ZETA (fusion reactor)
ZETA, short for "Zero Energy Thermonuclear Assembly", was a major experiment in the early history of fusion power research. Based on the pinch technique, it was much larger and more powerful than any fusion machine in the world at that time.
ZETA went into operation in 1957, and early in the experimental cycle began giving off bursts of about million neutrons per "shot". Measurements suggested it was reaching between 1 and 5 million degrees, a temperature that would produce rates of nuclear fusion reactions 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 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. This led Lyman Spitzer to publicly express his skepticism of the results, but 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 temperature was too low for fusion reactions to explain the number of neutrons being seen. The neutrons 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 The claim that ZETA had produced fusion had to be publicly withdrawn, an enormously embarrassing event that cast a chill over the entire fusion establishment. These instabilities appeared inherent to any similar design, and 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. More generally, studies of the instabilities in ZETA have led to several important theoretical advances that form the basis of modern plasma theory.
- 1 Conceptual development
- 2 Pinch concept
- 3 First machines
- 4 Early results
- 5 Stabilized pinch
- 6 ZETA begins construction
- 7 Soviet visit, push to declassify
- 8 Promising results
- 9 Prestige politics
- 10 Early concerns
- 11 Public release, worldwide interest
- 12 Further skepticism
- 13 Retraction of claims
- 14 Harwell in turmoil, ZETA soldiers on
- 15 Thomson scattering and tokamaks
- 16 Reversed field pinch
- 17 Demolition
- 18 Notes
- 19 References
- 20 External links
The basic understanding of nuclear fusion was developed using the then-new field of quantum mechanics. A key advance was made by George Gamow's 1928 demonstration of fusion's theoretical basis. Another was made by Fritz Houtermans and Robert Atkinson the next year, using Gamow's equations to demonstrate that expected reaction rates in the core of the sun supported Arthur Eddington's suggestion that the sun was fusion powered.
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 system allowed them to measure the nuclear cross section of various fusion reactions, and determined that the tritium-deuterium reaction occurred at a lower energy than any other fuel, peaking at about 100,000 electronvolts (100 keV). This would be the average energy of particles heated to of millions of degrees, and in 1944, Enrico Fermi calculated the reaction would be self-sustaining at about 50,000,000 Celcius.
Materials heated beyond a few tens of thousand degrees dissociate into their electrons and nuclei, producing a gas-like state of matter known as plasma. According to the ideal gas law, like any hot gas the plasma will have an internal pressure and thus want to expand. For a fusion reactor, the problem was keeping the plasma contained; any known substance would melt at these temperatures. But because a plasma is electrically conductive, it is subject to electric and magnetic fields. In a magnetic field, the electrons and nuclei orbit around the magnetic field lines, confining them to the area defined by the field.
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 magnetic field lines running down the center of the tube, and the plasma particles orbit these lines, preventing their motion towards the sides. Unfortunately, this arrangement would not confine the plasma along the length of the tube, and the plasma would be free to flow out the ends.
The obvious solution to this problem is to bend the tube around into a torus (a ring or donut) 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,[a] 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 center. Some additional force needs to counteract this drift, providing long-term confinement.
A potential solution to the confinement problem had been detailed in 1934 by Willard Harrison Bennett. Any electrical 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".
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, not unlike a fluorescent lamp. The disadvantage of this arrangement is that there is still no confinement along the length of the tube, so the plasma flows onto the electrodes. At the temperatures involved in fusion, the electrodes rapidly erode, although this is not a problem for a purely experimental machine, and there are ways to reduce this. 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 freely circulate.
The 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 or simply "Harwell") in 1945, Thomson repeatedly petitioned the director, John Cockcroft, for funds to develop an experimental machine. These requests were turned down every time. At the time there was no obvious military use, so the concept was left unclassified. This allowed Thomson and Moses Blackman to patent the idea in 1947, describing a device using just enough pinch current to confine the plasma while being heated by a microwave source.
