ZETA
Encyclopedia
ZETA, short for "Zero-Energy Toroidal (or 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 US's 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 neutron
s 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 that were causing all of the pinch experiments to fail. The ZETA claims had to be publicly withdrawn, casting a chill over the entire fusion establishment.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 laser
s 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.
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
gasses 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.
Gasses heated to that temperature will dissociate into their electron
s 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
. Solenoid create 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.
One potential solution to the confinement problem had already been discovered. As the plasma is electrically conducting, it is possible to pass an electrical 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 Lorenz 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 re-start 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
.
(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, and both appeared difficult to correct. 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.
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".
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 Lev Artsimovich
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.
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 neutron
s. 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.
, Lyman Spitzer, Jim Tuck and Arthur Edward Ruark
, all visited ZETA and concluded there was a "major probability" the neutrons were from fusion.
The glowing reviews of ZETA's results did not last long. 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 th 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.
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, but then got into the meat of the issue:
The reporters continued to press Cockroft on the neutron issue, and 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.
However, 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, and 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. 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 area, ejecting them from the plasma. These were the same sorts of instabilities seen in earlier machines, but in ZETA when they finally developed they were much more powerful. 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.
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 laser
s 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 on its tokamak designs with performance numbers 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.
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
.
Fusion power
Fusion power is the power generated by nuclear fusion processes. In fusion reactions two light atomic nuclei fuse together to form a heavier nucleus . In doing so they release a comparatively large amount of energy arising from the binding energy due to the strong nuclear force which is manifested...
research. It was the ultimate device in a series of UK designs using the Z-pinch
Z-pinch
In fusion power research, the Z-pinch, also known as zeta pinch or Bennett pinch , is a type of plasma confinement system that uses an electrical current in the plasma to generate a magnetic field that compresses it...
confinement technique, and the first large-scale fusion machine to be built. ZETA sparked an intense national rivalry with the US's pinch and stellarator
Stellarator
A stellarator is a device used to confine a hot plasma with magnetic fields in order to sustain a controlled nuclear fusion reaction. It is one of the earliest controlled fusion devices, first invented by Lyman Spitzer in 1950 and built the next year at what later became the Princeton Plasma...
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 neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s was measured. Neutrons are the most obvious results of nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
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
Jingoism
Jingoism is defined in the Oxford English Dictionary as extreme patriotism in the form of aggressive foreign policy. In practice, it is a country's advocation of the use of threats or actual force against other countries in order to safeguard what it perceives as its national interests...
, 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 that were causing all of the pinch experiments to fail. 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 laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s to more accurately measure the temperature was well developed at ZETA, and later used to confirm the results of the Soviet tokamak
Tokamak
A tokamak is a device using a magnetic field to confine a plasma in the shape of a torus . Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape...
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
Reversed field pinch
A reversed-field pinch is a device used to produce and contain near-thermonuclear plasmas. It is a toroidal pinch which uses a unique magnetic field configuration as a scheme to magnetically confine a plasma, primarily to study magnetic fusion energy. Its magnetic geometry is somewhat different...
concept, which sees continued development to this day.
Conceptual development
The basic understanding of nuclear fusionNuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
was developed using the new field of quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
through the 1930s. During the 1940s, physicists working on the atomic bomb at Los Alamos National Laboratory
Los Alamos National Laboratory
Los Alamos National Laboratory is a United States Department of Energy national laboratory, managed and operated by Los Alamos National Security , located in Los Alamos, New Mexico...
had worked through the equations and found that a 50–50 mix of tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
and deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
gasses 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.
Gasses heated to that temperature will dissociate into their electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s and nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
, producing a charged gas known as plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
. 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
Solenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...
. Solenoid create 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
Torus
In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle...
(donut) shape, eliminating the ends. However, as Enrico Fermi
Enrico Fermi
Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...
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
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....
into the plasma. Through the Lorenz 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
George Paget Thomson
Sir George Paget Thomson, FRS was an English physicist and Nobel laureate in physics recognised for his discovery with Clinton Davisson of the wave properties of the electron by electron diffraction.-Biography:...
of Imperial College London
Imperial College London
Imperial College London is a public research university located in London, United Kingdom, specialising in science, engineering, business and medicine...
