Theta pinch

The name refers to the configuration of currents used to confine the plasma fuel in the reactor, arranged to run around a cylinder in the direction normally denoted as theta in polar coordinate diagrams.

Theta-pinch was developed primarily in the United States, mostly at the Los Alamos National Laboratory (LANL) in a series of machines known as Scylla.

A design mistake led to Scyllac being unable to come anywhere near its desired performance, and the United States Atomic Energy Commission shut the program down in 1977 to focus on the tokamak and magnetic mirror.

George Gamow's 1928 paper on quantum tunnelling demonstrated that nuclear reactions could take place at much lower energies than classical theory predicted.

[13] When the new design became known within the energy labs, James L. Tuck of Los Alamos christened it theta-pinch[16] to distinguish it from the original pinch approach.

[13] Others also proved interested in the design; at General Electric (GE) a small team formed to consider the concept as the basis for a power-producing reactor.

Kurchatov's visit the year earlier had warned against being too hasty in concluding that neutrons in the system were the result of fusion, and that there were other reactions that could produce them.

[15] Unfortunately, there was simply not enough time; the team shipped Scylla I to the show in September and mentioned that it was generating about 20 million neutrons per shot,[21] but was careful to make no claim as to their origin.

A wide variety of experiments on the system demonstrated that the ions were thermalizing at about 15 million Kelvin, much hotter than ZETA and hot enough to explain the neutrons if they were from fusion reactions.

[23][24] Concerned about the ever-rising cost of the fusion program, Paul McDaniel, director of the Division of Research at the United States Atomic Energy Commission (AEC), decided that the FY 1963 budget should cancel one design of the many being developed at the labs.

[25] Tuck, Richard Taschek and Los Alamos' director Norris Bradbury were all convinced the lab needed a major machine.

In the short term, a set of minor improvements produced Scylla II, which was similar to the original but later upgraded from 35 kJ of capacitor power to 185.

As it appeared no breakthrough in performance was possible in the short term, moving forward with their research would require larger machines that they were not willing to build using internal funding alone.

A review of the field was published under the direction of Leslie Cook, which concluded "The likelihood of an economically successful fusion electricity station being developed in the foreseeable future is small."

This solution had not been considered very deeply given the simplicity of the stellarator concept compared to the more complex magnet layout required for the Meyer and Schmidt corrugated version.

On his return to New York he began reading all of the published materials on the theta-pinch concept and concluded that a dynamic stabilization system would likely not work and would be extremely complex even if it did.

He referred to the resulting system as a "high-beta stellarator", beta being a measure of the magnetic strength in the plasma, which would be much higher in a pinch device.

[34] Los Alamos proved extremely interested in Grad's work and proposed that he fully develop it with an eye to presenting it at the next triennial fusion research meeting, due to take place in August 1968 in Novosibirsk.

As the team continued working, several new and disturbing instabilities were revealed and it became clear the helical magnets were ultimately no more stable than the original Meyer-Schmidt concept.

In May 1969, AEC fusion division director Taschek wrote to Bishop stating his feeling the US should respond with their own devices that had the best chance of showing reasonable performance, and that "it is inescapable that they are the Scyllac and 2X!

As Taschek put it in mid-1970, "there may be some real tactical and impact merit in noting that a linear theta pinch... would provide a major contribution to the nτ derby which not seems to have arisen on a short time scale.

To do this, in February 1969 he outlined a plan in which a shorter 10 metres (33 ft) linear device would be built at the same time as a 120 degree sector of Scyllac which would be used to learn how to build the machine as a whole.

While the tokamak had excellent performance, the mirrors being developed at Lawrence Livermore would be far less expensive to build and operate, and these two devices became the focus of his plans.

To keep their design in the running, Los Alamos decided to rapidly move ahead with the toroidal section to prove their approach was also worthy of consideration.

By this time, Ken Thomassen of MIT had made additional calculations that showed feedback would not work at the radius of the current design.

Experiments on Scylla IV and the original segment ended as the entire team focused on the new enlarged design, so additional problems were not discovered.

The final blow was that the gross stability seen in the original segment in 1971 proved to be illusory; in the larger machine the plasma was seen to slowly drift.

Los Alamos attempted one more solution to save the system, re-commissioning Scylla IV with physical stoppers in the ends using light metals.

[43] In 1972, John Bryan Taylor published a series of papers on the topic of magnetic field conservation and flux reversals that had been seen on ZETA but not appreciated at the time.

[43] In the early 1970s, the Kurchatov Institute had demonstrated stable confinement over lengthy periods by reducing the pinch power and adding additional magnets at the end of the linear tube to aid field reversal.