The idea of using steady alternating current (AC) to create the pinch had a serious limitation. AC power drops to zero voltage twice per cycle, and during this time it would not induce a current in the plasma, briefly ending confinement. Direct current (DC) power avoids this issue, but steady current does not induce a magnetic field. However, a single pulse of such current, essentially half of an AC cycle, would work. Another key point was that the current in the plasma also heated it. In the original designs, a weak pinch current was used for confinement while the external heaters slowly heated the plasma. But if the current was also used as the heat source there was no practical limit to the heating rate, it was limited only by the power of the pulse. Combining the two concepts, a new reactor design emerged where the system operated in brief but extremely powerful pulses. Such a machine would demand a very large power supply.
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 received a chilly reception, especially from Fuchs. When this presentation also failed to gain funding, Thomson passed along his concepts to two graduate students at Imperial, Stan Cousins and Alan Ware. He added a report on a somewhat similar device built in Germany by Max Steenbeck called "Wirbelrohr" ("whirl tube"), a type of particle accelerator.
Later that year, Ware managed to build 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. However, 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 building a dedicated lab at the Associated Electrical Industries (AEI) lab that had recently been constructed at Aldermaston.
Ware discussed the experiments with anyone that was interested, including Jim Tuck who was working at Clarendon Laboratory at Oxford University. While working at Los Alamos during the war, Tuck and Stanislaw Ulam had built an unsuccessful colliding beam fusion system similar to Rutherford's pioneering experiment. Tuck was joined by Australian Peter Thonemann, who had worked on fusion theory, and the two arranged some 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 US, eventually returning to Los Alamos.
Thonemann continued working on the idea and began a rigorous program to explore the basic physics of plasmas in a magnetic field. Starting with linear tubes and mercury gas, not unlike a conventional fluorescent lamp, 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 on the outside of the tube, which pushed back against the current and kept it centered. 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 Cockroft visited and were duly impressed.
While all of this was ongoing, Cockcroft asked Herbert Skinner to review the concepts, which he did in April 1948. He was skeptical of Thompson's ideas for creating a current in the plasma, while Thonemann's ideas seemed more likely to work. He also pointed out that the entire field 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."
Meanwhile, at Los Alamos, Tuck acquainted the US researchers with the British efforts. In early 1951, Lyman Spitzer introduced his stellarator concept and was shopping the idea around the energy establishment looking for funding. Tuck was skeptical of Spitzer's enthusiasm and felt his development program was "incredibly ambitious". He proposed a much less aggressive program based on pinch. Both men presented their ideas in Washington in May 1951, which resulted in the Atomic Energy Commission giving Spitzer $50,000. Not to be outdone, Tuck convinced Norris Bradbury, the Los Alamos director, to give him $50,000 from the discretionary budget, using it to build the Perhapsatron.
In 1950 Fuchs admitted to turning UK and US atomic secrets over to the USSR. As fusion devices generated high energy neutrons, which could be used to enrich nuclear fuel for bombs, the UK immediately classified all their fusion research. This meant the teams could no longer work in the open environment of the universities. The Imperial team under Ware moved to the AEI labs at Aldermaston while the Oxford team under Thonemann moved to Harwell.
By early 1952 there were numerous pinch devices in operation; Cousins and Ware had built several follow-on machines under the name Scepter, and the Harwell team had built a series of ever-larger machines simply known as Mark I through Mark IV. In the US, Tuck built his Perhapsatron in January 1952. It was later learned that Fuchs had passed the UK work on to the Soviets, and they had started a fusion program as well.
By the spring of 1952, it was clear to all of these groups that something was seriously wrong in the pinch machines. As the current was applied, the plasma column inside the vacuum tube would become unstable and break up, ruining the compression. Further work identified two types of instabilities, nicknamed "kink" and "sausage". In the kink, the normally toroidal plasma would bend to the sides, eventually touching the edges of the vessel. In the sausage, the plasma would neck down at locations along the plasma column to form a pattern similar to a link of sausages.
Investigations demonstrated both were caused by the same underlying mechanism. When the pinch current was applied, any area of the gas that had a slightly higher density would create a slightly stronger magnetic field and collapse faster than the surrounding gas. This caused the localized area to have higher density, which created an even stronger pinch, and a runaway reaction would follow. The quick collapse in a single area would cause the column as a whole to break up.[b]
Early studies of the phenomenon suggested one solution to the problem was to increase the compression rate. In this approach, the compression would be started and stopped so rapidly that the bulk of the plasma would not have time to move; instead, a shock wave created by this rapid compression would be responsible for compressing the majority of the plasma. This approach became known as fast pinch. The Los Alamos team working on the Columbus linear machine designed an updated version to test this theory.
