ZETA, short for Zero Energy Thermonuclear Assembly, was a major experiment in the early history of fusion power research. Based on the pinch plasma confinement technique, and built at the Atomic Energy Research Establishment in the United Kingdom, ZETA was larger and more powerful than any fusion machine in the world at that time. Its goal was to produce large numbers of fusion reactions, although it was not large enough to produce net energy.
ZETA went into operation in August 1957 and by the end of the month it was giving off bursts of about a million neutrons per pulse. Measurements suggested the fuel was reaching between 1 and 5 million kelvins, a temperature that would produce nuclear fusion reactions, explaining the quantities of neutrons being seen. Early results were leaked to the press in September 1957, and the following January an extensive review was released. Front-page articles in newspapers around the world announced it as a breakthrough towards unlimited energy, a scientific advance for Britain greater than the recently launched Sputnik had been for the Soviet Union.
U.S. and Soviet experiments had also given off similar neutron bursts at temperatures that were not high enough for fusion. This led Lyman Spitzer to express his scepticism of the results, but his comments were dismissed by UK observers as jingoism. Further experiments on ZETA showed that the original temperature measurements were misleading; the bulk temperature was too low for fusion reactions to create the number of neutrons being seen. The claim that ZETA had produced fusion had to be publicly withdrawn, an embarrassing event that cast a chill over the entire fusion establishment. The neutrons were later explained as being the product of instabilities in the fuel. These instabilities appeared inherent to any similar design, and work on the basic pinch concept as a road to fusion power ended by 1961.
Despite ZETA's failure to achieve fusion, the device went 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 tested on ZETA, and was 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-stabilised after the power was turned off. This has led to the modern reversed field pinch concept. More generally, studies of the instabilities in ZETA have led to several important theoretical advances that form the basis of modern plasma theory.
Conceptual development The basic understanding of nuclear fusion was developed during the 1920s as physicists explored the new science of quantum mechanics. George Gamow's 1928 exploration of quantum tunnelling demonstrated that nuclear reactions could take place at lower energies than classical theory predicted. Using this theory, in 1929 Fritz Houtermans and Robert Atkinson demonstrated that expected reaction rates in the core of the Sun supported Arthur Eddington's 1920 suggestion that the Sun is powered by fusion.
In 1934, Mark Oliphant, Paul Harteck and Ernest Rutherford were the first to achieve fusion on Earth, using a particle accelerator to shoot deuterium nuclei into a metal foil containing deuterium, lithium or other elements. This allowed them to measure the nuclear cross section of various fusion reactions, and determined that the deuterium-deuterium reaction occurred at a lower energy than other reactions, peaking at about 100,000 electronvolts (100 keV).
This energy corresponds to the average energy of particles in a gas heated to thousands of millions of kelvins. Materials heated beyond a few tens of thousands of kelvins dissociate into their electrons and nuclei, producing a gas-like state of matter known as plasma. In any gas the particles have a wide range of energies, normally following the Maxwell–Boltzmann statistics. In such a mixture, a small number of particles will have much higher energy than the bulk.
This leads to an interesting possibility: even at temperatures well below 100,000 eV, some particles will randomly have enough energy to undergo fusion. Those reactions release huge amounts of energy. If that energy can be captured back into the plasma, it can heat other particles to that energy as well, making the reaction self-sustaining. In 1944, Enrico Fermi calculated this would occur at about 50,000,000 K.
Confinement
Taking advantage of this possibility requires the fuel plasma to be held together long enough that these random reactions have time to occur. Like any hot gas, the plasma has an internal pressure and thus tends to expand according to the ideal gas law. For a fusion reactor, the problem is keeping the plasma contained against this pressure; any known physical container would melt at these temperatures.
A plasma is electrically conductive, and is subject to electric and magnetic fields. In a magnetic field, the electrons and nuclei orbit the magnetic field lines. A simple confinement system is a plasma-filled tube placed inside the open core of a solenoid. The plasma naturally wants to expand outwards to the walls of the tube, as well as move along it, towards the ends. The solenoid creates a magnetic field running down the centre of the tube, which the particles will orbit, preventing their motion towards the sides. Unfortunately, this arrangement does not confine the plasma along the length of the tube, and the plasma is free to flow out the ends.
The obvious solution to this problem is to bend the tube around into a torus (a ring or doughnut shape). Motion towards the sides remains constrained as before, and while the particles remain free to move along the lines, in this case, they will simply circulate around the long axis of the tube. But, as Fermi pointed out, when the solenoid is bent into a ring, the electrical windings would be closer together on the inside than the outside. This would lead to an uneven field across the tube, and the fuel will slowly drift out of the centre. Some additional force needs to counteract this drift, providing long-term confinement.
