A critique by Rudolf Peierls noted several problems with the concept. Over the next few years, Thomson continued to suggest starting an experimental effort to study these issues, but was repeatedly denied as the underlying theory of plasma diffusion was not well developed. When similar concepts were suggested by Peter Thonemann that included a more practical heating arrangement, John Cockcroft began to take the concept more seriously, establishing small study groups at Harwell. Thomson adopted Thonemann's concept, abandoning the radio frequency system.
When the patent had still not been granted in early 1948, the Ministry of Supply inquired about Thomson's intentions. Thomson explained the problems he had getting a program started and that he did not want to hand off the rights until that was clarified. As the directors of the UK nuclear program, the Ministry quickly forced Harwell's hand to provide funding for Thomson's program. Thomson then released his rights the patent, which was granted late that year. Cockcroft also funded Thonemann's work, and with that, the UK fusion program began in earnest. After the news furor over the Huemul Project in February 1951, significant funding was released and led to rapid growth of the program in the early 1950s, and ultimately to the ZETA reactor of 1958.
This energy corresponds to the average energy of particles in a gas heated to a billion Kelvin. Materials heated beyond a few tens of thousand Kelvin dissociate into their electrons and nuclei, producing a gas-like state of matter known as plasma. In any gas the particles have a wide range of energies, normally following the Maxwell–Boltzmann statistics. In such a mixture, a small number of particles will have much higher energy than the bulk.[7]
This leads to an interesting possibility; even at temperatures well below 100,000 eV, some particles will randomly have enough energy to undergo fusion. Those reactions release huge amounts of energy. If that energy can be captured back into the plasma, it can heat other particles to that energy as well, making the reaction self-sustaining. In 1944, Enrico Fermi calculated this would occur at about 50,000,000 K.[8][9]
Confinement
Taking advantage of this possibility requires the fuel plasma to be held together long enough that these random reactions have time to occur. Like any hot gas, the plasma has an internal pressure and thus tends to expand according to the ideal gas law.[7] For a fusion reactor, the problem is keeping the plasma contained against this pressure; any known physical container would melt at temperatures in the thousands of Kelvin, far below the millions needed for fusion.[10]
A plasma is electrically conductive, and is subject to electric and magnetic fields. In a magnetic field, the electrons and nuclei orbit the magnetic field lines.[10][11][12] A simple confinement system is a plasma-filled tube placed inside the open core of a solenoid. The plasma naturally wants to expand outwards to the walls of the tube, as well as move along it, towards the ends. The solenoid creates a magnetic field running down the centre of the tube, which the particles will orbit, preventing their motion towards the sides. Unfortunately, this arrangement does not confine the plasma along the length of the tube, and the plasma is free to flow out the ends.[13]
Initial design
The obvious solution to this problem is to bend the tube, and solenoid, around to form a torus (a ring or doughnut shape).[14] Motion towards the sides remains constrained as before, and while the particles remain free to move along the lines, in this case, they will simply circulate around the long axis of the tube.[15] But, as Fermi pointed out,[lower-alpha 2] when the solenoid is bent into a ring, the electrical windings of the solenoid would be closer together on the inside than the outside. This would lead to an uneven field across the tube, and the fuel will slowly drift out of the centre. Some additional force needs to counteract this drift, providing long-term confinement.[15][17][18]
Thomson began development of his concept in February 1946. He noted that this arrangement caused the positively charged fuel ions to drift outward more rapidly than the negatively charged electrons. This would result in a negative area in the center of the chamber that would develop over a short period. This net negative charge would then produce an attractive force on the ions, keeping them from drifting too far from the center, and thus preventing them from drifting to the walls. It appeared this could provide long-term confinement.[19]
This leaves the issue of how to heat the fuel to the required temperatures. Thomson proposed injecting a cool plasma into the torus and then heating it with radio frequency signals beamed into the chamber. The electrons in the plasma would be "pumped" by this energy, transferring it to the ions though collisions. If the chamber held a plasma with densities on the order of 1014 to 1015 nuclei/cm3, it would take several minutes to reach the required temperatures.[19]
Filing a patent