Others started looking for ways to stabilize the plasma during compression, and by 1953 two concepts had come to the fore. One solution was to wrap the vacuum tube in a sheet of thin but highly conductive metal. If the plasma column began to move, the current in the plasma would induce a magnetic field in the sheet, one that, due to Lenz's law, would push back against the plasma. This was most effective against large, slow movements, like the entire plasma torus drifting within the chamber.
The second solution used additional electromagnets wrapped around the vacuum tube. The magnetic fields from these magnets mixed with the pinch field created by the current in the plasma. The result was that the paths of the particles within the plasma tube were no longer purely circular around the torus, but twisted like the stripes on a barber pole. In the US, this concept was known as giving the plasma a "backbone", suppressing small-scale, localized instabilities. Calculations showed that this stabilized pinch would dramatically improve confinement times, and the older systems "suddenly looked old fashion".
Marshall Rosenbluth, recently arrived at Los Alamos, begin a detailed theoretical study of the pinch concept. With his wife Arianna and Richard Garwin, he developed "motor theory", or "M-theory", published in 1954. One prediction of the theory was that the heating effect of the electric current was greatly increased with the power of the electric field. This suggested that the fast pinch concept would be more attractive, as it was easier to produce larger currents in these devices. When he incorporated the idea of stabilizing magnets into the theory a second phenomenon appeared; for a particular, and narrow, set of conditions based on the physical size of the reactor, the power of the stabilizing magnets and the amount of pinch, toroidal machines appeared to be naturally stable.
ZETA begins construction
US researchers planned to test both fast pinch and stabilized pinch by modifying their existing small-scale machines. In the UK, Thomson once again pressed for funding for a larger machine. This time he received a much warmer reception. Initial funding, £200,000, was provided in late 1954. Design continued through 1955, and in July 1955 the project gained the name ZETA. The name is illustrative; "zero energy" referred to the aim of producing copious numbers of fusion reactions, but releasing no net energy.
The design was finalized in early 1956. Metropolitan-Vickers was hired to build the machine, which included a 150 tonne pulse transformer, the largest built in England to that point. A serious issue arose when the required high-strength steels needed for the electrical components were in short supply, but a strike in the US electrical industry resulted in a sudden glut of material and this problem was avoided.
ZETA was the largest and most powerful fusion device in the world at the time of its construction.[c] Its aluminum torus had an internal bore of 1 metre (3 ft 3 in) diameter and a major radius of 1.6 metres (5 ft 3 in), over three times the size of any machine built to date. It was also the most powerful design, incorporating an enormous induction magnet that was originally designed to induce currents up to 100,000 amperes (amps) into the plasma, but later amended to 900,000 amps. It included both types of stabilization; its aluminum walls acted as the metal shield, and a series of secondary magnets ringed the torus. Windows placed in the gaps between the toroidal magnets allowed direct inspection of the plasma.
In July 1954, the AERE was reorganized into the United Kingdom Atomic Energy Authority, or AEA. Modifications to Harwell's Hangar 7 in order to house the machine began that year. Despite its advanced design, the price tag was modest, about US$1 million.[d] By late 1956 it was clear that ZETA was going to come online during the summer of 1957, beating the Model C and the newest versions of the Perhapsatron and Columbus. Because these projects were masked in secrecy, based on what little information was available the press concluded they were versions of the same conceptual device, and that the British were far ahead in the race to produce a working machine.
Soviet visit, push to declassify
From 1953 the US had increasingly concentrated on the fast pinch concept. Some of these machines had produced neutrons, and these were initially associated with fusion. There was so much excitement that several other researchers quickly entered the field as well. Among these was Stirling Colgate, but his experiments quickly led him to conclude that fusion was not taking place. According to M-theory, the only theoretical study of high-power plasmas at the time, the temperature of the plasma could be determined from the current flowing through it. When Colgate ran the calculation, the temperatures in the plasma were far below the requirements for fusion.
