Sceptre (fusion reactor)
Encyclopedia
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. When the UK's fusion work was classified in 1950, Ware's team was moved to the Associated Electrical Industries
(AEI) labs at Aldermaston
. The team worked on the problems associated with using metal tubes with high voltages, in support of the efforts at Harwell
. When Harwell's ZETA
machine apparently produced fusion, AEI quickly built a smaller machine, Sceptre, to test their results. Sceptre also produced neutrons, apparently confirming the ZETA experiment. It was later found that the neutrons were spurious, and UK work on Z-pinch ended in the early 1960s.
Fusion research in the UK started on a shoestring budget at Imperial College in 1946. When George Paget Thomson
failed to gain funding from John Cockcroft
's Atomic Energy Research Establishment
(AERE), he turned over the project to two students, Stan Cousins and Alan Ware. They started working on the concept in January 1947, using a glass tube and old radar parts. Their small experimental device was able to generate brief flashes of light. However, the nature of the light remained a mystery as they could not come up with a method of measuring its temperature.
Little interest was shown in the work, although it was noticed by Jim Tuck, who was interested in all things fusion. He, in turn, introduced the concepts to Peter Thonemann, and the two developed a similar small machine of their own at Oxford University's Clarendon Laboratory
. Tuck left for the University of Chicago
before the device was built. After moving to Los Alamos
, Tuck introduced the pinch concept there, and eventually built the Perhapsatron
along the same lines.
In early 1950 Klaus Fuchs
' admitted to turning UK and US atomic secrets over to the USSR. As fusion devices would generate copious amounts of neutron
s, which could be used to enrich nuclear fuel for atomic bombs, the UK immediately classified all their fusion work. The research was considered important enough to continue, but it was difficult to maintain in a university setting. The decision was made to move both teams to secure locations. Imperial team under Ware was set up at the Associated Electrical Industries
(AEI) labs at Aldermaston
in November while the Oxford team under Thonemann were moved to UKAEA 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 everyone that something was wrong in the machines. As current was applied, the plasma would first pinch down as expected, but would then develop a series of "kinks", evolving into a sinusoidal shape. When the outer portions hit the walls of the container, a small amount of the material would spall off into the plasma, cooling it and ruining the reaction. This so-called "kink instability" appeared to be a fundamental problem.
. Compared to the team at Harwell, the Aldermaston team decided to focus on faster pinch systems. Their power supply consisted of a large bank of capacitor
s with a total capacity of 66,000 Joules (when fully expanded) switched by spark gap
s that could dump the stored power into the system at high speeds. Harwell's devices used slower rising pinch currents, and had to be larger to reach the same conditions.
Allibone, originally from Metropolitan-Vickers
, had worked on metal-walled X-ray tubes that used small inserts of porcelain to insulate them electrically. He suggested trying the same thing for the fusion experiments, potentially leading to higher temperatures than the glass tubes could handle. They started with an all-porcelain tube of 20 cm major axis, and were able to induce 30 kA of current into the plasma before it broke up. Following this they built an aluminum version, which was split into two parts with mica inserts between them. This version suffered arcing between the two halves.
Convinced that the metal tube was the way ahead, the team then started a long series of experiments with different materials and construction techniques to solve the arcing problem. By 1955 they had developed one with 64 segments that showed promise, and using 60 kJ capacitor bank they were able to induce 80 kA discharges. Although the tube was an improvement, it also suffered from the same kink instabilities, and work on this approach was abandoned.
To better characterize the problem, the team started construction of a larger aluminum torus with a 12 inch bore and 45 inch diameter, and inserted two straight sections to stretch it into a racetrack shape. The straight sections, known as the "pepper pot", had a series of holes drilled in them, angled so they all pointed to a single focal point some distance from the apparatus. A camera placed at the focal point was able to image the entire plasma column, greatly improving their understanding of the instability process.
Studying the issue, Shavranov, Taylor and Rosenbluth all developed the idea of adding a second magnetic field to the system, a steady-state toroidal field generated by magnets circling the vacuum tube. Tuck referred to this concept as "giving the plasma a backbone". The second field would force the electrons and deuterons in the plasma to orbit the lines of force, reducing the effects of small imperfections in the field generated by the pinch itself. This sparked off considerable interest in both the US and UK. Thomson, armed with the possibility of a workable device and obvious interest in the US, won approval for a very large machine, ZETA.