In early March, Thomson sent a copy of his proposal to Rudolf Peierls, then at the University of Birmingham. Peierls immediately pointed out a concern; both Peierls and Thomson had been to meetings at the Los Alamos in 1944 where Edward Teller held several informal talks, including the one in which Fermi outlined the basic conditions needed for fusion. This was in the context of an H-bomb, or "the super" as it was then known. Peierls noted that the US might claim priority on such information and consider it highly secret, which meant that while Thomson was privy to the information, it was unlikely others at Imperial were.[20]
Considering the problem, Thomson decided to attempt to file a patent on the concept. This would ensure the origins of the concepts would be recorded, and prove that the ideas were due to efforts in the UK and not his previous work on the atom bomb. At the time, Thomson was not concerned with establishing personal priority for the concept nor generating income from it. At his suggestion, on 26 March 1946 they met with Arthur Block of the Ministry of Supply (MoS), which led to B.L. Russel, the MoS' patent agent, beginning to write a patent application that would be owned entirely by the government.[19]
Peierls' concerns
Peierls then followed up with a lengthy critique of the concept, noting three significant issues.[20]
The major concern was that the system as a whole used a toroidal field to confine the electrons, and the electric field resulting to confine the ions. Peierls pointed out that this "cross field" would cause the particles to be forced across the magnetic lines due to the right hand rule, causing the electrons to orbit around the chamber in the poloidal direction, eliminating the area of increased electrons in the center, and thereby allowing the ions to drift to the walls. Using Thomson's own figures for the conditions in an operating reactor, Peierls demonstrated that the resulting neutralized region would extend all the way to the walls, by less than the radius of the electrons in the field. There would be no confinement of the ions.[20]
He also included two additional concerns. One involved the issue of the deuterium fuel ions impacting with the walls of the chamber and the effects that would have, and the other that having electrons leave the plasma would cause an ion to be forced out to maintain charge balance, which would quickly "clean up" all of the gas in the chamber.[20][lower-alpha 3]
Pinch emerges
Thomson was not terribly concerned about the two minor problems but accepted that the primary one about the crossed fields was a serious issue. Considering the issue, a week later he wrote back with a modified concept. In this version, the external magnets producing the toroidal field were removed, and confinement was instead provided by running a current through the plasma. He proposed inducing this current using radio signals injected through slots cut into the torus at spaces that would create a wave moving around the torus similar to the system used in linear accelerators used to accelerate electrons.[20]
A provisional patent was filed on 8 May 1946, updated to use the new confinement system. In the patent, Thomson noted that the primary problem would be overcoming energy losses through bremsstrahlung. He calculated that a plasma density of 1015 would remain stable long enough for the energy of the pumped electrons to heat the D fuel to the required 100keV over the time of several minutes. Although the term "pinch effect" is not mentioned, except for the current generation concept, the description was similar to the pinch machines that would become widespread in the 1950s.[22]
Thomson described his concept, including several possible ways to drive the current. Peierls reiterated his earlier concerns, mentioning the observations by Mark Oliphant and Harrie Massey who had worked with David Bohm on isotopic separation at Berkeley.[22] Bohm had observed greatly increased rates of diffusion well beyond what classical diffusion would suggest, today known as Bohm diffusion. If this was inherent to such designs, Peierls suggested there was no way the device would work. He then added a highly prescient statement that there may be further unknown instabilities that would ruin confinement.[23]
Peierls concluded by suggesting initial studies on the pinch effect be carried out by Moon in Birmingham, where Moon had some experience in these sorts of devices and especially because Sayers was already planning experiments with powerful spark discharges in deuterium. There is no record that this work was carried out, although theoretical studies on the behaviour of plasma in a pinch was worked on.[23]
Early experiments
The main outcome of the meeting was to introduce Thomson to the wirbelrohr, a new type of particle accelerator built in 1944 in Germany. The wirbelrohr used a cyclotron-like arrangement to accelerate the electrons in a plasma, which its designer, Max Steenbeck, believed would cause them to "break away" from the ions and accelerate to very high speeds. The parallels between this device and Thomson's concept were obvious, but Steenbeck's acceleration mechanism was novel and presented a potentially more efficient heating system.[23]