Something else was creating the neutrons, and further work demonstrated that these were the result of instabilities in the fuel. Modifications failed to improve the situation and by 1956 the fast pinch concept had largely been abandoned. The US labs began turning their attention to the stabilized pinch concept, but by this time ZETA was almost complete and the US was well behind.
In 1956, while planning a well publicized state visit by Nikita Khrushchev and Nikolai Bulganin to the UK, the Harwell researchers received an offer from Soviet scientist Igor Kurchatov to give a talk. They were surprised when he began his talk on "the possibility of producing thermonuclear reactions in a gaseous discharge". Kurchatov's speech revealed the Soviet efforts to produce fast pinch devices similar to the American designs, and their problems with instabilities in the plasmas. Critically, Kurchatov noted that they had also seen neutrons being released, and had initially believed these to be from fusion. But as they examined the numbers, it became clear the plasma was not hot enough and concluded the neutrons were from other interactions.
Kurchatov's speech revealed that the three countries were all working on the same basic concepts and had all run up against the same sorts of problems. Cockcroft missed Kurchatov's visit because he had left for the US to press for declassification of the fusion work, specifically to avoid this sort of duplication of effort. There was a widespread belief on both sides of the Atlantic that sharing their findings would greatly improve progress. Now that it was known the Soviets were at the same basic development level, and that they were clearly interested in talking about it publicly, a wider effort started to release all fusion research at the 2nd Atoms for Peace conference in Geneva in September 1958.
In June 1957 the UK and US finalized their agreement to release data to each other sometime prior to the conference, which both the UK and the US planned on attending "in force". The final terms were reached on 27 November 1957, opening the projects to mutual inspection, and calling for a wide public release of all the data in January 1958.
ZETA started operation in mid-August 1957, initially with test gasses of hydrogen. These runs demonstrated that ZETA was not suffering from the same stability problems that earlier pinch machines had seen and their plasmas were lasting for milliseconds, up from microseconds. The length of the pulses allowed the plasma temperature to be measured using spectrographic means; although the light given off was broadband, the Doppler shifting of the spectral lines of slight impurities in the gas (oxygen in particular) led to calculable temperatures.
Even in early experimental runs, the team started introducing deuterium gas to the mix and began increasing the current to 200,000 amps. On the evening of 30 August the machine generated huge numbers of neutrons, on the order of one million per "shot". Considering Kurchatov's comments, a hurried effort to duplicate the results and eliminate possible measurement failure followed. Much of these depended on the temperature of the plasma; if the temperature was low the neutrons would not be fusion related. Spectrographic measurements suggested plasma temperatures between 1 and 5 million degrees, much lower than the 100 million degrees needed for high rates of fusion, but within a factor of two of theoretical predictions of the fusion rate at that temperature. It appeared that ZETA had finally reached the long-sought goal of producing small numbers of fusion reactions, exactly what it was designed to do.
US efforts, meanwhile, had suffered a string of minor technical setbacks that delayed their experiments by about a year; both the new Perhapsatron S-3 and Columbus II started operating around the same time as ZETA in spite of being much smaller experiments. Nevertheless, as these experiments finally came online in the summer of 1957, they too began generating neutrons. By September, both these machines and a new design, DCX at Oak Ridge National Laboratory, appeared so promising that Edward Gardner reported that:
…there is a distinct possibility that either the machine at Oak Ridge or the one at Los Alamos will have confirmed by January 1958 the production of thermonuclear neutrons.
Although the British and US had agreed to release their data in full, at this point the overall director of the US program, Lewis Strauss, decided to hold back the release. He claimed that releasing the data while the new reactors were apparently making great strides would be premature, and he decided to delay the US data until these machines either confirmed or denied the ZETA results. This argument had been initially presented by Tuck, who stated that stabilized pinch looked so promising that releasing data before the researchers knew for sure one way or the other would be premature.
The news was too good to keep bottled up, and tantalizing leaks started as early as September. In October, Thonemann, Cockcroft and William P. Thompson hinted that interesting results would be following, and in November an AEA spokesman noted "The indications are that fusion has been achieved". Based on these hints, the Financial Times dedicated an entire two-column article to the issue. Between then and early 1958, the British press published an average of two articles a week on ZETA. Even the US papers picked up the story; on 17 November The New York Times reported on the hints of success.
