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1987 USSR stamp, commemorating thermonuclear fusion research on Tokamak

A tokamak is a type of machine that uses a magnetic field to confine a plasma in the shape of a torus (donut). Achieving a stable plasma equilibrium requires magnetic field lines that move around the torus in a helical shape. Such a helical field can be generated by adding a toroidal field (traveling around the torus in circles) and a poloidal field (traveling in circles orthogonal to the toroidal field). In a tokamak, the toroidal field is produced by electromagnets that surround the torus, and the poloidal field is the result of a toroidal electric current that flows inside the plasma. This current is induced inside the plasma with a second set of electromagnets.

The tokamak is one of several types of magnetic confinement devices, and is one of the most-researched candidates for producing controlled thermonuclear fusion power. Magnetic fields are used for confinement since no solid material could withstand the extremely high temperature of the plasma. An alternative to the tokamak is the stellarator.

Tokamaks were invented in the 1950s by Soviet physicists Igor Yevgenyevich Tamm and Andrei Sakharov, inspired by an original idea of Oleg Lavrentyev[1].

The word tokamak is a transliteration of the Russian word токамак, an acronym of either "тороидальная камера с магнитными катушками" (toroidal'naya kamera s magnitnymi katushkami)—toroidal chamber with magnetic coils, or (toroidal'naya kamera s aksial'nym magnitnym polem)—toroidal chamber with axial magnetic field.[2]

Contents

History

Although nuclear fusion research began soon after World War II, the programs in various countries were each initially classified as secret. It was not until after the 1955 United Nations International Conference on the Peaceful Uses of Atomic Energy in Geneva that programs were declassified and international scientific collaboration could take place.

Experimental research of tokamak systems started in 1956 in Kurchatov Institute, Moscow by a group of Soviet scientists led by Lev Artsimovich. The group constructed the first tokamaks, the most successful being T-3 and its larger version T-4. T-4 was tested in 1968 in Novosibirsk, conducting the first ever quasistationary thermonuclear fusion reaction.[3]

In 1968, at the third IAEA International Conference on Plasma Physics and Controlled Nuclear Fusion Research at Novosibirsk, Soviet scientists announced that they had achieved electron temperatures of over 1000 eV in a tokamak device. This met with disbelief from British and American scientists, who were far from reaching that benchmark; they remained suspicious until laser scattering tests confirmed the findings a few years later.

Toroidal design

Tokamak magnetic field and current. Shown is the toroidal field and the coils (blue) that produce it, the plasma current (red) and the poloidal field produced by it, and the resulting twisted field when these are overlaid.

Positively charged ions and negatively charged electrons in a fusion plasma are at very high temperatures, and have correspondingly large velocities. In order to maintain the fusion process, particles from the hot plasma must be confined in the central region, or the plasma will rapidly cool. Magnetic confinement fusion devices exploit the fact that charged particles in a magnetic field feel a Lorentz force and follow helical paths along the field lines.

Early fusion research devices were variants on the Z-pinch and used electrical current to generate a poloidal magnetic field to contain the plasma along a linear axis between two points. Researchers discovered that a simple toroidal field, in which the magnetic field lines run in circles around an axis of symmetry, confines a plasma hardly better than no field at all. This can be understood by looking at the orbits of individual particles. The particles not only spiral around the field lines, they also drift across the field. Since a toroidal field is curved and decreases in strength moving away from the axis of rotation, the ions and the electrons move parallel to the axis, but in opposite directions. The charge separation leads to an electric field and an additional drift, in this case outward (away from the axis of rotation) for both ions and electrons. Alternatively, the plasma can be viewed as a torus of fluid with a magnetic field frozen in. The plasma pressure results in a force that tends to expand the torus. The magnetic field outside the plasma cannot prevent this expansion. The plasma simply slips between the field lines.

For a toroidal plasma to be effectively confined by a magnetic field, there must be a twist to the field lines. There are then no longer flux tubes that simply encircle the axis, but, if there is sufficient symmetry in the twist, flux surfaces. Some of the plasma in a flux surface will be on the outside (larger major radius, or "low-field side") of the torus and will drift to other flux surfaces farther from the circular axis of the torus. Other portions of the plasma in the flux surface will be on the inside (smaller major radius, or "high-field side"). Since some of the outward drift is compensated by an inward drift on the same flux surface, there is a macroscopic equilibrium with much improved confinement. Another way to look at the effect of twisting the field lines is that the electric field between the top and the bottom of the torus, which tends to cause the outward drift, is shorted out because there are now field lines connecting the top to the bottom.

When the problem is considered even more closely, the need for a vertical (parallel to the axis of rotation) component of the magnetic field arises. The Lorentz force of the toroidal plasma current in the vertical field provides the inward force that holds the plasma torus in equilibrium.

Plasma heating

In an operating fusion reactor, part of the energy generated will serve to maintain the plasma temperature as fresh deuterium and tritium are introduced. However, in the startup of a reactor, either initially or after a temporary shutdown, the plasma will have to be heated to its operating temperature of greater than 10 keV (over 100 million degrees Celsius). In current tokamak (and other) magnetic fusion experiments, insufficient fusion energy is produced to maintain the plasma temperature.

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Ohmic heating

Since the plasma is an electrical conductor, it is possible to heat the plasma by inducing a current through it; in fact, the induced current that heats the plasma usually provides most of the poloidal field. The current is induced by slowly increasing the current through an electromagnetic winding linked with the plasma torus: the plasma can be viewed as the secondary winding of a transformer. This is inherently a pulsed process because there is a limit to the current through the primary (there are also other limitations on long pulses). Tokamaks must therefore either operate for short periods or rely on other means of heating and current drive. The heating caused by the induced current is called ohmic (or resistive) heating; it is the same kind of heating that occurs in an electric light bulb or in an electric heater. The heat generated depends on the resistance of the plasma and the current. But as the temperature of heated plasma rises, the resistance decreases and ohmic heating becomes less effective. It appears that the maximum plasma temperature attainable by ohmic heating in a tokamak is 20-30 million degrees Celsius. To obtain still higher temperatures, additional heating methods must be used.

