ZETA (fusion reactor)

ZETA, short for "Zero Energy Thermonuclear Assembly", was a major experiment in the early history of fusion power research. It was the ultimate device in a series of UK designs using the pinch confinement technique and the first large-scale fusion machine to be built. It was much larger and more powerful than any fusion machine in the world at that time, and the construction of ZETA sparked an intense national rivalry with the United States' pinch and stellarator programs.

ZETA went into operation in 1957, and very quickly in the experimental cycle began giving off bursts of about million neutrons per "shot". Neutrons are the most obvious results of nuclear fusion reactions. Temperature measurements suggested the reactor was operating between 1 and 5 million degrees, a temperature that would produce rates of fusion just about explaining the quantities of neutrons being seen. Early results leaked to the press in September 1957, and the following January an extensive review was released with great fanfare. Front-page articles in major newspapers around the world announced the breakthrough as a major step on the road to unlimited power, a scientific advance for Britain greater than the recently launched Sputnik had been for the Soviet Union.

Both US and Soviet experiments gave off similar neutron bursts at temperatures that were clearly not high enough for fusion, and this led Lyman Spitzer to publicly express his skepticism of the results. His comments were initially dismissed by UK observers as jingoism. Continued experiments on ZETA showed that the original temperature measurements were only accounting for the hottest portions of the fuel, and the bulk of the system was much cooler. The bulk temperature was too low to account for the number of neutrons being seen, and these were later explained as the byproduct of instabilities in the fuel. The claim that ZETA had produced fusion had to be publicly withdrawn, an enormously embarrassing event that cast a chill over the entire fusion establishment. Most work on the basic pinch concept as a road to fusion had ended by 1961.

In spite of ZETA's failure to achieve fusion, the device would go on to have a long experimental lifetime and produced numerous important advances in the field. In one line of development, the use of lasers to more accurately measure the temperature was developed on ZETA, and later used to confirm the results of the Soviet tokamak approach. In another, while examining ZETA test runs it was noticed that the plasma self-stabilized after the power was turned off. This has led to the modern reversed field pinch concept, which sees continued development to this day. More generally, studies of the instabilities in ZETA have led to several important theoretical advances that form the basis of modern plasma theory.

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