As the matter became better known in the press, on 26 November the publication issue was raised in House of Commons. Responding to a question by the opposition, the leader of the house announced the results publicly while explaining the delay in publication due to the UK–US agreement. The UK press interpreted this differently, claiming that the US was dragging its feet because it was unable to replicate the British results.
Some people have suggested darkly to me that the real reason for this American reluctance to have this momentous news released is politics. They point to the loss of prestige which the Administration would suffer if they had to admit that Britain, as well as Russia, was ahead of America in scientific development. I prefer to believe this attitude stems from a slavish and misguided application of security. But, whatever may be the reason, it shows a deplorable misconception in Washington of the true meaning of Western partnership and the real nature of the Soviet threat.
The article resulted in a flurry of activity in the Macmillan administration. Having originally planned on releasing their results at a scheduled meeting of the Royal Society, there was great concern over whether to invite the Americans and Soviets, especially as they believed the Americans would be greatly upset if the Soviets arrived, but likely just as upset if they weren't invited and the event was all-British. The affair eventually led to the AEA making a public announcement that the US was not holding back the ZETA results, but this infuriated the local press, which continued to claim the US was delaying to give them to catch up.[e]
When the information-sharing agreement was signed in November a further benefit was realized; teams from the various labs were allowed to visit each other. The US team, including Stirling Colgate, Lyman Spitzer, Jim Tuck and Arthur Edward Ruark, all visited ZETA and concluded there was a "major probability" the neutrons were from fusion.
On his return to the US, Spitzer was working the numbers and concluded something was wrong with the ZETA results. He noticed that the apparent temperature, 5 million degrees, would not have time to develop during the short firing times. ZETA simply didn't discharge enough energy into the plasma to heat it to those temperatures that quickly. And if the temperature was increasing at the relatively slow rate his calculations suggested, fusion would not be taking place early in the reaction, and could not be adding energy that might make up the difference. Spitzer suspected the temperature reading was not accurate. Since it was the temperature reading that suggested the neutrons were from fusion, if the temperature were really lower, it implied the neutrons were non-fusion in origin.
Colgate had reached similar conclusions. Joined by Harold Furth and John Ferguson, in early 1958 the three started an extensive study of the results from all known pinch machines. Instead of inferring temperature from neutron energy, they used the conductivity of the plasma itself, based on the well-understood relationships between temperature and conductivity. They concluded that the machines were producing temperatures perhaps 1⁄10 what the neutrons were suggesting, nowhere near hot enough to explain the number of neutrons being produced, regardless of their energy.
By this time the latest versions of the US pinch devices, Perhapsatron S-3 and Columbus S-4, were producing neutrons of their own. The fusion research world reached a high point. In January, results from pinch experiments in the US and UK would both announce that neutrons were being released, and that fusion had apparently been achieved. The misgivings of Spitzer and Colgate were ignored.
Public release, worldwide interest
The long-planned release of fusion data was pre-announced to the public in mid-January. Considerable material from the UK's ZETA and Sceptre devices would be released in-depth in the 25 January 1958 edition of Nature, which would also include results from Los Alamos' Perhapsatron S-3, Columbus II and Columbus S-2. The UK press was livid. The Observer noted that "Admiral Strauss' tactics have soured what should be an exciting announcement of scientific progress so that it has become a sordid episode of prestige politics."
The results were typical of the normally sober scientific language, and although the neutrons were noted, there were no strong claims as to their source. However, the day before the release, Cockcroft, the overall director at Harwell, called a press conference to introduce the British press to the results. Some indication of the importance of the event can be seen in the presence of a BBC television field crew, a rare occurrence at that time. He began by introducing the fusion program and the ZETA machine, and then noted:
In all experiments on toroidal discharges neutrons have been observed in about the numbers to be expected if thermonuclear reactions were proceeding. It is well known, however, from previous experiments carried out in Russian and other laboratories that instabilities in the current channel can give rise to strong electric fields which accelerated deuterons and can produce neutrons. So in no case have the neutrons been definitely proved to be due to the random motion of the deuterium associated with a temperature on the order of five million degrees ... Their origin, will, however, become clear as soon as the number of neutrons produced can be increased by increasing current and temperatures.
