In June 1983, Professor Barry Green recalls the pivotal moment when the JET fusion laboratory in Oxford embarked on its inaugural experiment. Over the next forty years, this European project diligently pursued the dream of nuclear fusion, offering the potential for abundant, eco-friendly energy.

However, this Saturday marks the culmination of JET's remarkable journey, as the world's most successful fusion reactor concludes its final test.

The concept of nuclear fusion was first explored in the 1920s, with early research initially oriented towards its application in nuclear weaponry. When the United States declassified its fusion-related war research in 1958, it triggered a global race involving Russia, the UK, Europe, Japan, and the US to harness fusion for energy production.

Fusion stands as the ultimate goal in energy generation, emitting copious amounts of energy without any greenhouse gas emissions. It's the same process that powers celestial bodies like the Sun and other stars. In fusion, light atoms are fused together, a stark contrast to nuclear fission, where heavy atoms are split apart.

The UK and Europe joined forces, giving birth to the Joint European Torus (JET) facility. Researchers from across the continent gathered at Culham in Oxfordshire, with Professor Green among them.

Originally an Australian scientist specializing in plasma physics working in Germany, he assumed the role of lead engineer at the site, responsible for its design and construction. The chosen model, a tokamak, utilizes magnetic fields to confine the plasma—a hot, ionized gas—within a vessel. This plasma enables the fusion of light elements, yielding energy.

It was also designed to work with a mix of deuterium-tritium - radioactive elements of hydrogen - rather than just one, which proved a crucial decision. It has been identified as the most efficient reaction for fusion reactors.

The first experiment in the world with this fuel mix took place at JET in 1991. Subsequent experiments have achieved higher energy yields, and the site holds the world record for the most energy produced from a fusion experiment - 59 megajoules (MJ) during a five-second pulse.

Despite the records, the JET site faced many difficulties and delays, with experiments suspended for nearly a decade in the mid-2000s while the internal structure was replaced, according to Fernanda Rimini, JET's senior exploitation manager.

And the hope of producing enough energy to power homes remains a long way off - 59 MJ is only enough to boil about 60 kettles' worth of water.

Joelle Mailloux is the JET science programme leader overseeing the third round of deuterium-tritium experiments which end on Saturday.

She says the key challenges they are focusing on are making the plasma more stable, spreading the power load, and looking at improving materials in the reactor to withstand the conditions.

Once the experiments end, scientists will still have a lot to learn from JET.

"The decommissioning will look at analyzing what has happened to the [reactor] materials and how they have changed. This will help better maintain other fusion sites," Ms Rimini said.

One of the sites benefiting from JET's research will be the new Iter reactor in southern France. It is the world's largest fusion project and is a consortium of many countries including the EU, Russia, the US, and China - but a few weeks ago the UK government confirmed the UK would not play a role.

"In line with the preferences of the UK fusion sector, the UK has decided to pursue a domestic fusion energy strategy instead of associating with the EU's Euratom program," the government said.

The UK government has committed to spending £650m on an alternative UK fusion program between now and 2027. This includes a new prototype fusion energy plant in Nottinghamshire called STEP.

Paul Methven, STEP program director at the UK Atomic Energy Agency, told the BBC: "On endeavors like this, you need to be simultaneously really ambitious and also realistic.

"We are driving pretty hard towards our first operations to be in the early 2040s."

Noted