However, before either was completed, the ZETA
team at Harwell had already achieved stable plasmas in August 1957. The Aldermaston team raced to complete their larger photographic system. Electrical arcing and shorting between the tube segments became a problem, but the team had already learned that "dry firing" the apparatus hundreds of times would reduce this effect. After addressing the arcing, further experiments demonstrated temperatures around 1 million degrees. The system worked as expected, producing clear images of the kink instabilities using high-speed photography and argon gas so as to produce a bright image.
The team then removed the straight sections, added stabilization magnets, and re-christened the machine Sceptre III. In December they started experimental runs like those on ZETA. By measuring the spectral lines of oxygen, they calculated interior temperatures of 2 to 3.5 million degrees. Photographs through a slit in the side showed the plasma column remaining stable for 300 to 400 microseconds, a dramatic improvement on previous efforts. Working backward, the team calculated that the plasma had an electrical resistivity around 100 times that of copper, and was able to carry 200 kA of current for 500 microseconds in total. When the current was over 70 kA, neutrons were observed in roughly the same numbers as ZETA.
As in the case of ZETA, it was soon learned that the neutrons were being produced by a spurious source, and the temperatures were due to turbulence in the plasma, not the average temperature.
However, none of these techniques helped. Sceptre IV proved to have the same performance problems as the earlier machines. Sceptre IV proved to be the last major "classic" pinch device built in the UK.
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...
device based 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...
concept of 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...
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
Imperial College London
Imperial College London is a public research university located in London, United Kingdom, specialising in science, engineering, business and medicine...
by Cousins and Ware in 1947. When the UK's fusion work was classified in 1950, Ware's team was moved to the Associated Electrical Industries
Associated 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...
. The team worked on the problems associated with using metal tubes with high voltages, in support of the efforts at Harwell
Harwell Science and Innovation Campus
The Harwell Science and Innovation Campus is a science and technology campus near the villages of Harwell and Chilton, Oxfordshire, England. The site is about south of Oxford...
. When Harwell's ZETA
ZETA
ZETA, short for "Zero-Energy Toroidal 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...
machine apparently produced fusion, AEI quickly built a smaller machine, Sceptre, to test their results. Sceptre also produced neutrons, apparently confirming the ZETA experiment. It was later found that the neutrons were spurious, and UK work on Z-pinch ended in the early 1960s.
Background
- For a detailed history of pinch in the UK, see ZETAZETAZETA, short for "Zero-Energy Toroidal 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...
Fusion research in the UK started on a shoestring budget at Imperial College in 1946. When 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:...
failed to gain funding from 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....
's 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), he turned over the project to two students, Stan Cousins and Alan Ware. They started working on the concept in January 1947, using a glass tube and old radar parts. Their small experimental device was able to generate brief flashes of light. However, the nature of the light remained a mystery as they could not come up with a method of measuring its temperature.
Little interest was shown in the work, although it was noticed by Jim Tuck, who was interested in all things fusion. He, in turn, introduced the concepts to Peter Thonemann, and the two developed a similar small machine of their own at Oxford University's 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...
. Tuck left for the University of Chicago
University of Chicago
The University of Chicago is a private research university in Chicago, Illinois, USA. It was founded by the American Baptist Education Society with a donation from oil magnate and philanthropist John D. Rockefeller and incorporated in 1890...
before the device was built. After moving to Los Alamos
Los Alamos
-United States:*Los Alamos, California*Los Alamos, New Mexico**Los Alamos Ranch School, boys' school**Los Alamos National Laboratory**Los Alamos County, New Mexico**Los Alamos Museum, unofficial name of the Bradbury Science Museum was a large floating dry dock...
, Tuck introduced the pinch concept there, and eventually built 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...
along the same lines.
In early 1950 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...
' admitted to turning UK and US atomic secrets over to the USSR. As fusion devices would generate copious amounts 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, which could be used to enrich nuclear fuel for atomic bombs, the UK immediately classified all their fusion work. The research was considered important enough to continue, but it was difficult to maintain in a university setting. The decision was made to move both teams to secure locations. Imperial team under Ware was set up at the Associated Electrical Industries
Associated 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...
in November while the Oxford team under Thonemann were moved to UKAEA 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 everyone that something was wrong in the machines. As current was applied, the plasma would first pinch down as expected, but would then develop a series of "kinks", evolving into a sinusoidal shape. When the outer portions hit the walls of the container, a small amount of the material would spall off into the plasma, cooling it and ruining the reaction. This so-called "kink instability" appeared to be a fundamental problem.
Practical work
At Aldermaston, the Imperial team was put under the direction of Thomas AlliboneThomas Allibone
Thomas Edward Allibone, CBE, FRS was an English physicist, his work included important research into particle physics, X-rays, high voltage equipment, and electron microscopes.-Early life:...