When he returned to London after the meeting, Thomson had two PhD students put on the project, with Alan Ware tasked with building a wirbelrohr and Stanley Cousins starting a mathematical study on diffusion of plasma in a magnetic field.[23] Ware build a device using 3cm tube bent around into a 25cm wide torus. Using a wide variety of gas pressures and currents up to 13,000Amps, Ware was able to show some evidence of the pinching of the plasma, but failed, as had the Germans, to find any evidence of the break away electrons. With this limited success, Ware and Cousins built a second device at 40cm and up to 27,000Amps. Once again, no evidence of electron break away was seen, but this time a new high-speed rotating-mirror camera was able to directly image the plasma during the discharge and was able to conclusively show the plasma was indeed being pinched.[24]
Classification concerns
While Cousins and Ware began their work, in April 1947 Thomson filed a more complete patent application. This described a larger 4 metres (13ft) wide torus with many ports for injecting and removing gas and to inject the radio frequency energy to drive the current. The entire system was then placed within a large magnet that produced a moderate 0.15T vertical magnetic field across the entire torus, which kept the electrons confined. He predicted that a power input of 1.9MW would be needed and calculated that the D-D and D-T reactions would generate 9MW of fusion energy, of which 1.9MW was in the form of neutrons. He suggested that the neutrons could be used as a power source, but also if the system was surrounded by natural uranium, mostly 238U, the neutrons would transmute it into plutonium-239, a major component of atomic bombs.[24]
It was this last part that raised new concerns. If, as Thomson described, one could make a relatively simple device that could produce plutonium there was an obvious nuclear security concern and such work would need to be secret. Neither Thomson or Harwell were happy performing secret work at the university. Considering the problem, Thomson suggested moving this work to RAF Aldermaston. Associated Electrical Industries (AEI) was outgrowing their existing labs in Rugby and Trafford Park, and had already suggested building a new secure lab at Aldermaston. AEI was looking to break into the emerging nuclear power field, and its director of research, Thomas Allibone, was a friend of Thomson's. Allibone strongly supported Thomson's suggestion, and further backing was received from Nobel winnerJames Chadwick. Cockcroft, on the other hand, believed it was too early to start the large program Thomson was suggesting, and continued to delay.[25]
Thonemann's concept
Around the same time, Cockcroft learned of similar work carried out independently by Peter Thonemann at Clarendon, triggering a small theoretical program at Harwell to consider it. But all suggestions of a larger development program continued to be rejected.[25]
Thonemann's concept was to replace the radio frequency injection used by Thonemann and arrange the reactor like a betatron, that is, wrapping the torus in a large magnet and using its field to induce a current in the torus in a fashion similar to an electrical transformer. Betatrons had a natural limitation that the number of electrons in them was limited due to their self-repulsion, known as the space charge limit. Some had suggested introducing a gas to the chamber; when ionized by the accelerated electrons, the leftover ions would produce a positive charge that would help neutralize the chamber as a whole. Experiments to this end instead showed that collisions between the electrons and ions would scatter so rapidly that the number of electrons remaining was actually lower than before. This effect, however, was precisely what was desired in a fusion reactor, where the collisions would heat the deuterium ions.[25]
At an accidental meeting at Clarendon, Thonemann ended up describing his idea to Thomson. Thonemann was not aware he was talking to Thomson, nor of Thomson's work on similar ideas. Thomson followed up with Skinner, who strongly supported Thonemann's concept over Thomson's. Skinner then wrote a paper on the topic, "Thermonuclear Reactions by Electrical Means", and presented it to the Atomic Energy Commission on 8 April 1948. He clearly pointed out where the unknowns were in the concepts, and especially the possibility of destructive instabilities that would ruin confinement. He concluded that it would be "useless to do much further planning" before further study on the instability issues.[26]
It was at this point that a curious bit of legality comes into the events. In February 1948, Thompson's original patent filing had not been granted as the Ministry of Supply was not sure about his intentions on assigning the rights. Blackman was ill with malaria in South Africa, and the issue was put off for a time. It was raised again in May when he returned, resulting in a mid-July meeting. Thompson complained that Harwell was not supporting their efforts, and that as none of this was classified, he wanted to remain open to turning to private funding. In that case, he was hesitant to assign the rights to the Ministry. The Ministry, who was in charge of the nuclear labs including Harwell, quickly arranged for Cockroft to fund Thompson's development program. The program was approved in November, and the patent was assigned to the Ministry by the end of the year.[27]
Move to AEI
The work on fusion at Harwell and Imperial remained relatively low-level until 1951, when two events occurred that changed the nature of the program significantly.
