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30-513: LDX ceased operations in November 2011 when its funding from the Department of Energy ended as resources were being diverted to tokamak research. The concept of the levitated dipole as a fusion reactor was first theorized by Akira Hasegawa in 1987. The concept was later proposed as an experiment by Jay Kesner of MIT and Michael Mauel of Columbia University in 1997. The pair assembled

60-570: A control test in February 2007 and replaced in May 2007. The replacement coil was inferior, a copper wound electromagnet , that was also water cooled. Scientific results, including the observation of an inward turbulent pinch, were reported in Nature Physics . This experiment needed a special free-floating electromagnet, which created the unique "toilet-bowl" magnetic field. The magnetic field

90-445: A different and sometimes much larger effective neutron absorption cross-section for a given nuclide than fast neutrons, and can therefore often be absorbed more easily by an atomic nucleus , creating a heavier, often unstable isotope of the chemical element as a result. This event is called neutron activation . Fast neutrons are produced by nuclear processes: Fast neutrons are usually undesirable in

120-425: A larger number of neutrons, so a fast breeder reactor can potentially "breed" more fissile fuel than it consumes. Fast reactor control cannot depend solely on Doppler broadening or on negative void coefficient from a moderator. However, thermal expansion of the fuel itself can provide quick negative feedback. Perennially expected to be the wave of the future, fast reactor development has been nearly dormant with only

150-501: A speed of 2.19 km/s), which is the energy corresponding to the most probable speed at a temperature of 290 K (17 °C or 62 °F), the mode of the Maxwell–Boltzmann distribution for this temperature, E peak = k T. After a number of collisions with nuclei ( scattering ) in a medium ( neutron moderator ) at this temperature, those neutrons which are not absorbed reach about this energy level. Thermal neutrons have

180-421: A steady-state nuclear reactor because most fissile fuel has a higher reaction rate with thermal neutrons. Fast neutrons can be rapidly changed into thermal neutrons via a process called moderation. This is done through numerous collisions with (in general) slower-moving and thus lower-temperature particles like atomic nuclei and other neutrons. These collisions will generally speed up the other particle and slow down

210-596: A superconducting ring. The ring produced a 5.7 T peak field. This superconductor was encased inside a liquid helium cryostat, which kept the electromagnet below 10 kelvins . This design is similar to the D20 dipole experiment at Berkeley and the RT-1 experiment at the University of Tokyo. The dipole was suspended inside a "squashed-pumpkin"-shaped vacuum chamber, which was about 5.2 meters in diameter and ~3 meters high. At

240-412: A team and raised money to build the machine. They achieved first plasma on Friday, August 13, 2004, at 12:53 PM. First plasma was done by (1) successfully levitating the dipole magnet and (2) RF heating the plasma. The LDX team has since successfully conducted several levitation tests, including a 40-minute suspension of the superconducting coil on February 9, 2007. Shortly after, the coil was damaged in

270-488: A theoretical basis for a thermonuclear reactor, where the plasma would have the shape of a torus and be held by a magnetic field. The first tokamak was built in 1954, and for over a decade this technology existed only in the USSR. In 1968 the electronic plasma temperature of 1 keV was reached on the tokamak T-3, built at the I. V. Kurchatov Institute of Atomic Energy under the leadership of academician L. A. Artsimovich. By

300-958: Is a transliteration of the Russian word токамак , an acronym of either: то роидальная to roidal'naya to roidal ка мера ka mera cha mber с s with ма гнитными ma gnitnymi ma gnetic к атушками k atushkami c oils то роидальная ка мера с ма гнитными к атушками to roidal'naya ka mera s ma gnitnymi k atushkami to roidal cha mber with ma gnetic c oils or: то роидальная to roidal'naya to roidal кам ера kam era cham ber с s with ак сиальным ak sial'nym ax ial магнитным magnitnym magnetic полем polem field то роидальная кам ера с ак сиальным магнитным полем to roidal'naya kam era s ak sial'nym magnitnym polem to roidal cham ber with ax ial magnetic field Fast neutrons The neutron detection temperature , also called

330-409: Is confined by the dipole magnetic field. Single particles corkscrew along the field lines of the dipole magnet at the cyclotron resonance frequency while completing poloidal orbits. The electron population was shown to have a peaked pressure and density profile as a result of the turbulent pinch phenomenon. There were two modes of operation observed: These had been proposed by Nicholas Krall in

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360-464: Is currently one of the leading candidates for a practical fusion reactor . The proposal to use controlled thermonuclear fusion for industrial purposes and a specific scheme using thermal insulation of high-temperature plasma by an electric field was first formulated by the Soviet physicist Oleg Lavrentiev in a mid-1950 paper. In 1951, Andrei Sakharov and Igor Tamm modified the scheme by proposing

390-459: Is far more expensive than deuterium. This is because tritium is rare. It has a short half-life making it hard to produce and store. It is also considered a hazardous material, increasing difficulties with storage and handling. Finally, tritium and deuterium produces fast neutrons which means any reactor burning it would require heavy radiation shielding for its magnets. As the floating dipole magnet cannot have services (such as cooling) connected from

420-423: Is the uranium-233 of the thorium cycle , which has a good fission/capture ratio at all neutron energies. Fast-neutron reactors use unmoderated fast neutrons to sustain the reaction, and require the fuel to contain a higher concentration of fissile material relative to fertile material (uranium-238). However, fast neutrons have a better fission/capture ratio for many nuclides, and each fast fission releases