Neutral-beam injection

Neutral-beam injection involves the introduction of high-energy (rapidly moving) atoms into the ohmically-heated, magnetically-confined plasma. The atoms are ionized as they pass through the plasma and are trapped by the magnetic field. The high-energy ions then transfer part of their energy to the plasma particles in repeated collisions, increasing the plasma temperature.

Magnetic compression

A gas can be heated by sudden compression. In the same way, the temperature of a plasma is increased if it is compressed rapidly by increasing the confining magnetic field. In a tokamak system this compression is achieved simply by moving the plasma into a region of higher magnetic field (i.e., radially inward). Since plasma compression brings the ions closer together, the process has the additional benefit of facilitating attainment of the required density for a fusion reactor.

Set of hyperfrequency tubes (84 GHz and 118 GHz) for plasma heating by electron cyclotron waves on the Tokamak à Configuration Variable (TCV). Courtesy of CRPP-EPFL, Association Suisse-Euratom.

Radio-frequency heating

High-frequency electromagnetic waves are generated by oscillators (often by gyrotrons or klystrons) outside the torus. If the waves have the correct frequency (or wavelength) and polarization, their energy can be transferred to the charged particles in the plasma, which in turn collide with other plasma particles, thus increasing the temperature of the bulk plasma. Various techniques exist including electron cyclotron resonance heating (ECRH) and ion cyclotron resonance heating.

Tokamak cooling

The fusion reactions in the plasma spiraling around a tokamak reactor produce large amounds of high energy neutrons. These neutrons, being electrically neutral, are no longer held in the stream of plasma by the toroidal magnets and continue until stopped by the inside wall of the tokamak. This is a large advantage of tokamak reactors since these freed neutrons provide a simple way to extract heat from the plasma stream; this is how the fusion reactor generates usable energy. The inside wall of the tokamak must be cooled because these neutrons are at a high enough temperature to melt the walls of the reactor. A cryogenic system is used to prevent heat loss from the superconducting magnets. Mostly liquid helium and liquid nitrogen are used as refrigerants.[4] Ceramic plates specifically designed to withstand hot temperatures are also placed on the inside reactor wall to protect the magnets and reactor.

Experimental tokamaks

Currently in operation

(in chronological order of start of operations)

Previously operated

  • LT-1, Australia National University's plasma physics group built the first tokamak outside of Russia circa 1963
  • T-3, in Kurchatov Institute, Moscow, Russia (formerly Soviet Union);
  • T-4, in Kurchatov Institute, Moscow, Russia (formerly Soviet Union); in operation in 1968
  • Texas Turbulent Tokamak, University of Texas, USA; in operation from 1971 to 1980.
  • Alcator A and Alcator C, MIT, USA; in operation from 1975 until 1982 and from 1982 until 1988, respectively.
  • TFTR, Princeton University, USA; in operation from 1982 until 1997
  • CASTOR,[8] in Prague, Czech Republic; in operation from 1983 after reconstruction from Soviet TM-1-MH until 2006
  • T-15, in Kurchatov Institute, Moscow, Russia (formerly Soviet Union); 10 MW; in operation from 1988 until 2005
  • UCLA Electric Tokamak, in Los Angeles, United States; in operation from 1999 to 2005
  • Tokamak de Varennes; Varennes, Canada; in operation from 1987 until 1999; operated by Hydro-Québec and used by researchers from Institut de Recherche en Électricité du Québec (IREQ) and the Institut National de la Recherche Scientifique (INRS)
  • START in Culham, United Kingdom; in operation from 1991 until 1998
  • COMPASS in Culham; in operation until 2001

Planned

See also

Notes

  1. ^ Bondarenko B D "Role played by O. A. Lavrent'ev in the formulation of the problem and the initiation of research into controlled nuclear fusion in the USSR" Phys. Usp. 44 844 (2001) available online
  2. ^ Merriam-Webster Online
  3. ^ Great Soviet Encyclopedia, 3rd edition, entry on "Токамак", available online here
  4. ^ Tokamak Cryogenics reference
  5. ^ Ramos J., Meléndez L. et al., Diseño del Tokamak Novillo, Rev. Mex. Fís. 29 (4), 551, 1983
  6. ^ Tore Supra
  7. ^ DIII-D (video)
  8. ^ a b TOKAMAK DEPARTMENT
  9. ^ ISTTOK
  10. ^ Alcator C-Mod
  11. ^ ITER press release June 2008
  12. ^ The SST-1 Tokamak Page

References

  • Braams, C.M., Stott, P.E. (2002). Nuclear Fusion: Half a Century of Magnetic Confinement Research. Institute of Physics Publishing. ISBN 0-7503-0705-6. 
  • Dolan, Thomas J. (1982). Fusion Research, Volume 1 - Principles. Pergamon Press. LCC QC791.D64. 
  • Nishikawa, K., Wakatani, M. (2000). Plasma Physics. Springer-Verlag. ISBN 3-540-65285-X. 
  • Raeder, J.; et al (1986). Controlled Nuclear Fusion. John Wiley & Sons. ISBN 0-471-10312-8. 
  • Wesson, John (2000). The Science of JET. http://www.jet.efda.org/documents/wesson/wesson.html. 
  • Wesson, John; et al (2004). Tokamaks. Oxford University Press. ISBN 0-19-850922-7. 

External links


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