. With the formation of the Atomic Energy Research Establishment
Atomic Energy Research Establishment
The Atomic Energy Research Establishment near Harwell, Oxfordshire, was the main centre for atomic energy research and development in the United Kingdom from the 1940s to the 1990s.-Founding:...
(AERE or "Harwell") in 1945, Thomson repeatedly petitioned the director, John Cockcroft
John Cockcroft
Sir John Douglas Cockcroft OM KCB CBE FRS was a British physicist. He shared the Nobel Prize in Physics for splitting the atomic nucleus with Ernest Walton, and was instrumental in the development of nuclear power....
, 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
Moses Blackman
Moses Blackman was a South African-born British crystallographer.His father was a minister of religion, Rev. Joseph Blackman.-Education:...
wrote a patent on the idea in 1946, exploring a device using microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
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
Klaus Fuchs
Klaus Emil Julius Fuchs was a German theoretical physicist and atomic spy who in 1950 was convicted of supplying information from the American, British and Canadian atomic bomb research to the USSR during and shortly after World War II...
. 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
Max Steenbeck
Max Christian Theodor Steenbeck was a German physicist who worked at the Siemens-Schuckertwerke in his early career, during which time he invented the betatron in 1934. He was taken to the Soviet Union after World War II , and he contributed to the Soviet atomic bomb project...
, better known for his work on the betatron
Betatron
A betatron is a cyclotron developed by Donald Kerst at the University of Illinois in 1940 to accelerate electrons, but the concepts ultimately originate from Rolf Widerøe and previous development occurred in Germany through Max Steenbeck in the 1930s. The betatron is essentially a transformer with...
, 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
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
.
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 re-start the Clarendon Laboratory
Clarendon Laboratory
The Clarendon Laboratory, located on Parks Road with the Science Area in Oxford, England , is part of the Physics Department at Oxford University...
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
Lyman Spitzer
Lyman Strong Spitzer, Jr. was an American theoretical physicist and astronomer best known for his research in star formation, plasma physics, and in 1946, for conceiving the idea of telescopes operating in outer space...
had introduced his stellarator
Stellarator
A stellarator is a device used to confine a hot plasma with magnetic fields in order to sustain a controlled nuclear fusion reaction. It is one of the earliest controlled fusion devices, first invented by Lyman Spitzer in 1950 and built the next year at what later became the Princeton Plasma...
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
United States Atomic Energy Commission
The United States Atomic Energy Commission was an agency of the United States government established after World War II by Congress to foster and control the peace time development of atomic science and technology. President Harry S...
giving Spitzer $50,000. Not to be outdone, Tuck convinced Norris Bradbury
Norris Bradbury
Norris Edwin Bradbury , was an American physicist who was born in Santa Barbara, California. He served as director of the Los Alamos National Laboratory for 25 years , succeeding J. Robert Oppenheimer, who personally chose Bradbury for the position of director after working closely with him on the...
, the Los Alamos director, to give him $50,000 from the discretionary budget, using it to build the Perhapsatron
Perhapsatron
The Perhapsatron was an early fusion power device based on the pinch concept. Dreamt up by James Tuck while working at Los Alamos National Laboratory , he named the device whimsically on the off chance that it might be able to create fusion reactions.The first example was built in the winter of...
.
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 IndustriesAssociated Electrical Industries
Associated Electrical Industries was a British holding company formed in 1928 through the merger of the British Thomson-Houston Company and Metropolitan-Vickers electrical engineering companies...
(AEI) labs at Aldermaston
Atomic Weapons Establishment
The Atomic Weapons Establishment is responsible for the design, manufacture and support of warheads for the United Kingdom's nuclear deterrent. AWE plc is responsible for the day-to-day operations of AWE...
, 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, and both appeared difficult to correct. 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.
Stabilized pinch
By 1953 two stabilization 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".