Pinch concept
A potential solution to the confinement problem had been detailed in 1934 by Willard Harrison Bennett. Any electric current creates a magnetic field, and due to the Lorentz force, this causes an inward directed force. This was first noticed in lightning rods. Bennett showed that the same effect would cause a current to "self-focus" a plasma into a thin column. A second paper by Lewi Tonks in 1937 considered the issue again, introducing the name "pinch effect". It was followed by a paper by Tonks and William Allis.
Applying a pinch current in a plasma can be used to counteract expansion and confine the plasma. A simple way to do this is to put the plasma in a linear tube and pass a current through it using electrodes at either end, like a fluorescent lamp. This arrangement still produces no confinement along the length of the tube, so the plasma flows onto the electrodes, rapidly eroding them. This is not a problem for a purely experimental machine, and there are ways to reduce the rate. Another solution is to place a magnet next to the tube; when the magnetic field changes, the fluctuations cause an electric current to be induced in the plasma. The major advantage of this arrangement is that there are no physical objects within the tube, so it can be formed into a torus and allow the plasma to circulate freely.
The toroidal pinch concept as a route to fusion was explored in the UK during the mid-1940s, especially by George Paget Thomson of Imperial College London. With the formation of the Atomic Energy Research Establishment (AERE) at Harwell, Oxfordshire, in 1945, Thomson repeatedly petitioned the director, John Cockcroft, for funds to develop an experimental machine. These requests were turned down. At the time there was no obvious military use, so the concept was left unclassified. This allowed Thomson and Moses Blackman to file a patent on the idea in 1946, describing a device using just enough pinch current to ionise and briefly confine the plasma while being heated by a microwave source that would also continually drive the current.
As a practical device there is an additional requirement, that the reaction conditions last long enough to burn a reasonable amount of the fuel. In the original Thomson and Blackman design, it was the job of the microwave injection to drive the electrons to maintain the current and produce pinches that lasted on the order of one minute, allowing the plasma to reach 500 million K. The current in the plasma also heated it; if the current was also used as the heat source, the only limit to the heating was the power of the pulse. This led to a new reactor design where the system operated in brief but very powerful pulses. Such a machine would demand a very large power supply.
First machines In 1947, Cockcroft arranged a meeting of several Harwell physicists to study Thomson's latest concepts, including Harwell's director of theoretical physics, Klaus Fuchs. Thomson's concepts were poorly received, especially by Fuchs. When this presentation also failed to gain funding, Thomson passed along his concepts to two graduate students at Imperial, Stanley (Stan) W. Cousins and Alan Alfred Ware (1924-2010). He added a report on a type of toroidal particle accelerator known as the "Wirbelrohr" ("whirl tube"), designed in Germany by Max Steenbeck. The Wirbelrohr consisted of a transformer with a torus-shaped vacuum tube as its secondary coil, similar in concept to the toroidal pinch devices.
Later that year, Ware built a small machine out of old radar equipment and was able to induce powerful currents. When they did, the plasma gave off flashes of light, but he could not devise a way to measure the temperature of the plasma. Thomson continued to pressure the government to allow him to build a full-scale device, using his considerable political currency to argue for the creation of a dedicated experimental station at the Associated Electrical Industries (AEI) lab that had recently been constructed at Aldermaston.
Ware discussed the experiments with anyone who was interested, including Jim Tuck of Clarendon Laboratory at Oxford University. While working at Los Alamos during the war, Tuck and Stanislaw Ulam had built an unsuccessful fusion system using shaped charge explosives, but it did not work. Tuck was joined by Australian Peter Thonemann, who had worked on fusion theory, and the two arranged funding through Clarendon to build a small device like the one at Imperial. But before this work started, Tuck was offered a job in the U.S., eventually returning to Los Alamos.
Thonemann continued working on the idea and began a rigorous programme to explore the basic physics of plasmas in a magnetic field. Starting with linear tubes and mercury gas, he found that the current tended to expand outward through the plasma until it touched the walls of the container (see skin effect). He countered this with the addition of small electromagnets outside the tube, which pushed back against the current and kept it centred. By 1949, he had moved on from the glass tubes to a larger copper torus, in which he was able to demonstrate a stable pinched plasma. Frederick Lindemann and Cockcroft visited and were duly impressed.
Cockcroft asked Herbert Skinner to review the concepts, which he did in April 1948. Skinner was sceptical of Thomson's ideas for creating a current in the plasma and thought Thonemann's ideas seemed more likely to work. He also pointed out that the behaviour of plasmas in a magnetic field was not well understood, and that "it is useless to do much further planning before this doubt is resolved."