Editing Magnetic mirror (section)

===Thermal barriers and MFTF===
Even before the tandem mirror concept emerged, what was by this time the [[Department of Energy]] had agreed to fund the construction of a much larger mirror known as the [[Mirror Fusion Test Facility]] (MFTF). At the time, the plan for MFTF was to simply be the largest yin-yang magnet anyone could figure out how to build. With the success of the TMX concept, the design was modified to become MFTF-B, using two of the largest yin-yang magnets anyone could figure out how to build in an enormous tandem configuration. The goal was to meet ''Q''=5. Through late 1978 when the teams began to actually consider the steps in scaling up the TMX, it became clear that it simply would not hit the required goals.<ref>"Summary of results from the tandem mirror experiment, TMX group, February 26, 1981</ref> In January 1979, Fowler stopped the work, stating that some improvement would have to be found.{{sfn|Heppenheimer|1984|p=93}}

During experiments on the TMX, it was found to everyone's surprise that the law introduced by [[Lyman Spitzer]] in the 1950s was not holding; in TMX at least, [[electron]]s on any single magnetic line were found to be in a wide variety of speeds, which was entirely unexpected. Further work by John Clauser demonstrated that this was due to the warm plasma injection used to suppress DCLC. Logan took these results and used them to come up with an entirely new way to confine the plasma; with the careful arrangement of these electrons, one could produce a region with a large number of "cool" electrons that would attract the positively charged ions. Dave Baldwin then demonstrated this could be enhanced through the neutral beams. Fowler referred to the result as a "[[thermal barrier]]", as the hotter fuel was repelled from these regions. It appeared it could maintain confinement using much less energy than the pure TMX concept.{{sfn|Heppenheimer|1984|p=95}}

This result suggested that MFTF would not just meet an arbitrary ''Q''=5, but make it a real competitor to the tokamaks, which were promising much higher ''Q'' values. Fowler began the design of another version of MFTF, still called MFTF-B, based on the thermal barrier concept. the lab decided they should begin construction, lacking any experimental evidence that the concept worked, in order to get a competitive machine out around the same time as TFTR. While this huge machine was being built, TMX would be modified to test the concept.{{sfn|Heppenheimer|1984|p=96}}

On 28 January 1980, Fowler and his team presented their results to the DOE. Demonstrating that TMX had worked, and armed with additional data from the Soviets as well as computer simulations, they presented a plan to begin construction on a $226 million MFTF while upgrading TMX to add the thermal barriers in the $14 million TMX-U. The proposal was accepted and construction on both systems began, with TMX shutting down in September 1980 for conversion.{{sfn|Heppenheimer|1984|p=97}}