Background

By the early 1960s, the fusion research community was in the "doldrums". Many initially promising experimental paths had all failed to produce useful results, and the latest experiments suggested performance was stalled at the Bohm diffusion limit, far below what would be needed for a practical fusion generator.^[7]

In 1968, the Soviets held the periodic meeting of fusion researchers in Novosibirsk, where they introduced data from their T-3 tokamak. This represented a dramatic leap in fusion performance, at least 10 times what the best machines in the world had produced to that point. The results were so good that some dismissed them as faulty measurements. To counter this, the Soviets invited a team from the UK to independently test their machine. Their 1969 report confirmed the Soviet results, resulting in a "veritable stampede" of tokamak construction around the world.^[8][9]

A key issue in tokamak designs was that they did not generate enough of an electric current in their plasma to provide enough heating to bring the fuel to fusion conditions. Some sort of external heating would be required. There was no shortage of ideas for this, and in the mid-1970s a series of machines were built around the world to explore these concepts. One of these, the Princeton Large Torus (PLT) demonstrated that neutral beam injection was a workable concept, using it to reach record temperatures well over the 50 million K that is the minimum needed for a practical reactor.^[10]

With the PLT's success, the path to scientific breakeven finally appeared possible after decades of effort. Scientific breakeven is the point where the power produced by the fusion reactions is equal to the amount of power injected to heat the plasma. Once breakeven is achieved, even small improvements from that point begin to rapidly increase the amount of net energy being released. Teams around the world began planning for a new generation of machines combining PLT's injectors with superconducting magnets and vacuum vessels that could hold deuterium-tritium fuel instead of the test fuels containing pure deuterium or hydrogen that had been used up to that point.^[11]

European design

In 1971, the member states of the European Atomic Energy Community (Euratom) decided in favour of a robust fusion programme and provided the necessary legal framework for a European fusion device to be developed.^[12] In 1975, the first proposals for the JET machine were completed. Detailed design took three years.^[13] At the end of 1977, after a long debate, Culham was chosen as the host site for the new design. Funding was approved on 1 April 1978 as the "JET Joint Undertaking" legal entity.^[14]

The reactor was built at a new site next to the Culham Centre for Fusion Energy, the UK's fusion research laboratory which opened in 1965. The construction of the buildings was undertaken by Tarmac Construction,^[15] starting in 1978 with the Torus Hall. The Hall was completed in January 1982 and construction of the JET machine itself began immediately after the completion of the Torus Hall.^[14] The cost was 198.8 Million European Units of Account (predecessor to the Euro)^[16] or 438 million in 2014 US dollars.^[17]

JET was one of only two tokamak models designed to work with a real deuterium-tritium fuel mix, the other being the US-built TFTR. Both were built with the hope of reaching scientific breakeven where the "fusion energy gain factor" or Q = 1.0.^{[18][19][20][1]}

JET achieved its first plasma on 25 June 1983.^[14] It was officially opened on 9 April 1984 by Queen Elizabeth II.^[21] On 9 November 1991, JET performed the world's first deuterium-tritium experiment.^[22] This beat the US's machine, TFTR, by a full two years.^[23]

Upgrades

Although very successful, JET and its counterpart TFTR failed to reach scientific breakeven. This was due to a variety of effects that had not been seen in previous machines operating at lower densities and pressures. Based on these results, and a number of advances in plasma shaping and divertor design, a new tokamak layout emerged, sometimes known as an "advanced tokamak". An advanced tokamak capable of reaching scientific breakeven would have to be very large and very expensive, which led to the international effort ITER.^[24]

In 1991, the first experiments including tritium were made, allowing JET to run on the production fuel of a 50–50 mix of tritium and deuterium.^[3] It was also decided at this time to add a divertor, allowing removal of waste material from the plasma.^[25] Performance was significantly improved, allowing JET to set many records in terms of confinement time, temperature and fusion triple product. In 1997, JET set the record for the closest approach to scientific breakeven. It attained Q = 0.67, producing 16 MW of fusion energy while injecting 24 MW of thermal power to heat the fuel,^[26] a record that endured until 2021.^[27][28] This was also the record for greatest fusion power produced.^[29][30]

In 1998, JET's engineers developed a remote handling system with which, for the first time, it was possible to exchange certain components using artificial hands only. A "Remote Handling" system is, in general, an essential tool for any subsequent fusion power plant and especially for the International Thermonuclear Experimental Reactor (ITER) being developed at Saint-Paul-lès-Durance, in Provence, southern France. This Remote Handling system was later to lead on to become RACE (Remote Applications in Challenging Environments).^[31]

In 1999, the European Fusion Development Agreement (EFDA) was established with responsibility for the future collective use of JET.^[32]

ITER design work

In October 2009, a 15-month shutdown period was started to rebuild many parts of the JET to adopt concepts being used in the development of the ITER project in Saint-Paul-lès-Durance, in Provence, southern France.^[3] This including replacing carbon components in the vacuum vessel with tungsten and beryllium ones.^[33]

In mid-May 2011, the shutdown reached its end.^[34] The first experimental campaign after the installation of the "ITER-Like Wall" started on 2 September 2011.^[35]

On 14 July 2014, the European Commission signed a contract worth €283m for another 5-year extension so more advanced higher energy research can be performed at JET.^[36]

Post-Brexit

Brexit threw the plans for JET in doubt. As part of its plan to leave the EU, the UK was to leave Euratom, which provides the funding for JET.^[37] Talks on the funding after 2018, when the 5-year plan expired, commenced and a new agreement to extend JET's operation until 2019 or 2020 appeared to be largely complete. These talks were put on hold after the Brexit announcement.^[12] However, in March 2019, the UK Government and European Commission signed a contract extension for JET.^[38] This guaranteed JET operations until the end of 2024 regardless of Brexit situation.^[39] In December 2020, a JET upgrade commenced using tritium, as part of its contribution to ITER.^[4]

On 21 December 2021, JET produced 59 megajoules using deuterium-tritium fuel while sustaining fusion during a five-second pulse, beating its previous record of 21.7 megajoules with Q = 0.33, set in 1997.^[28][40]

In October 2023, JET set its final fusion energy record, producing 69.29 megajoules over 6 seconds from only 0.21 mg of D-T fuel.^[41][42] In November 2023, a petition asking that JET not be closed was started, with scientists fearing a research time gap and personnel loss between JET's closure and the start of ITER's operations.^[5] Operations ceased in December, after performing 105,842 pulses, with decommissioning expected to last until 2040.^[6] The final pulses were used to operate JET outside of its design capabilities.^[43] The decommissioning and repurposing process is expected to last until 2040.^[6]

Petition against the closure of JET

Immediately after the announcement of JET's closure at the IAEA conference in London, October 2023, the group "Scientists for JET" launched a petition to call for a review of the decision to close JET. ^[5] The scientists are concerned that JET's end date was set assuming that ITER would be up and running by that date to continue fusion experiments, but with ITER's startup being postponed and ITER's deuterium-tritium (D-T) reactions only scheduled for 2035, that there will be a gap of many years with no fusion research.^[5]