner core.

"Of course, there are other (start-up) fusion companies in both the UK and the US, so it is not impossible that their methods, both similar and dissimilar to ours, could prove more effective. However, the experiments that we have carried out clearly suggest that the largely European method could work best and that this form of reactor can be developed further," Jõgi said.

There is still room for improvement. Last year's experiments already recovered about a third of the energy used to start the reaction. "We're not getting another third of the energy back yet, but it's still a significant amount. This gave us an idea of what happens when most of the energy comes from the process itself. Previous experiments have all been where the energy itself does not come from the process," he said.

Scientists hope to recover more energy than is put into the process in the ITER full-scale fusion reactor to be built in France. It is scheduled to start operating in ten years' time. It is there that the knowledge gained today could be put to practical use.

"If the process starts to reproduce itself in ITER, we may also need different control mechanisms. Physical processes change. The experiments have helped us to learn more about this," he said.

Although there will be no more experiments like JET in Europe for the next decade, Jõe and his Estonian colleagues have a lot of work ahead of them. "Now that JET has been shut down, my own laboratory, among others, will be able this year to start looking more closely at how to measure the amount of radioactive tritium that has now been deposited in the walls. This is a step towards preparing for ITER," he explained.

Although the ITER reactor weighs more than the Tallinn TV Tower, it must be cleaned after 700 grams of tritium have been left in it. This requires the ability to take very precise measurements. If the team is successful in demonstrating the soundness of its approach, it might be repeated at ITER.

Also, researchers at the University of Tartu are designing materials for fusion reactor walls, such as window materials and concrete, to better absorb hazardous neutron radiation. The latter could prove especially valuable in ITER's first fusion power plant.

40 years of fusion experiments

The JET reactor was finished at the end of the 1970s, with an estimated lifetime of only 10 years. However, with the help of smaller and larger accomplishments it was continuously improved, eventually becoming the world's best fusion reactor in terms of performance. Compared to experiments carried out in 1997, energy yields tripled in the following decades.

JET has been the largest and most successful fusion experiment in the world, and a central research facility of the European Fusion Program. "The JET reactor has a long 40 years behind it. We managed to get new results from it and to advance fusion knowledge until the last moment," Jõgi said.

The machine is based at the UKAEA campus in Culham, UK and has been a collective facility used by European fusion researchers under the management of the EUROfusion consortium, co-funded by the European Commission.

JET's successor, ITER, is scheduled to become operational in France in December 2027. It could reach full power by 2035.

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