Editing Magnetic mirror (section)

===Q-enhancement and tandem mirrors===
[[File:The Tandem Mirror Experiment.jpg|thumbnail|The Tandem Mirror Experiment (TMX) in 1979. One of the two yin-yang mirrors can be seen exposed on the end closer to the camera.]]

In July 1975, the 2XIIB team presented their results for nT at 7x10<sup>10</sup>, an order of magnitude better than 2XII and close enough to Dean's requirements.{{sfn|Heppenheimer|1984|p=82}} By this time, the [[Princeton Large Torus]] had come online and was setting record after record, prompting Hirsch to begin planning for even larger machines for the early 1980s with the explicit goal of hitting [[breakeven (fusion)|breakeven]], or ''Q''=1. This became known as the [[Tokamak Fusion Test Reactor]] (TFTR), whose goal was to run on [[deuterium]]-[[tritium]] fuel and reach ''Q''=1, while future machines would be ''Q''>10.{{sfn|Heppenheimer|1984|p=85}}

With the latest results on 2XIIB, it appeared that a larger yin-yang design would also improve performance. However, calculations showed it would only reach ''Q''=0.03. Even the most developed versions of the basic concept, with leakage at the absolute lower limit allowed by theory, could only reach ''Q''=1.2. This made these designs largely useless for power production, and Hirsch demanded that this be improved if the program were to continue. This problem became known as "Q-enhancement".{{sfn|Heppenheimer|1984|p=85}}

In March 1976, the Livermore team decided to organize a working group on the topic of Q-enhancement at the October 1976 international fusion meeting in Germany. Over the July 4th weekend, Fowler and Post came up with the idea of the tandem mirror, a system consisting of two mirrors at either end of a large chamber that held large amounts fusion fuel at lower magnetic pressure. They returned to LLNL on Monday to find the idea had been developed independently by a staff physicist, Grant Logan. They brought further developed versions of these ideas to Germany to find a Soviet researcher proposing exactly the same solution.{{sfn|Heppenheimer|1984|p=89}}

Upon their return from the meeting, Dean met with the team and decided to shut down the Baseball II system and direct its funding to a tandem mirror project. This emerged as the [[Tandem Mirror Experiment]], or TMX.<ref>"TMX Major Project proposal" Fred Coensgen, January 12, 1977</ref> The final design was presented and approved in January 1977. Construction of what was then the largest experiment at Livermore was completed by October 1978. By July 1979, experiments were demonstrating that TMX was operating as expected.{{sfn|Heppenheimer|1984|p=91}}