ZETA
Given the apparently enormous leap that stabilized pinch represented, the US planned a development program based on the modification of existing small devices. Thomson once again pressed Harwell for funding for a larger machine, and this time 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 Lev Artsimovich
Lev Artsimovich
Lev Andreevich Artsimovich was a Soviet physicist, academician of the Soviet Academy of Sciences , member of the Presidium of the Soviet Academy of Sciences , and Hero of Socialist Labor .- Academic research :Artsimovich worked on the...
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
Atoms for Peace
"Atoms for Peace" was the title of a speech delivered by U.S. President Dwight D. Eisenhower to the UN General Assembly in New York City on December 8, 1953....
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.
Fusion!
ZETA started full operation in mid-August 1957, initially with test gasses of hydrogen. These runs demonstrated that ZETA was not suffering from the same problems that earlier pinch machines had seen (the so-called "sausage" and "kink" instabilities) and their plasmas were lasting for milliseconds, up from microseconds. The length of the pulses allowed the plasma temperature to be measured using spectrographicSpectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
means; although the light given off was broadband, the Doppler shifting
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
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 neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s. 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
Financial Times
The Financial Times is an international business newspaper. It is a morning daily newspaper published in London and printed in 24 cities around the world. Its primary rival is the Wall Street Journal, published in New York City....
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.
...or is it?
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 ColgateStirling Colgate
Stirling Colgate is an American physicist at Los Alamos National Laboratory and a professor emeritus of physics at the New Mexico Institute of Mining and Technology . He was America's premier diagnostician of thermonuclear weapons during the early years at the Lawrence Livermore National...
, Lyman Spitzer, Jim Tuck and Arthur Edward Ruark
Arthur Edward Ruark
Arthur Edward Ruark was an American physicist who actively played a role in the development of quantum mechanics. He wrote the book "Atoms, Molecules, and Quanta" with Nobel Prize in Chemistry winner Harold Clayton Urey in 1930, and is the author of numerous scientific papers on quantum...
, all visited ZETA and concluded there was a "major probability" the neutrons were from fusion.
The glowing reviews of ZETA's results did not last long. On his return to the US, Lyman Spitzer
Lyman Spitzer
Lyman Strong Spitzer, Jr. was an American theoretical physicist and astronomer best known for his research in star formation, plasma physics, and in 1946, for conceiving the idea of telescopes operating in outer space...
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
Harold Furth
Harold P. Furth was an Austrian-American physicist.Furth emigrated to the United States in 1941. He graduated from Harvard University with a Bachelor's degree in 1951 and received his Ph.D. from Harvard in 1960...
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 th 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.
Release
The long-planned release of fusion data was pre-announced to the public in mid-January. Considerable material from the UK's ZETA and SceptreSceptre (fusion reactor)
Sceptre was an early fusion power device based the Z-pinch concept of plasma confinement, built in the UK starting in 1957. They were the ultimate versions of a series of devices tracing their history to the original pinch machines, built at Imperial College London by Cousins and Ware in 1947...
devices would be released in-depth in the 25 January 1958 edition of Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
, which would also include results from Los Alamos' Perhapsatron S-3, Columbus II and Columbus S-2. The UK press was livid. The Observer
The Observer
The Observer is a British newspaper, published on Sundays. In the same place on the political spectrum as its daily sister paper The Guardian, which acquired it in 1993, it takes a liberal or social democratic line on most issues. It is the world's oldest Sunday newspaper.-Origins:The first issue,...
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, but then got into the meat of the issue:
The reporters continued to press Cockroft on the neutron issue, and 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
The New York Times
The New York Times is an American daily newspaper founded and continuously published in New York City since 1851. The New York Times has won 106 Pulitzer Prizes, the most of any news organization...
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
Voice of Russia
Voice of Russia is the Russian government's international radio broadcasting service owned by the All-Russia State Television and Radio Company. Its predecessor Radio Moscow was the official international broadcasting station of the Union of Soviet Socialist Republics.-Early years: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
Osaka University
, or , is a major national university located in Osaka, Japan. It is the sixth oldest university in Japan as the Osaka Prefectural Medical College, and formerly one of the Imperial Universities of Japan...