The reporters at the meeting were not satisfied with this assessment and continued to press Cockcroft on the neutron issue. After being asked several times, he eventually stated that in his opinion, he was "90 percent certain" they were from fusion. This was unwise; a statement of opinion from the man that won the Nobel prize for splitting the atom was taken as a statement of fact. The next day, the Sunday newspapers were covered with the news that fusion had been achieved in ZETA, often with claims about how the UK was now far in the lead in fusion research. On television following the release, Cockcroft stated "To Britain this discovery is greater than the Russian Sputnik".
As planned, the US also released a large batch of results from their smaller pinch machines. Many of them were also giving off neutrons, although ZETA was stabilized for much longer periods and generating more neutrons, by a factor of about 1000. When questioned about the success in the UK, Strauss denied that the US was behind in the fusion race. When reporting on the releases, The New York Times chose to focus on Los Alamos' Columbus II, only mentioning ZETA later in the article, and then concluded the two countries were "neck and neck". Other reports from the US generally gave equal support to both programs. Newspapers from the rest of the world were more favourable to the UK; Radio Moscow went so far to publicly congratulate the UK while failing to mention the US results at all.
As ZETA continued to generate positive results, plans were made to build a follow-on machine. The new design was announced in May; ZETA II would be a significantly larger US$14 million machine whose explicit goal would be to reach 100 million degrees, and generate net power. This announcement gathered praise even in the US; The New York Times ran a story about the new version. Meanwhile, machines similar to ZETA were being announced around the world; Osaka University announced their pinch machine was even more successful than ZETA, the Aldermaston team announced positive results from their Sceptre machine of only US$28,000, and a new reactor was built in Uppsala University that was presented publicly later that year. The Efremov Institute in Leningrad began construction of a smaller version of ZETA, although still larger than most, known as Alpha.
Spitzer had already concluded that known theory suggested that the ZETA was nowhere near the temperatures the ZETA team was claiming, and as part of the publicity surrounding the release of the work suggested that "Some unknown mechanism would appear to be involved". Other researchers in the US, notably Furth and Colgate, were far more critical, telling anyone who would listen that the results were bunk.
In the Soviet Union, Lev Artsimovich rushed to have the Nature article translated, and after reading it, declared "Chush sobachi!" (dog shit). Experiments with pinch in the USSR had already shown similar neutron releases, but the asymmetry in the directions they came out of the apparatus convinced him they were not created by fusion reactions. Neutrons from fusion reactions should be symmetric, but the neutrons being seen had a clear preference along the direction of the pinch current. Kurchatov had talked about this with the UK researchers in 1956.
Retraction of claims
Critically, Cockcroft had stated that they were receiving too few neutrons from the device to measure their spectrum or their direction.
In the same converted hangar that housed ZETA was the Harwell Synchrocyclotron effort run by Basil Rose. This project also constructed a sensitive high-pressure diffusion cloud chamber as the cyclotron's main detector. Rose was convinced it would be able to directly measure the neutron energies and trajectories. In a series of experiments, he showed that the neutrons had a high directionality, at odds with a fusion origin which would be expected to be randomly directed. To further demonstrate this he had the machine run "backward", with the electric current running in the opposite direction. This demonstrated a clear difference in the number of neutrons and their energy, which suggested they were a result of the electrical current itself, not fusion reactions inside the plasma.
This was followed by similar experiments on Perhapsatron and Columbus, demonstrating the same problems. When instabilities developed, areas of enormous electrical potential developed, rapidly accelerating protons in the area. These sometimes collided with neutrons in the plasma or the container walls, ejecting them through neutron spallation. These were the same sorts of instabilities seen in earlier machines, and precisely the problem Cockcroft had mentioned during the press releases. But in ZETA they were much more powerful and the neutrons appeared to be fusion related until further work demonstrated their nature. The promise of stabilized pinch disappeared.
Cockcroft was forced to publish a humiliating retraction on 16 May 1958, claiming "It is doing exactly the job we expected it would do and is functioning exactly the way we hoped it would." Le Monde raised the issue to a front-page headline in June, noting "Contrary to what was announced six months ago at Harwell - British experts confirm that thermonuclear energy has not been 'domesticated'". The event cast a chill over the entire field; it was not only the British who looked foolish, every other country involved in fusion research had been quick to jump on the bandwagon.
