The world’s largest operating tokamak for nearly four decades, JET set successive records for deuterium–tritium fusion energy and provided the experimental foundation for ITER.
The Joint European Torus (JET), located at the Culham Centre for Fusion Energy in Oxfordshire, United Kingdom, was the flagship experiment of the European fusion programme for almost 40 years. Operational from 1983 to December 2023, JET was the only tokamak in the world equipped to handle tritium fuel during the latter decades of the magnetic fusion programme, and it set successive world records for fusion energy production.[1]
JET’s original carbon-walled interior was replaced in 2011 with an ITER-Like Wall (ILW) comprising beryllium first-wall tiles and a tungsten divertor, mirroring the material choices planned for ITER. This upgrade provided critical data on fuel retention, impurity behaviour, and divertor performance in conditions directly relevant to ITER’s design.[2]
JET’s record of verified experimental results in deuterium–tritium fusion is unmatched among tokamaks:
1991 — First controlled D–T fusion in a tokamak. JET’s Preliminary Tritium Experiment produced approximately 1.7 MW of fusion power, demonstrating that tritium could be safely handled and burned in a magnetic confinement device.[3]
1997 — Record peak fusion power. JET achieved 16.1 MW of peak D–T fusion power (Q ≈ 0.67), a world record for a magnetically confined plasma that stood for over 20 years.[1]
2021 — Record sustained fusion energy. In deuterium–tritium experiments conducted in late 2021 and announced in February 2022, JET produced 59 MJ of fusion energy over a five-second sustained pulse, more than doubling its own 1997 record of 21.7 MJ. This verified result confirmed plasma scenarios designed for ITER-relevant conditions with the metallic ITER-Like Wall.[4]
Beyond its headline fusion records, JET made foundational contributions to tokamak physics. It was among the first large tokamaks to access H-mode (high-confinement mode), discovered at ASDEX in 1982, and to study edge-localised modes (ELMs)—periodic plasma instabilities that expel energy to the vessel wall. JET developed and tested ELM mitigation techniques, including resonant magnetic perturbations and pellet-pacing, that inform ITER’s ELM-control strategy. JET also advanced understanding of isotope effects on confinement, demonstrating that D–T plasmas exhibit measurably better energy confinement than pure deuterium plasmas—a result with direct implications for ITER performance projections.[5]
JET conducted its final plasma experiments in December 2023 after approximately 100,000 individual plasma pulses over its 40-year lifetime. The facility entered a decommissioning and repurposing phase managed by the United Kingdom Atomic Energy Authority (UKAEA) starting in 2024. The decommissioning programme itself is expected to advance remote-handling and waste-management technologies relevant to future fusion power plants.
JET’s scientific legacy is substantial. It produced the only large-scale D–T experimental dataset available to the fusion community, validated the ITER-Like Wall concept under reactor-relevant conditions, and trained a generation of fusion scientists and engineers across Europe. Its results underpin many of the physics assumptions in the ITER design basis and will continue to be analysed for years to come.