The first was the January 1950 confession by Klaus Fuchs that he had been passing atomic information to the Soviets.[28] His confession led to immediate and sweeping classification of almost anything nuclear related. This included all fusion related work, as the previous fears about the possibility of using fusion as a neutron source to produce plutonium now seemed like a serious issue. The earlier plans to move the team from Imperial were put into effect immediately, with the AEI labs being set up at the former Aldermaston and opening in April. This lab soon became the Atomic Weapons Research Establishment.[29]
The second was the February 1951 announcement that Argentina had successfully produced fusion in its Huemul Project. Physicists around the world quickly dismissed it as impossible, which was revealed to be the case by 1952. However, it also had the effect of making politicians learn of the concept of fusion, and its potential as an energy source. Physicists working on the concept suddenly found themselves able to talk to high-ranking politicians, who proved rather receptive to increasing their budgets. Within weeks, programs in the US, UK and USSR were seeing dramatic expansion.[30]
By the summer of 1952, the UK fusion program was developing several machines based on Thonemann's overall design, and Thomson's original RF-concept was put aside.[31]
↑ Andrei Sakharov came to the same conclusion as Fermi in 1950, but his paper on the topic was not known in the West until 1958.[16]
↑ This effect would be seen in a number of later designs, where it was known as "pump out".[21]
Related Research Articles
Nuclear fusion is a reaction in which two or more atomic nuclei, usually deuterium and tritium, combine to form one or more different atomic nuclei and subatomic particles. The difference in mass between the reactants and products is manifested as either the release or absorption of energy. This difference in mass arises due to the difference in nuclear binding energy between the atomic nuclei before and after the reaction. Nuclear fusion is the process that powers active or main-sequence stars and other high-magnitude stars, where large amounts of energy are released.
A stellarator is a plasma device that relies primarily on external magnets to confine a plasma. Scientists researching magnetic confinement fusion aim to use stellarator devices as a vessel for nuclear fusion reactions. The name refers to the possibility of harnessing the power source of the stars, such as the Sun. It is one of the earliest fusion power devices, along with the z-pinch and magnetic mirror.
A tokamak is a device which uses a powerful magnetic field to confine plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power. As of 2016, it was the leading candidate for a practical fusion reactor. The word "tokamak" is derived from a Russian acronym meaning "toroidal chamber with magnetic coils".
A fusor is a device that uses an electric field to heat ions to a temperature in which they undergo nuclear fusion. The machine induces a voltage between two metal cages, inside a vacuum. Positive ions fall down this voltage drop, building up speed. If they collide in the center, they can fuse. This is one kind of an inertial electrostatic confinement device – a branch of fusion research.
Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion reactions. In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, while releasing energy. Devices designed to harness this energy are known as fusion reactors. Research into fusion reactors began in the 1940s, but as of 2023, no device has reached net power.
This timeline of nuclear fusion is an incomplete chronological summary of significant events in the study and use of nuclear fusion.
Inertial electrostatic confinement, or IEC, is a class of fusion power devices that use electric fields to confine the plasma rather than the more common approach using magnetic fields found in magnetic confinement fusion (MCF) designs. Most IEC devices directly accelerate their fuel to fusion conditions, thereby avoiding energy losses seen during the longer heating stages of MCF devices. In theory, this makes them more suitable for using alternative aneutronic fusion fuels, which offer a number of major practical benefits and makes IEC devices one of the more widely studied approaches to fusion.
The Lawson criterion is a figure of merit used in nuclear fusion research. It compares the rate of energy being generated by fusion reactions within the fusion fuel to the rate of energy losses to the environment. When the rate of production is higher than the rate of loss, the system will produce net energy. If enough of that energy is captured by the fuel, the system will become self-sustaining and is said to be ignited.
