A nuclear fusion world record has been set in the US, marking another step on the long road towards the unlocking of limitless clean energy.

A team at the Massachusetts Institute of Technology (MIT) created the highest plasma pressure ever recorded, using its Alcator C-Mod tokamak reactor. High pressures and extreme temperatures are vital in forcing atoms together to release huge amounts of energy.

Nuclear fusion powers the sun and has long been touted as the ultimate solution to powering the world while halting climate change. But, as fusion sceptics often say, the reality has stubbornly remained a decade or two away for many years.

Now MIT scientists have increased the record plasma pressure to more than two atmospheres, a 16% increase on the previous record set in 2005, at a temperature of 35 million C and lasting for two seconds. The breakthrough was presented at the International Atomic Energy Agency’s fusion summit in Japan on Monday.

Successful fusion means getting more energy out than is put in and this requires the combination of pressure, temperature and time to pass a critical value at which point the reaction becomes self-sustaining. This remains elusive but the MIT record shows that using very high magnetic fields to contain the plasma may be the most promising route to practical nuclear fusion reactors.

“This is a remarkable achievement,” said Dale Meade, former deputy director at the Princeton Plasma Physics Laboratory. “The record plasma pressure validates the high-magnetic-field approach as an attractive path to practical fusion energy.”

Prof Riccardo Betti, at the University of Rochester, New York, said: “This result confirms that the high pressures required for a burning plasma can be best achieved with high-magnetic-field tokamaks such as Alcator C-Mod.”

However, the world record was achieved on the last day of the MIT tokamak’s operation, because funding from the US Department of Energy has now ended. The US, along with the EU, China, India, South Korea, Russia and Japan, are now ploughing their fusion funding into a huge fusion reactor called ITER.

The giant, seven-storey-high tokamak is being built in southern France, with magnets weighing about the same as a Boeing 747. The volume of ITER’s tokamak will be 800 times bigger than the MIT vessel. ITER should be completed in 15-20 years and aims to deliver 500MW of power, about the same as today’s large fission reactors. But the project has been hampered by delays.

In the meantime, there are numerous private companies hoping to develop small scale nuclear fusion reactors. One is Tokamak Energy, a spin-off from the UK’s national fusion lab, which uses high-temperature superconductors to create the magnetic field to contain the fusion plasma. The MIT tokamak used copper magnets, which require use more power.

Dr David Kingham, chief executive of Tokamak Energy, said the important aspect of the MIT world record was that it showed extreme conditions can be created in small tokamaks: the volume of the MIT device is just one cubic metre. “The conventional view is that tokamaks have to be huge [like ITER] to be powerful,” he said. “The MIT people disagree with that view, as do we.” Kingham’s target is for his company’s compact reactors to produce their first electricity by 2025.

Rival companies also backing small fusion reactors include Lockheed Martin’s famous Skunk Works team. In 2014 said they would produce a truck-sized fusion plant in a decade but attracted criticism for providing few details.

Others in the field include Tri Alpha Energy, which harnesses particle accelerator technology and is backed by Paul Allen, Microsoft’s co-founder. General Fusion, which uses a vortex of molten lead and lithium to contain the plasma, is backed by Amazon’s Jeff Bezos. Helion Energy, First Light Fusion and the University of Washington’s Dynomak are all also chasing the fusion dream.

Prof Dennis Whyte, director of MIT’s Plasma Science and Fusion Center, said small, non-tokamak approaches, though less familiar, could be promising: “Compact, high-field tokamaks provide an exciting opportunity for accelerating fusion energy development, so that it’s available soon enough to make a difference to problems like climate change and the future of clean energy, goals I think we all share.”