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 Z-pinch confinement technique, and the first large-scale fusion machine to be built. ZETA sparked an intense national rivalry with the United States pinch and stellarator programs, and as ZETA was much larger and more powerful than US machines, it was expected that it would put the UK in the lead in the fusion race.
ZETA went into operation in 1957, and on each experimental run a burst of neutrons was measured. Neutrons are the most obvious results of nuclear fusion reactions, which was a positive development. Temperature measurements suggested the reactor was operating between 1 and 5 million degrees, a temperature that would produce low rates of fusion just about perfectly explaining the quantities of neutrons being seen. Early results were released in September 1957, and the following January an extensive review was released with great fanfare. Front-page articles in major newspapers announced the breakthrough as a major step on the road to unlimited power.
US researchers questioned ZETA's results, which was initially dismissed by UK observers as jingoism, but over time similar US experiments demonstrated the same neutron bursts at temperatures that were clearly not high enough for fusion. Further experiments demonstrated that the temperature measurements were accounting only for the hottest portions of the fuel, and the bulk of the system was much cooler. The neutrons were later explained as the byproduct of instabilities in the fuel. The ZETA claims had to be publicly withdrawn, casting a chill over the entire fusion establishment. Most work on the z-pinch concept as a road to fusion had ended by 1961.
In spite of ZETA's failure to achieve fusion, and the PR disaster that it created, 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 well developed at ZETA, and later used to confirm the results of the Soviet tokamak approach. In 1974, while poring over 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.
- 1 History
- 2 Notes
- 3 References
The basic understanding of nuclear fusion was developed using the new field of quantum mechanics through the 1930s. During the 1940s, physicists working on the atomic bomb at Los Alamos National Laboratory had worked through the equations and found that a 50–50 mix of tritium and deuterium gases would begin to fuse at a rapid rate if heated to a temperature of about 100 million degrees Celsius. The problem would be containing the gas at that temperature; any known substance would melt and mix with the fuel, ruining the reaction.
Gases heated to that temperature will dissociate into their electrons and nuclei, producing a charged gas known as plasma. In a magnetic field, the charged electrons and nuclei would orbit around the direction of the magnetic field, being confined to a small volume, which meant that a magnetic system would be able to confine the plasma. The simplest device to understand is a tube placed inside the open core of a solenoid. A solenoid creates a linear magnetic field which can be arranged running down the center of the tube. An electric charge passed through the gas will turn it into a low temperature plasma, and the plasma will follow the magnetic lines, confining itself to the center of the tube.
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 of the solenoid. The obvious solution to this problem is to bend the tube around into a torus (donut) shape, eliminating the ends. However, as Enrico Fermi pointed out, when the solenoid is bent around the tube, the windings would be closer together on the inside than the outside. This would lead to an uneven field across the tube, and the electrons would drift one way while the nuclei would drift the other.
The pinch concept
One potential solution to the confinement problem had already been discovered. As the plasma is electrically conducting, it is possible to pass an electric current through it. In an enclosed tube this can be arranged by placing a magnet next to the toroidal tube; when the magnet is energized, an electric current will be induced into the plasma. Through the Lorentz force the current in the plasma will create magnetic forces that attracts the plasma around it, forcing the plasma inward, "pinching" in on itself.
The pinch concept as a route to fusion had first been 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 "Harwell") in 1945, Thomson repeatedly petitioned the director, John Cockcroft, for funds to develop a large experimental pinch machine. These requests were turned down every time. At the time there was no obvious military use, so the concept was left unclassified. Thomson and Moses Blackman wrote a patent on the idea in 1946, exploring a device using microwave heating and a steady current flow.
In 1947, Cockcroft arranged a meeting of several Harwell physicists to study Thomson's work, including Harwell's director of theoretical physics, Klaus Fuchs. Thomson's concepts received a chilly reception, especially from Fuchs. At the same meeting, information returned from wartime Germany on a similar device was also presented. Max Steenbeck, better known for his work on the betatron, had been working on a toroidal pinch device he called the "Wirbelrohr" ("whirl tube") in an effort to produce a new type of particle accelerator.
When this presentation also failed to gain funding at Harwell, Thomson passed along his concepts and the Wirbelrohr report to two graduate students at Imperial, Stan Cousins and Alan Ware. Later that year, Ware managed to build a small machine out of old radar equipment, and was able to induce powerful currents into the linear tube. When they did, the plasma gave off flashes of light. However, he could not devise a way to measure the temperature of the plasma.
Ware discussed the experiments with anyone that proved interested, including Jim Tuck who was helping restart the Clarendon Laboratory at Oxford University. Tuck had started some early work at Los Alamos on an unsuccessful colliding beam fusion system. Tuck also knew of an Australian who had worked on fusion, Peter Thonemann, and the two arranged some funding through Clarendon to build a small device like the one at Imperial. However, before this work started, Tuck was offered a job in the US, eventually returning to Los Alamos.