. Compared to the team at Harwell, the Aldermaston team decided to focus on faster pinch systems. Their power supply consisted of a large bank of capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s with a total capacity of 66,000 Joules (when fully expanded) switched by spark gap
Spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms,...
s that could dump the stored power into the system at high speeds. Harwell's devices used slower rising pinch currents, and had to be larger to reach the same conditions.
Allibone, originally from Metropolitan-Vickers
Metropolitan-Vickers
Metropolitan-Vickers, Metrovick, or Metrovicks, was a British heavy electrical engineering company of the early-to-mid 20th century formerly known as British Westinghouse. Highly diversified, they were particularly well known for their industrial electrical equipment such as generators, steam...
, had worked on metal-walled X-ray tubes that used small inserts of porcelain to insulate them electrically. He suggested trying the same thing for the fusion experiments, potentially leading to higher temperatures than the glass tubes could handle. They started with an all-porcelain tube of 20 cm major axis, and were able to induce 30 kA of current into the plasma before it broke up. Following this they built an aluminum version, which was split into two parts with mica inserts between them. This version suffered arcing between the two halves.
Convinced that the metal tube was the way ahead, the team then started a long series of experiments with different materials and construction techniques to solve the arcing problem. By 1955 they had developed one with 64 segments that showed promise, and using 60 kJ capacitor bank they were able to induce 80 kA discharges. Although the tube was an improvement, it also suffered from the same kink instabilities, and work on this approach was abandoned.
To better characterize the problem, the team started construction of a larger aluminum torus with a 12 inch bore and 45 inch diameter, and inserted two straight sections to stretch it into a racetrack shape. The straight sections, known as the "pepper pot", had a series of holes drilled in them, angled so they all pointed to a single focal point some distance from the apparatus. A camera placed at the focal point was able to image the entire plasma column, greatly improving their understanding of the instability process.
Studying the issue, Shavranov, Taylor and Rosenbluth all developed the idea of adding a second magnetic field to the system, a steady-state toroidal field generated by magnets circling the vacuum tube. Tuck referred to this concept as "giving the plasma a backbone". The second field would force the electrons and deuterons in the plasma to orbit the lines of force, reducing the effects of small imperfections in the field generated by the pinch itself. This sparked off considerable interest in both the US and UK. Thomson, armed with the possibility of a workable device and obvious interest in the US, won approval for a very large machine, ZETA.
Sceptre
At Aldermaston, using the same information, Ware's team calculated that with the 60 kJ available in the existing capacitor bank, they would reach the required conditions in a copper-covered quartz tube 2 inches in bore and 10 inches in diameter, or an all-copper version 2 inches in bore and 18 inches across. Work on both started in parallel, as Sceptre I and II.However, before either was completed, the ZETA
ZETA
ZETA, short for "Zero-Energy Toroidal 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...
team at Harwell had already achieved stable plasmas in August 1957. The Aldermaston team raced to complete their larger photographic system. Electrical arcing and shorting between the tube segments became a problem, but the team had already learned that "dry firing" the apparatus hundreds of times would reduce this effect. After addressing the arcing, further experiments demonstrated temperatures around 1 million degrees. The system worked as expected, producing clear images of the kink instabilities using high-speed photography and argon gas so as to produce a bright image.
The team then removed the straight sections, added stabilization magnets, and re-christened the machine Sceptre III. In December they started experimental runs like those on ZETA. By measuring the spectral lines of oxygen, they calculated interior temperatures of 2 to 3.5 million degrees. Photographs through a slit in the side showed the plasma column remaining stable for 300 to 400 microseconds, a dramatic improvement on previous efforts. Working backward, the team calculated that the plasma had an electrical resistivity around 100 times that of copper, and was able to carry 200 kA of current for 500 microseconds in total. When the current was over 70 kA, neutrons were observed in roughly the same numbers as ZETA.
As in the case of ZETA, it was soon learned that the neutrons were being produced by a spurious source, and the temperatures were due to turbulence in the plasma, not the average temperature.
Sceptre IV
As the ZETA debacle played out in 1958, solutions to the problems seen in ZETA and Sceptre IIIA were hoped to be simple: a better tube, higher vacuum, and denser plasma. As the Sceptre machine was much less expensive and the high-power capacitor bank already existed, the decision was made to test these concepts with a new device, Sceptre IV.However, none of these techniques helped. Sceptre IV proved to have the same performance problems as the earlier machines. Sceptre IV proved to be the last major "classic" pinch device built in the UK.