The Tokamak Fusion Test Reactor (TFTR) was an experimental tokamak built at Princeton Plasma Physics Laboratory (PPPL) circa 1980 and entering service in 1982. TFTR was designed with the explicit goal of reaching scientific breakeven, the point where the heat being released from the fusion reactions in the plasma is equal or greater than the heating being supplied to the plasma by external devices to warm it up.
Aneutronic fusion is any form of fusion power in which very little of the energy released is carried by neutrons. While the lowest-threshold nuclear fusion reactions release up to 80% of their energy in the form of neutrons, aneutronic reactions release energy in the form of charged particles, typically protons or alpha particles. Successful aneutronic fusion would greatly reduce problems associated with neutron radiation such as damaging ionizing radiation, neutron activation, reactor maintenance, and requirements for biological shielding, remote handling and safety.
Magnetic confinement fusion (MCF) is an approach to generate thermonuclear fusion power that uses magnetic fields to confine fusion fuel in the form of a plasma. Magnetic confinement is one of two major branches of controlled fusion research, along with inertial confinement fusion.
The Madison Symmetric Torus (MST) is a reversed field pinch (RFP) physics experiment with applications to both fusion energy research and astrophysical plasmas.
ZETA, short for Zero Energy Thermonuclear Assembly, was a major experiment in the early history of fusion power research. Based on the pinch plasma confinement technique, and built at the Atomic Energy Research Establishment in the United Kingdom, ZETA was larger and more powerful than any fusion machine in the world at that time. Its goal was to produce large numbers of fusion reactions, although it was not large enough to produce net energy.
Migma, sometimes migmatron or migmacell, was a proposed colliding beam fusion reactor designed by Bogdan Maglich in 1969. Migma uses self-intersecting beams of ions from small particle accelerators to force the ions to fuse. Similar systems using larger collections of particles, up to microscopic dust sized, were referred to as "macrons". Migma was an area of some research in the 1970s and early 1980s, but lack of funding precluded further development.
The polywell is a design for a fusion reactor based on two ideas: heating ions by concentrating (-) charge to accelerate the ions and trapping a diamagnetic plasma inside a cusp field.
Sceptre was a series of early fusion power devices based on the Z-pinch concept of plasma confinement, built in the UK starting in 1956. 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.
The bumpy torus is a class of magnetic fusion energy devices that consist of a series of magnetic mirrors connected end-to-end to form a closed torus. It is based on a discovery made by a team headed by Ray Dandl at Oak Ridge National Laboratory in the 1960s.
Colliding beam fusion (CBF), or colliding beam fusion reactor (CBFR), is a class of fusion power concepts that are based on two or more intersecting beams of fusion fuel ions that are independently accelerated to fusion energies using a variety of particle accelerator designs or other means. One of the beams may be replaced by a static target, in which case the approach is termed accelerator based fusion or beam-target fusion, but the physics is the same as colliding beams.
The Diffusion Inhibitor is the first known attempt to build a working fusion power device. It was designed and built at the National Advisory Committee for Aeronautics' (NACA) Langley Memorial Aeronautical Laboratory beginning in the spring of 1938. The basic concept was developed by Arthur Kantrowitz and his boss, Eastman Jacobs. They deliberately picked a misleading name to avoid the project being detected by NACA's headquarters in Washington, D.C., as they believed it would immediately be cancelled if their superiors learned of it.
Theta-pinch, or θ-pinch, is a type of fusion power reactor design. The name refers to the configuration of currents used to confine the plasma fuel in the reactor, arranged to run around a cylinder in the direction normally denoted as theta in polar coordinate diagrams. The name was chosen to differentiate it from machines based on the pinch effect that arranged their currents running down the centre of the cylinder; these became known as z-pinch machines, referring to the Z-axis in cartesian coordinates.
Goodman, Michael (July 2005). "Who Is Trying to Keep What Secret From Whom and Why? MI5-FBI Relations and the Klaus Fuchs Case". Journal of Cold War Studies. 7 (3): 124–146. doi:10.1162/1520397054377160. ISSN1531-3298. S2CID57560118.
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