tokamak



A tokamak is a machine producing a toroidal (doughnut-shaped) magnetic field for confining a plasma. It is one of several types of magnetic confinement devices and the leading candidate for producing fusion energy.

The term Tokamak is a transliteration of the Russian word Токамак which itself is an acronym made from the Russian words: "тороидальная камера в магнитных катушках" (toroidal'naya kamera v magnitnykh katushkakh) — toroidal chamber in magnetic coils (Tochamac)). It was invented in the 1950s by Soviet physicists Igor Yevgenyevich Tamm and Andrei Sakharov (who were in turn inspired by an original idea of Oleg Lavrentyev).

The tokamak is characterized by azimuthal (rotational) symmetry and the use of the plasma current to generate the helical component of the magnetic field necessary for stable equilibrium. This can be contrasted to another toroidal magnetic confinement device, the stellarator, which has a discrete (e.g. five-fold) rotational symmetry and in which all of the confining magnetic fields are produced by external coils with a negligible current flowing through the plasma.

History

While research into nuclear fusion began soon after World War II, the programs were initially classified. 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 of them 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.[1]

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 stunned British and American scientists, who were far away from reaching that benchmark. They remained suspicious until tests were done with laser scattering a few years later, confirming the original temperature measurements.

Since this performance was far superior to any obtained in their existing devices, most fusion research programs quickly switched to using tokamaks. The tokamak continues to be the most promising device for generating net power from nuclear fusion, reflected in the design of the next generation ITER device.

Toroidal design

Enlarge picture
Tokamak magnetic field and current


Ions and electrons in the centre of 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 a poloidal field to contain the plasma (See figure; the center graphic shows the poloidal field). Researchers discovered that such plasmas are prone to rapid instabilities and quickly lose confinement. The tokamak introduces a toroidal field (see figure, top) that makes the plasma stable enough that sustained fusion burn is feasible. The particles can stream parallel (but not perpendicular) to this magnetic field, confining them on a toroidal surface.

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.

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. Tokamaks must therefore either operate for short periods or rely on other means of heating and current drive (although there are also other limitations on long pulses). 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.

Radio-frequency heating

Enlarge picture
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.
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.

Experimental tokamaks

Currently in operation

(in chronological order of start of operations)

Previously operated

  • 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
  • T-15, in Kurchatov Institute, Moscow, Russia (formerly Soviet Union); 10 MW; in operation from 1988 until 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

Planned

See also

Notes

1. ^ Great Soviet Encyclopedia, 3rd edition, entry on "Токамак", available online here
2. ^ CASTOR
3. ^ Tore Supra
4. ^ DIII-D (video)
5. ^ Alcator C-Mod
6. ^ [1]

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. 
  • Wesson, John; et al (2004). Tokamaks. Oxford University Press. ISBN 0-19-850922-7. 

External links


Fusion power
    [ e]
Atomic nucleus| Nuclear fusion| Nuclear power| Nuclear reactor| Timeline of nuclear fusion| Plasma physics| Magnetohydrodynamics| Neutron flux| Fusion energy gain factor| Lawson criterion
Methods of fusing nuclei
Magnetic confinement: – Tokamak – Spheromak – Stellarator – Reversed field pinch – Field-Reversed Configuration – Levitated Dipole
Inertial confinement: –
Laser driven – Z-pinch – Bubble fusion (acoustic confinement) – Fusor (electrostatic confinement)
Other forms of fusion: –
Muon-catalyzed fusion – Pyroelectric fusion – Migma – Polywell
List of fusion experiments
Magnetic confinement devices
ITER (International)|
JET (European)| JT-60 (Japan)| Large Helical Device (Japan)| KSTAR (Korea)| EAST (China)| T-15 (Russia)| DIII-D (USA)| Tore Supra (France)| TFTR (USA)| NSTX (USA)| NCSX (USA)| UCLA ET (USA)| Alcator C-Mod (USA)| LDX (USA)| H-1NF (Australia)| MAST (UK)| START (UK)| ASDEX Upgrade (Germany)| Wendelstein 7-X (Germany)| TCV (Switzerland)| DEMO (Commercial)
Inertial confinement devices
Laser driven:NIF (USA)|
OMEGA laser (USA)| Nova laser (USA)| Novette laser (USA)| Nike laser (USA)| Shiva laser (USA)| Argus laser (USA)| Cyclops laser (USA)| Janus laser (USA)| Long path laser (USA)| 4 pi laser (USA)| LMJ (France) | Luli2000 (France)| GEKKO XII (Japan)| ISKRA lasers (Russia)| Vulcan laser (UK)| Asterix IV laser (Czech Republic)| HiPER laser (European)
Non-laser driven: Z machine (USA)|
PACER (USA)

See also: International Fusion Materials Irradiation Facility
Tokamak Game Physics SDK is an open-source physics engine.

At its beginnings, Tokamak was free for non commercial uses only. Since May 2007, it has become open sourced under a BSD License.

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Tokamak features a unique iterative method for solving constraints.
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Tokamak can refer to:

In Physics:
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In Video Games:
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In geometry, a torus (pl. tori) is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle, which does not touch the circle. Examples of tori include the surfaces of doughnuts and inner tubes.
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magnetic field is a field that permeates space and which exerts a magnetic force on moving electric charges and magnetic dipoles. Magnetic fields surround electric currents, magnetic dipoles, and changing electric fields.
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An important field of plasma physics is the stability of the plasma. It usually only makes sense to analyze the stability of a plasma once it has been established that the plasma is in equilibrium. "Equilibrium" asks whether there are net forces that will accelerate any part of the plasma.
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plasma is typically an ionized gas. Plasma is considered to be a distinct state of matter, apart from gases, because of its unique properties. "Ionized" refers to presence of one or more free electrons, which are not bound to an atom or molecule.
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Magnetic confinement fusion is an approach to generating fusion energy that uses magnetic fields to confine the fusion fuel in the form of a plasma. Magnetic confinement is one of two major branches of fusion energy research, the other being inertial confinement fusion.
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Fusion power refers to power generated by nuclear fusion reactions. In this kind of reaction, two light atomic nuclei fuse together to form a heavier nucleus and release energy.
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Andrei Sakharov
Андрей Сахаров


Andrei Sakharov, 1943
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magnetic field is a field that permeates space and which exerts a magnetic force on moving electric charges and magnetic dipoles. Magnetic fields surround electric currents, magnetic dipoles, and changing electric fields.
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A stellarator is a device used to confine a hot plasma with magnetic fields in order to sustain a controlled nuclear fusion reaction. The magnetic field necessary to confine the plasma is completely generated by external coils.
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nuclear fusion is the process by which multiple atomic particles join together to form a heavier nucleus. It is accompanied by the release or absorption of energy. Iron and nickel nuclei have the largest binding energies per nucleon of all nuclei and therefore are the most stable.
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