450-546: The International Thermonuclear Experimental Reactor (ITER) effort emerged and remains the primary international effort to develop practical fusion power. Many smaller designs, and offshoots like the spherical tokamak , continue to be used to investigate performance parameters and other issues. As of 2024 , JET remains the record holder for fusion output, with 69 MJ of energy output over a 5-second period. The word tokamak

480-478: The Joint European Torus (JET) and Tokamak Fusion Test Reactor (TFTR), had the explicit goal of reaching breakeven. Instead, these machines demonstrated new problems that limited their performance. Solving these would require a much larger and more expensive machine, beyond the abilities of any one country. After an initial agreement between Ronald Reagan and Mikhail Gorbachev in November 1985,

510-462: The neutron energy , indicates a free neutron 's kinetic energy , usually given in electron volts . The term temperature is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is then adapted to the Maxwell distribution known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy of

540-402: The z-pinch and stellarator had attempted this, but demonstrated serious instabilities. It was the development of the concept now known as the safety factor (labelled q in mathematical notation) that guided tokamak development; by arranging the reactor so this critical factor q was always greater than 1, the tokamaks strongly suppressed the instabilities which plagued earlier designs. By

570-543: The 1960s. In the case of deuterium fusion (the cheapest and most straightforward fusion fuel) the geometry of the LDX has the unique advantage over other concepts. Deuterium fusion makes two products, that occur with near equal probability: In this machine, the secondary tritium could be partially removed, a unique property of the dipole. Another fuel choice is tritium and deuterium. This reaction can be done at lower heats and pressures. But it has several drawbacks. First, tritium

600-569: The Soviets invited British scientists from the laboratory in Culham Centre for Fusion Energy (Nicol Peacock et al.) to the USSR with their equipment. Measurements on the T-3 confirmed the results, spurring a worldwide stampede of tokamak construction. It had been demonstrated that a stable plasma equilibrium requires magnetic field lines that wind around the torus in a helix . Devices like

630-422: The base of the chamber was a charging coil. This coil is used to charge the dipole, using induction . Next, the dipole is raised into the center of the chamber using a launcher-rather system running through the bore of the dipole magnet. A copper magnet fixed on top of the chamber produced a magnetic field which attracted the floating dipole magnet. This external field would interact with the dipole field, suspending

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660-406: The dipole. The magnetic field produce by the floating dipole magnet is used to confine the plasma. The plasma forms around the dipole and inside the chamber. The plasma is formed by heating a low pressure gas using a radio frequency , essentially microwaving the plasma in a ~15-kilowatt field. The machine was monitored using diagnostics fairly standard to all of fusion. These included: The plasma

690-734: The fission cross section for fissile nuclei such as uranium-235 or plutonium-239 . In addition, uranium-238 has a much lower capture cross section for thermal neutrons, allowing more neutrons to cause fission of fissile nuclei and propagate the chain reaction, rather than being captured by U. The combination of these effects allows light water reactors to use low-enriched uranium . Heavy water reactors and graphite-moderated reactors can even use natural uranium as these moderators have much lower neutron capture cross sections than light water. An increase in fuel temperature also raises uranium-238's thermal neutron absorption by Doppler broadening , providing negative feedback to help control

720-429: The free neutrons. The momentum and wavelength of the neutron are related through the de Broglie relation . The long wavelength of slow neutrons allows for the large cross section. But different ranges with different names are observed in other sources. The following is a detailed classification: A thermal neutron is a free neutron with a kinetic energy of about 0.025 eV (about 4.0×10 J or 2.4 MJ/kg, hence

750-411: The mid-1960s, the tokamak designs began to show greatly improved performance. The initial results were released in 1965, but were ignored; Lyman Spitzer dismissed them out of hand after noting potential problems in their system for measuring temperatures. A second set of results was published in 1968, this time claiming performance far in advance of any other machine. When these were also met skeptically,

780-428: The mid-1970s, dozens of tokamaks were in use around the world. By the late 1970s, these machines had reached all of the conditions needed for practical fusion , although not at the same time nor in a single reactor . With the goal of breakeven (a fusion energy gain factor equal to 1) now in sight, a new series of machines were designed that would run on a fusion fuel of deuterium and tritium . These machines, notably

810-400: The neutron and scatter it. Ideally, a room temperature neutron moderator is used for this process. In reactors, heavy water , light water , or graphite are typically used to moderate neutrons. Most fission reactors are thermal-neutron reactors that use a neutron moderator to slow down (" thermalize ") the neutrons produced by nuclear fission . Moderation substantially increases

840-506: The outside world, this makes thermal management of the floating magnet much harder in a D-T machine. Tokamak A tokamak ( / ˈ t oʊ k ə m æ k / ; Russian : токамáк ) is a device which uses a powerful magnetic field generated by external magnets to confine plasma in the shape of an axially symmetrical torus . The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power . The tokamak concept

870-467: The reactor. When the coolant is a liquid that also contributes to moderation and absorption (light water or heavy water), boiling of the coolant will reduce the moderator density, which can provide positive or negative feedback (a positive or negative void coefficient ), depending on whether the reactor is under- or over-moderated. Intermediate-energy neutrons have poorer fission/capture ratios than either fast or thermal neutrons for most fuels. An exception

900-506: Was originally made of three coils. Each coil contained a 19-strand niobium-tin Rutherford cable (common in low-temperature superconducting magnets). These looped around inside an inconel structure; creating a magnet that looked like an oversized donut. The donut was charged using induction . Once charged, it generated a magnetic field for roughly an 8-hour period. Overall, the ring weighed 560 kilograms and levitated 1.6 meters above

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