The UK’s JET machine, one of the world’s biggest and most powerful fusion reactors, achieved a new record for energy output on Thursday during its last run before being permanently shut down.

High fusion power was consistently produced from the 40-year-old reactor for five seconds, the UK Government said, resulting in a record 69 megajoules of energy output from just 0.2 milligrams of fuel.

European scientists working on the project said they had achieved things “never done before”. UK Minister for Nuclear and Networks Andrew Bowie called the record just before the reactor’s retirement a “fitting swansong”.

Professor Ambrogio Fasoli, programme manager at EUROfusion, said: “Our successful demonstration of operational scenarios for future fusion machines, validated by the new energy record, instil greater confidence in the development of fusion energy. Beyond setting a new record, we achieved things we have never done before and deepened our understanding of fusion physics.”

JET, located in Oxfordshire, began operating in 1983. During its run, it temporarily became the hottest point in the solar system, reaching 150 million degrees.

Nuclear fusion is the process that powers stars. Unlike already-commercialised nuclear fission reactions that work by splitting atoms, fusion works by heating and forcing tiny particles together to make a heavier one that releases huge amounts of energy.

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If successfully harnessed and scaled, fusion could in theory produce limitless clean energy with no harmful waste, unlike fission power. It would be a constant reliable source of electricity, unlike current renewable options such as wind and solar.

However, it has been notoriously difficult to make progress in the sector due to the sheer amount of energy and heat required to force and sustain the reaction.

Dr Aneeqa Khan, research fellow in nuclear fusion at the University of Manchester, told BBC News: “In order for the atoms to fuse together on Earth, we need temperatures ten-times hotter than the Sun – around 100 million celsius, and we need a high enough density of the atoms and for a long enough time.”

Although based in the UK, JET has been funded predominantly by the EU nuclear research programme, Euratom, and is operated by the UK Atomic Energy Agency (UKAEA). For four decades it hosted scientists from the UK, Europe, Switzerland and Ukraine.

Since Brexit, the UK has been shut out of the Euratom programme and last year it made the decision not to rejoin, making its position in the European collaboration on fusion development unclear. Bowie said that the government will commit £650m to national research programmes instead.

The reactor was originally only meant to be operational for around a decade, but repeated successes saw its life extended. The record hit on Thursday is triple what was achieved in similar tests back in 1997.

Professor Sir Ian Chapman, UKAEA CEO, said: “JET has operated as close to powerplant conditions as is possible with today’s facilities, and its legacy will be pervasive in all future powerplants. It has a critical role in bringing us closer to a safe and sustainable future.”

He added: “JET’s research findings have critical implications not only for ITER – a fusion research mega-project being built in the south of France – but also for the UK’s STEP prototype powerplant, Europe’s demonstration powerplant, DEMO, and other global fusion projects, pursuing a future of safe, low-carbon and sustainable energy.”