Harwell in turmoil, ZETA soldiers on
Beginning in 1955, Cockcroft had pressed for the establishment of a new site for the construction of multiple prototype power-producing fission reactors. This was strongly opposed by Christopher Hinton, and a furious debate broke out within the AEA over the issue. Cockcroft eventually won the debate, and in late 1958 the AEA formed AEE Winfrith in Dorset, where they eventually built several experimental reactor designs.
Cockcroft had also pressed for the ZETA II reactor to be housed at the new site. He argued that Winfrith would be better suited to build the large reactor, and the unclassified site would better suit the now-unclassified research. This led to what has been described as "as close to a rebellion that the individualistic scientists at Harwell could possibly mount". Thonemann made it clear he was not interested in moving to Dorset and suggested that several other high-ranking members would also quit rather than move. He then went on sabbatical to Princeton University for a year. The entire affair was a major strain on Basil Schonland, who took over the Research division when Cockcroft left in October 1959 to become the Master of the newly formed Churchill College, Cambridge.
While this was taking place, the original ZETA II proposal had been growing ever-larger, eventually specifying currents as powerful as the Joint European Torus that was built years later. As it seemed this was beyond the state-of-the-art, the project was eventually cancelled in February 1959. A new proposal soon took its place, the Intermediate-Current Stability Experiment (ICSE). ICSE was designed to take advantage of further stabilizing effects noticed in M-theory, which suggested that very fast pinches would cause the current to flow only in the outer layer of the plasma, which should be much more stable. Over time, this machine grew to be about the same size as ZETA II; ICSE had a 6 m major diameter and 1 m minor diameter, powered by bank of capacitors storing 10 MJ at 100 kV.
Harwell was as unsuited to ICSE as it was for ZETA II, so Schonland approached the government with the idea of a new site for fusion research located close to Harwell. He was surprised to find they were happy with the idea, as this would limit employment at Harwell. When further study demonstrated that the cost of building an entirely new site was offset by the savings in keeping the site close, in May 1959, the AEA purchased RNAS Culham, about 10 miles (16 km) from Harwell. ICSE construction began later that year, starting with a one-acre building to house it known as "D-1". Through this period there was an increasing understanding that the instabilities being seen in the plasmas were apparently due to the mistaken assumption that they were ideal conducting fluids. When the new head of the AEA, William Penny, heard that the ICSE design also assumed these conditions, he canceled the project in August 1960. Parts for the partially-assembled reactor were scavenged by other teams.
Thonemann had returned by this point and found much to disagree with on ICSE. He demanded to be allowed to set up a new fusion group to remain at Harwell on ZETA. ZETA remained the largest toroidal machine in the world for some time. ZETA soldiered on at Harwell and would go on to have a very productive career for just over a decade. But in spite of later successes, ZETA was always known as an example of British folly.
Thomson scattering and tokamaks
ZETA's failure was due to limited information; using the best available measurements, ZETA was returning several signals that suggested the neutrons were due to fusion. Over the next decade, ZETA was used almost continually in an effort to develop better diagnostic tools to resolve these problems.
This work eventually developed a method that is used to this day. The original temperature measures were made by examining the Doppler shifting of the spectral lines of the atoms in the plasma. However, the inaccuracy of the measurement and spurious results caused by electron impacts with the container led to misleading results. The introduction of lasers provided a new solution. Lasers have extremely accurate and stable frequency control, and the light they emit interacts strongly with free electrons. A laser shone into the plasma will be reflected off the electrons, and will be Doppler shifted by the electrons' movement, a British discovery known as Thomson scattering. The speed of the electrons is a function of their temperature, so by comparing the frequency before and after collisions, the temperature of the electrons could be measured with an extremely high degree of accuracy.
Through the 1960s ZETA was not the only experiment to suffer from unexpected performance problems. Problems with plasma diffusion across the magnetic fields plagued both the magnetic mirror and stellarator programs, at rates that classical theory could not address. Adding more fields did not appear to correct the problems in any of the existing designs. Work slowed dramatically as teams around the world tried to better understand the physics of the plasmas in their devices. Pfirsch and Schluter were the first to make a significant advance, suggesting that much larger and more powerful machines would be needed to correct these problems.
