The Joint European Torus (JET) was a magnetically confined plasma physics experiment, located at Culham Centre for Fusion Energy in Oxfordshire, UK. Based on a tokamak design, the fusion research facility was a joint European project with the main purpose of opening the way to future nuclear fusion grid energy. At the time of its design JET was larger than any comparable machine. JET began operation in 1983 and spent most of the next decade increasing its performance in a lengthy series of experiments and upgrades. In 1991 the first experiments including tritium were made, making JET the first reactor in the world to run on the production fuel mix of 50–50 tritium and deuterium. It was also decided to add a divertor design to JET, which occurred between 1991 and 1993. Performance was significantly improved, and in 1997 JET set the record for the closest approach to scientific breakeven, reaching Q = 0.67 in 1997, producing 16 MW of fusion power while injecting 24 MW of thermal power to heat the fuel. Between 2009 and 2011, JET was shut down to rebuild many of its parts, to adopt concepts being used in the development of the ITER project in Saint-Paul-lès-Durance, in Provence, southern France. In December 2020, a JET upgrade commenced using tritium, as part of its contribution to ITER. 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, with scientists fearing a research time gap and personnel loss between JET's closure and the start of ITER's operations. JET finished operations in December 2023, with decommissioning expected to last until 2040.
Purpose As a large tokamak experiment, JET was designed to study plasma behaviour in conditions and dimensions approaching those required in a fusion reactor. The principal aims of the experiment were to investigate:
the scaling of plasma behaviour as parameters approach the reactor range, the plasma-wall interaction in these conditions, the additional plasma heating (neutral beam injection, lower hybrid resonance, and ion cyclotron resonance), the production of alpha particles from fusion reactions, their confinement, and the consequent plasma heating (fusion self-heating).
History
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. 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. 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. 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.
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. In 1975, the first proposals for the JET machine were completed. Detailed design took three years. 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. 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, 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. The cost was 198.8 million European Units of Account (a predecessor of the euro) or 438 million in 2014 US dollars. 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. JET achieved its first plasma on 25 June 1983. It was officially opened on 9 April 1984 by Queen Elizabeth II. On 9 November 1991, JET performed the world's first deuterium-tritium experiment. This beat the US's machine, TFTR, by a full two years.
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