Editing Stellarator (section)

=== Tokamak stampede ===
In 1968, scientists in the [[Soviet Union]] released the results of their [[tokamak]] machines, notably their newest example, T-3. The results were so startling that there was widespread scepticism. To address this, the Soviets invited a team of experts from the United Kingdom to test the machines for themselves. Their tests, made using a [[laser]]-based system developed for the [[ZETA (fusion reactor)|ZETA]] reactor in England, verified the Soviet claims of electron temperatures of 1,000&nbsp;eV. What followed was a "veritable stampede" of tokamak construction worldwide.{{sfn|Kenward|1979b}}

At first the US labs ignored the tokamak; Spitzer himself dismissed it out of hand as experimental error. However, as new results came in, especially the UK reports, Princeton found itself in the position of trying to defend the stellarator as a useful experimental machine while other groups from around the US were clamoring for funds to build tokamaks. In July 1969 Gottlieb had a change of heart, offering to convert the Model C to a tokamak layout. In December it was shut down and reopened in May as the [[Symmetric Tokamak]] (ST).

The ST immediately matched the performance being seen in the Soviet machines, besting the Model C's results by over ten times. From that point, PPPL was the primary developer of the tokamak approach in the US, introducing a series of machines to test various designs and modifications. The [[Princeton Large Torus]] of 1975 quickly hit several performance numbers that were required for a commercial machine, and it was widely believed the critical threshold of [[breakeven (fusion)|breakeven]] would be reached in the early 1980s. What was needed was larger machines and more powerful systems to heat the plasma to fusion temperatures.

Tokamaks are a type of pinch machine, differing from earlier designs primarily in the amount of current in the plasma: above a certain threshold known as the ''[[safety factor]]'', or ''q'', the plasma is much more stable. ZETA ran at a ''q'' around {{frac|3}}, while experiments on tokamaks demonstrated it needs to be at least 1. Machines following this rule showed dramatically improved performance. However, by the mid-1980s the easy path to fusion disappeared; as the amount of current in the new machines began to increase, a new set of instabilities in the plasma appeared. These could be addressed, but only by greatly increasing the power of the magnetic fields, requiring [[superconducting]] magnets and huge confinement volumes. The cost of such a machine was such that the involved parties banded together to begin the [[ITER]] project.