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
Uppsala University
Uppsala University is a research university in Uppsala, Sweden, and is the oldest university in Scandinavia, founded in 1477. It consistently ranks among the best universities in Northern Europe in international rankings and is generally considered one of the most prestigious institutions of...
.
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
Critically, Cockcroft had stated that they were receiving too few neutrons from the device to measure their spectrum or their direction.However, in the same converted hangar that housed ZETA was the Harwell Synchrocyclotron
Harwell Synchrocyclotron
The Harwell Synchrocyclotron was a particle accelerator based at the Atomic Energy Research Establishment campus near Harwell, Oxfordshire. Construction of the accelerator began in 1946 and it was completed in 1949. The machine was of the synchrocyclotron design, with a 1.62T magnet of diameter...
effort run by Basil Rose. This project also constructed a sensitive high-pressure diffusion cloud chamber
Cloud chamber
The cloud chamber, also known as the Wilson chamber, is a particle detector used for detecting ionizing radiation. In its most basic form, a cloud chamber is a sealed environment containing a supersaturated vapor of water or alcohol. When a charged particle interacts with the mixture, it ionizes it...
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, and 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. 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 area, ejecting them from the plasma. These were the same sorts of instabilities seen in earlier machines, but in ZETA when they finally developed they were much more powerful. 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
Tokamak
A tokamak is a device using a magnetic field to confine a plasma in the shape of a torus . Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape...
designs.
Success through failure
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 laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s 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
Thomson scattering
Thomson scattering is the elastic scattering of electromagnetic radiation by a free charged particle, as described by classical electromagnetism. It is just the low-energy limit of Compton scattering: the particle kinetic energy and photon frequency are the same before and after the 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 on its tokamak designs with performance numbers 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
Kurchatov Institute
The Kurchatov Institute is Russia's leading research and development institution in the field of nuclear energy. In the Soviet Union it was known as I. V. Kurchatov Institute of Atomic Energy , abbreviated KIAE . It is named after Igor Kurchatov....
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 TaylorJohn Bryan Taylor
John Bryan Taylor is a British physicist known for his important contributions to plasma physics and their application in the field of fusion energy. Notable among these is the development of the "Taylor state", describing a minimum-energy configuration that conserves magnetic helicity...
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
Lawson criterion
In nuclear fusion research, the Lawson criterion, first derived on fusion reactors by John D. Lawson in 1955 and published in 1957, is an important general measure of a system that defines the conditions needed for a fusion reactor to reach ignition, that is, that the heating of the plasma by the...
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
Reversed field pinch
A reversed-field pinch is a device used to produce and contain near-thermonuclear plasmas. It is a toroidal pinch which uses a unique magnetic field configuration as a scheme to magnetically confine a plasma, primarily to study magnetic fusion energy. Its magnetic geometry is somewhat different...
(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
Magnetic helicity
In plasma physics, magnetic helicity is the extent to which a magnetic field "wraps around itself". It is a generalization of the topological concept of linking number to the differential quantities required to describe the magnetic field...
and minimum energy states, greatly advancing the understanding of plasma dynamics. The minimum-energy state, known as the "Taylor state
Taylor state
In plasma physics, a Taylor state is the minimum energy state of a plasma satisfying the constraint of conserving magnetic helicity.- Derivation :...
", is particularly important in the understanding of new fusion approaches in the compact toroid
Compact toroid
Compact toroids are a class of toroidal plasma configurations that are self-stable, and whose configuration does not require magnet coils running through the center of the toroid. They are studied primarily in the field of fusion energy, where the lack of complex magnets and a simple geometry may...
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
James Clerk Maxwell Prize in Plasma Physics
The James Clerk Maxwell Prize in Plasma Physics is an annual American Physical Society award given in recognition of outstanding contributions to the field of Plasma Physics. It was established in 1975 by Maxwell Technologies, Inc, in honor of the Scottish physicist James Clerk Maxwell. It is...
.