Over the course of its 40 years of operation, JET has produced numerous fusion milestones, including becoming the first reactor to use a 50/50 mixture of deuterium (hydrogen with an extra neutron) and tritium (hydrogen with two extra neutrons) atoms — now considered a standard fusion fuel. 

"We can reliably create fusion plasmas using the same fuel mixture to be used by commercial fusion energy powerplants, showcasing the advanced expertise developed over time," Fernanda Rimini, JET's scientific operations leader, said in a statement. 

However, this record will be JET's last. The project was decommissioned in December 2023, shortly after the record-breaking test took place. Researchers have now begun the arduous process of taking the reactor apart, a task that is expected to last until 2040. But over that time, they hope to learn even more about what made JET tick, including how renegade plasma blasts affected the tokamak's internal structure. It will also help scientists develop safer strategies to dispose of radioactive waste, the team said.

JET's legacy will live on in the International Thermonuclear Experimental Reactor (ITER), a massive tokamak in southern France scheduled to start up in 2025. The $22 billion dollar project will use a very similar fusion strategy, but at a much larger scale. 

"Throughout its lifecycle, JET has been remarkably helpful as a precursor to ITER," Pietro Barabaschi, ITER's director-general, said in the statement. "The results obtained here will directly and positively impact ITER, validating the way forward and enabling us to progress faster toward our performance goals once operation begins."