At Los Alamos, Tuck acquainted the US researchers with the British efforts. By this point Lyman Spitzer had 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", and 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.
Early pinch results
In 1950 Fuchs admitted to turning UK and US atomic secrets over to the USSR. As fusion devices generated copious amounts of neutrons, which could be used to enrich nuclear fuel for bombs, the UK immediately classified all their fusion research. The Imperial team under Ware was set up at the Associated Electrical Industries (AEI) labs at Aldermaston, while the Oxford team under Thonemann were moved to Harwell. By 1951 there were numerous pinch devices in operation; Cousins and Ware had built several follow-on machines, Tuck built his Perhapsatron, and another team at Los Alamos built a linear machine known as Columbus. It was later learned that Fuchs had passed on the UK work to the Soviets, and they had started a pinch program as well.
By 1952 it was clear to all of these researchers 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 collapse, ruining the compression. Further work identified two sources of the instabilities, the kink and sausage. Both were caused by the same underlying mechanism. When the pinch field 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. These effects would later be used to understand similar processes on the surface of the sun.
Some researchers believed that the solution to this problem was to increase the compression rate; the idea was that if the system operated quickly enough, the instabilities in the plasma would not have time to develop. This approach became known as "fast pinch", with the existing systems retroactively becoming "slow". The Los Alamos team was already working on a fast pinch device, Columbus, and designed an improved version to test this theory.
Others started looking for ways to stabilize the plasma during compression. By 1953 two concepts had started to become widely known; one solution was to wrap the vacuum tube in a sheet of thin metal, which formed a magnetic field that would keep the plasma centred in the tube, the other used a second set of magnets to produce a similar stabilizing field.
The new set of magnets ringed the tube to produce a field running linearly down the center of the tube, parallel to the pinch current. The pinch current generated a magnetic field running around the plasma, parallel to the new magnets. The two fields were at right angles to each other, and when they were both energized, they mixed to produce a single field running in a helix around the inside of the tube, like the stripes on a barber pole. The result was the "stabilized pinch".
When plasma was moving in such a field, the particles would alternately find themselves on the inside of the confinement area, then the outside. As a result, the plasma was mixed as it moved about the system, preventing the bunching up that characterized the instabilities seen in earlier devices. This was precisely the idea behind the stellarator, but that device used a complex mechanical layout instead of the stabilized pinch's relatively simple set of magnets. Calculations showed that the stability of the system would be dramatically improved, and the older systems "suddenly looked old fashion".
The US researchers planned to test both fast pinch and stabilized pinch by modifying their existing small-scale machines. Given the apparently enormous leap that stabilized pinch represented, Thomson once again pressed Harwell for funding for a larger machine. This time he received a much warmer reception, gaining funding for his aggressive design, "ZETA". The name is illustrative; "zero energy" refers to the aim of producing copious numbers of fusion reactions, but releasing no net energy.
ZETA was the largest and most power fusion device in the world at the time of its construction. Its aluminum torus had an internal bore of 1 meter diameter and a major radius of 3 meters, over three times the size of previous devices. It was also the most powerful design, incorporating an enormous pinch magnet that could induce currents up to 200,000 Amps into the plasma. It included both types of stabilization; its aluminum walls acted as the metal shield, and a series of secondary magnets ringed the torus. Small gaps between the toroidal magnets allowed direct inspection of the plasma.
Construction of ZETA started in 1954, starting with changes to Harwell's Hangar 7 that would house the device. Despite its advanced design, the price tag was modest, about US$1 million. By 1956 it was clear that ZETA was going to come online during the summer of 1957, beating the US's Model C stellarator and the newest versions of the Perhapsatron and Columbus. Because these projects were masked in secrecy, and they looked similar from the outside (large toroids wrapped in magnet coils), 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. The rivalry between the US and UK teams intensified throughout the year.
At this point the work was still classified, but a declassification effort was underway. This had started with a surprising speech by Soviet scientist Igor Kurchatov at Harwell in 1956, which outlined their efforts to produce pinch devices and their problems with instabilities. The US and UK had already been considering sharing their work between each other, and now that it appeared the Soviets were at the same basic level, a wider effort started to release all research at the 2nd Atoms for Peace conference in Geneva in September 1958. In June 1957 the UK and US had reached an agreement to release their data to each other, prior to the conference, which both the UK and US planned on attending "in force". The final terms were reached on 27 November, opening the projects to mutual inspection, and calling for a wide public release of all the data in January 1958.
Promising early results
ZETA started full operation in mid-August 1957, initially with test gases of hydrogen. These runs demonstrated that ZETA was not suffering from the same 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 runs the team started introducing deuterium gas. On the evening of 30 August the machine generated neutrons. A hurried effort to duplicate the results and eliminate possible measurement failure followed. 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 high enough to explain the small numbers of neutrons they were seeing. The numbers were within a factor of two of theoretical predictions of the 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.