But then, in a surprising announcement, in 1968 the USSR released data on its tokamak designs, with performance numbers that no other experiment was close to matching. The latest of their designs, the T3, was producing electron temperatures of 1000 eV, compared to about 10 eV in ZETA.) To avoid the possibility of another ZETA in the making, Artsimovitch invited the AEA team to bring their laser system to the Kurchatov Institute and independently measure the performance. They became known as "the Culham five". The resulting paper in 1969 re-invigorated the fusion world, and led to the tokamak becoming the most studied device in the fusion field.
Tokamaks are pinch machines. The key difference is the relative strengths of the fields. In the stabilized pinch machines, most of the magnetic field in the plasma was generated by the current induced in it. The strength of the external stabilization fields was much lower. The tokamak reversed this; the external magnets were much more powerful and the plasma current greatly reduced in comparison. Artsimovitch put it this way:
The longitudinal field intensity must be many times greater than the intensity of the azimuthal field produced by the current. This constitutes the principle difference between tokamak devices and systems with relatively weak longitudinal fields, such as the well-known English Zeta device.
The result of the Culham team's work not only convinced the Soviets that their device was indeed producing fusion but the rest of the world as well. The result was a "veritable stampede" of tokamak construction around the world.
Reversed field pinch
In 1965, the newly opened Culham hosted the now periodic meeting of international fusion researchers. Of all the work presented, only two papers on stabilized pinch were present, both on ZETA. Spitzer didn't even bother mentioning them during the opening comments.
In spite of this tepid response, one of the papers described something very interesting in ZETA data. Normally, the pulse of electricity sent into ZETA formed a current pulse with a shape similar to a Poisson distribution, ramping up quickly then trailing off. They noticed that the plasma stability reached a maximum just after the current began to taper off, and lasted longer than the current pulse itself. This curious phenomenon was dubbed "quiescence".
Three years later, in 1968, the international meeting was being held in Novosibirsk, where Soviet results with the T3 tokamak were first released. This overshadowed a paper by Robinson and King that had examined the quiescence period and determined it was due to the original toroidal magnetic field reversing itself, creating a more stable configuration.
John Bryan Taylor took up the issue and began a detailed theoretical study of the concept, publishing a groundbreaking 1974 article on the topic. He demonstrated that as the magnetic field that generated the pinch was relaxing, it interacted with the pre-existing stabilizing fields, creating a self-stable magnetic field. The phenomenon was driven by the system's desire to preserve magnetic helicity, which suggested a number of ways to improve the confinement time.
Although the stabilizing force was lower than the force available in the pinch, it lasted considerably longer. It appeared that a reactor could be built that would approach the Lawson criterion from a different direction; through extended confinement times rather than increased density. This was similar to the stellarator approach in concept, and although it would have lower field strength than those machines, the energy needed to maintain the confinement was much lower. Today this approach is known as the reversed field pinch (RFP) and has been a field of continued study.[f]
Taylor's study of the relaxation into the reversed state led to his development of a broader theoretical understanding of the role of magnetic helicity and minimum energy states, greatly advancing the understanding of plasma dynamics. The minimum-energy state, known as the "Taylor state", is particularly important in the understanding of new fusion approaches in the compact toroid class. Taylor went on to study the ballooning transformation, solving a mystery found in high-performance toroidal machines. His work won him the 1999 James Clerk Maxwell Prize in Plasma Physics.
Culham officially opened in 1965, and various teams began leaving the former sites through this period. A team kept ZETA operational until September 1968. Hangar 7, which housed ZETA and other machines, was demolished during financial year 2005/2006.
- Andrei Sakharov also came to the same conclusion as Fermi as early as 1950, but his paper on the topic was not known in the west until 1958.
- These effects would later be used to understand similar processes seen on the surface of the sun.
- One can see the comparatively large size of ZETA in a review of all the machines presented in Geneva in 1958; ZETA had a major radius of 160 cm, the next closest machine was 100, and the next only 62. The rest were much smaller.
- In comparison to ZETA's ~$1 million price, the contemporary Model C stellarator was $23 million.
- Hill covers the furor over the release in considerable depth.
- A comparison of modern toroidal confinement techniques in Bellan illustrates the close relationship between the RFP and stabilized pinch layout.
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