Editing Tokamak Fusion Test Reactor (section)

===TFTR concept===
[[Image:U.S. Department of Energy - Science - 114 035 002 (14281232230).jpg|thumb|Inside the TFTR plasma vessel]]
After the success of PLT and other follow-on designs, the basic concept was considered well understood. PPPL began the design of a much larger successor to PLT that would demonstrate plasma burning in pulsed operation.<ref name=princeton/>

In July 1974, the [[United States Department of Energy|Department of Energy]] (DOE) held a large meeting that was attended by all the major fusion labs. Notable among the attendees was [[Marshall Rosenbluth]], a theorist who had a habit of studying machines and finding a variety of new instabilities that would ruin confinement. To everyone's surprise, at this meeting he failed to raise any new concerns. It appeared that the path to break-even was clear.<ref name=search>{{cite book |title= Search for the Ultimate Energy Source |first= Stephen |last=Dean |url=https://books.google.com/books?id=KSA_AAAAQBAJ&pg=PA44 |page=44 |publisher=Springer |date=2013|isbn= 9781461460374 }}</ref>

The last step before the attack on break-even would be to make a reactor that ran on a mixture of [[deuterium]] and [[tritium]], as opposed to earlier machines which ran on deuterium alone. This was because tritium was both [[radioactive]] and easily absorbed in the body, presenting safety issues that made it expensive to use. It was widely believed that the performance of a machine running on deuterium alone would be basically identical to one running on D-T, but this assumption needed to be tested. Looking over the designs presented at the meeting, the DOE team chose the Princeton design.<ref name=search/>

[[Bob Hirsch]], who recently took over the DOE steering committee, wanted to build the test machine at [[Oak Ridge National Laboratory]] (ORNL), but others in the department convinced him it would make more sense to do so at PPPL. They argued that a Princeton team would be more involved than an ORNL team running someone else's design. If an engineering prototype of a commercial system followed, that could be built at Oak Ridge. They gave the project the name TFTR and went to Congress for funding, which was granted in January 1975. Conceptual design work was carried out throughout 1975, and detailed design began the next year.<ref name=search/>

TFTR would be the largest tokamak in the world; for comparison, the original ST had a plasma diameter of {{convert|12|inch}}, while the follow-on PLT design was {{convert|36|inch}}, and the TFTR was designed to be {{convert|86|inch}}.<ref name=princeton>{{cite magazine |url=https://books.google.com/books?id=AAEAAAAAMBAJ&pg=PA71 |title=Princeton tokamak heats up the race for fusion power |magazine= Popular Science |date= December 1978 |pages=69–71, 150}}</ref> This made it roughly double the size of other large-scale machines of the era; the 1978 [[Joint European Torus]] and roughly concurrent [[JT-60]] were both about half the diameter.<ref>{{cite tech report |url=https://physics.fjfi.cvut.cz/publications/FTTF/BP_Martin_Kubic.pdf |title=Review of plasma parameters of the JET tokamak in various regimes of its operation |first=Martin |last=Kubiˇc |publisher=Czech Technical University |date=31 July 2007}}</ref>

As PLT continued to generate better and better results, in 1978 and 79, additional funding was added and the design amended to reach the long-sought goal of "scientific breakeven" when the amount of power produced by the fusion reactions in the plasma was equal to the amount of power being fed into it to heat it to operating temperatures. Also known as ''Q'' = 1, this is an important step on the road to useful power-producing designs.{{sfn|Meade|1988|p=107}} To meet this requirement, the heating system was upgraded to 50&nbsp;MW, and finally to 80&nbsp;MW.<ref>{{cite tech report |vauthors=Ehlers KW, Berkner KH, Cooper WS, Hooper B, Pyle RV, Stearns JW |date=17 November 1975 |title=Conceptual Design of a Neutral-Beam Injection System for the TFTR |publisher=Lawrence Berkeley Laboratory |url=http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/07/243/7243157.pdf}}</ref>