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 due to worries that the British team would appear to be well ahead of its US counterparts. He claimed that releasing the data while the new reactors were apparently making great strides would be premature. The US would be bringing several new pinch devices online over the next year, and he decided to delay the US data until these machines either confirmed or denied the ZETA results. This position had been brought forward by Tuck himself, who stated that stabilized pinch looked so promising that releasing data before we knew one way or the other was premature. The British press interpreted this differently, claiming that the US was dragging its feet because it was unable to replicate the British results, while its own stellarator program was far more expensive and achieving worse results.
Nevertheless the news was too good to keep bottled up, and tantalizing leaks started as early as September. In October, Thonemann, Cockroft and William P. Thompson hinted that interesting results would be following, and in November a UKAEA 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. On 26 November the issue was made public in the House of Commons; the leader of the house responded to a question about Harwell, and announced the results publicly while explaining the delay in publication due to the UK–US agreement. In December the UKAEA denied that the US was holding back the ZETA results, but this infuriated the local press, which continued to claim the US was delaying to allow it to catch up.
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 teams, 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, Lyman 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 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 well into their construction stage, based on the same stabilizing principles as ZETA. When these experiments started producing neutrons of their own only a few weeks later, 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.
Press release and 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. He began by introducing the 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 accurate assessment, and continued to press Cockroft on the neutron issue. He eventually stated that he was "90 percent certain" they were from fusion. He went on to caution that practical applications were 10 to 20 years in the future, and that the initial results on ZETA would be scaled up over the years into a practical power-producing machine through a four-stage process. The next day the Sunday newspapers were covered with the news, often with claims about how the UK was now far in the lead in fusion research. On television following the release, Cockcroft stated that "To Britain this discovery is greater than the Russian Sputnik". Days later they announced plans to modify ZETA to reach 25 million degrees.
As planned, the US also released a large batch of their results, using smaller pinch machines. Many of the US pinch machines were also giving off neutrons, although the UK machines were stabilized for much longer periods and generating many more neutrons, by a factor of about 1000. When questioned about the major publicity in the UK, Strauss denied that the US was behind in the fusion race. The New York Times chose to give precedence to Los Alamos' Columbus II, and then concluded the two countries were "neck and neck". Papers from the rest of the world ignored the US efforts, 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.
Other researchers were more skeptical of the ZETA results. Spitzer had already concluded that known theory suggested that the ZETA was nowhere near the temperatures they were claiming, and publicly suggested that "Some unknown mechanism would appear to be involved". Artsimovich rushed to have the Nature article translated, and after reading it, declared "Chush sobachi!" (dog shit). His 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. Nevertheless, other teams in the USSR started working on a stabilized pinch machine similar to ZETA.
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 "backwards", with the electric current running in the opposite direction that the external magnets would want in order to pinch to fusion conditions. Sure enough, the directionality of the neutrons also reversed, and Rose concluded they were not fusion related.
This was followed by similar experiments on Perhapsatron and Columbus, demonstrating the same problems. Further work by all of the teams demonstrated a new mechanism that rapidly ejected particles from the edges of the instabilities. When the instabilities developed, areas of enormous electrical potential developed, rapidly accelerating protons in the area. These sometimes collided with neutrons in the plasma, ejecting them from the fuel. 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 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, but tried to put a good face on the issue by claiming "It is doing exactly the job we expected it would do and is functioning exact the way we hoped it would." Le Monde raised the issue to a front-page headline in June. Plans to build ZETA II ended in 1960, along with a freeze on any further development for at least three years. Despite a decade of further useful research, ZETA was always known as an example of British folly. ZETA operated until 1968, when the majority of the fusion world moved on to the more fruitful tokamak designs.
Thomson scattering developments
ZETA's failure was one of 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 mirror and stellarator programs, at rates that classical theory could not address. No amount of additional fields appeared 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, the USSR released data (in 1968) on its tokamak designs with performance numbers (electron temperatures of 1000 eV) that no other experiment was close to matching. The numbers were so impressive that many in the US and UK thought it might be another ZETA in the making. To avoid such a problem, Lev Artsimovitch invited the UKAEA team (now based at Culham Laboratory) to bring their laser system to the Kurchatov Institute and independently measure the performance. The resulting paper in 1969 re-invigorated the fusion world, and led to the tokamak becoming the most studied device today.
Reversed field pinch
In 1974, John Bryan Taylor was re-examining the ZETA data with an eye to solving an oddity that had been noticed but not understood; after the device was "fired" and the experimental run had ostensibly come to an end, the plasma often entered an extended period of stability. Calling this period "quiescence", Taylor started a detailed theoretical study of the issue. He demonstrated that as the magnetic field that generated the pinch was relaxing, it interacted with the pre-existing stabilizing fields. This led to a curious situation where the magnetic fields on the inside of the plasma were in the opposite direction from the outside, slowing their decay considerably, and creating a self-stable magnetic field.
Although the stabilizing force was dramatically lower than the force available in the pinch, the situation 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.
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, considered the last major contribution to plasma physics in the fusion area. His work won him the 1999 James Clerk Maxwell Prize in Plasma Physics.
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