Tokamak 31439 224702198 2008-07-10T01:06:04Z Lightbot 7178666 Units/dates/other {{otheruse|this=the fusion reactor device|use1=other uses}} [[Image:Soviet Union stamp 1987 CPA 5891.jpg|thumb|150px|1987 [[USSR]] stamp, commemorating thermonuclear fusion research on Tokamak]] A '''tokamak''' is a machine producing a [[torus|toroidal]] [[magnetic field]] for [[Plasma equilibria and stability|confining]] a [[Plasma (physics)|plasma]]. It is one of several types of [[magnetic fusion energy|magnetic confinement devices]] and the most researched candidate for producing [[fusion power|fusion energy]]. The term '''Tokamak''' is a transliteration of the [[Russian language|Russian]] word '''Токамак''' which itself is an acronym made from the Russian words: "'''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 ('''Tochamac'''). It was invented in the 1950s by [[Soviet Union|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. fivefold) 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 [[nuclear fusion]] research 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 (tokamak)|T-3]] and its larger version [[T-4 (tokamak)|T-4]]. T-4 was tested in 1968 in [[Novosibirsk]], conducting the first ever quasistationary thermonuclear fusion reaction.<ref>[[Great Soviet Encyclopedia]], 3rd edition, entry on "Токамак", available online [http://slovari.yandex.ru/art.xml?art=bse/00079/49400.htm here] </ref> In 1968, at the third [[International Atomic Energy Agency|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 == [[Image:Tokamak fields lg.png|thumb|200px|right|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 electrical current to generate a ''poloidal'' magnetic field to contain the plasma along a linear axis between two points. Researchers discovered that plasmas confined in a ''toroidal'' shape (see figure; top graphic), in which the magnetic lines run parallel to the axis of the toroid, are prone to rapid instabilities and quickly lose confinement. The tokamak and the similar [[stellarator]] designs combine a ''poloidal'' field (see figure; the center graphic shows the poloidal field) with the ''toroidal'' field to stabilize the plasma, making sustained fusion burn feasible. The particles stream parallel (but not perpendicular) to the magnetic field; in a toroidal-poloidal magnetic field, particles twist in a helical path along the toroidal axis (see figure; bottom graphic). ==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 (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. ===Radio-frequency heating=== [[Image:Gyrotron plateforme.jpg|thumb|Set of hyperfrequency tubes (84 GHz and 118 GHz) for plasma heating by electron cyclotron waves on the [[Tokamak à configuration variable|Tokamak à Configuration Variable]] (TCV). Courtesy of CRPP-EPFL, Association Suisse-Euratom.]] High-frequency electromagnetic waves are generated by oscillators (often by [[gyrotron]]s or [[klystron]]s) 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=== <small> (in chronological order of start of operations) </small> * [[T-10 (tokamak)|T-10]], in [[Kurchatov Institute]], [[Moscow]], [[Russia]] (formerly [[Soviet Union]]); 2 MW; in operation since 1975 * [[Forschungszentrum_Jülich#TEXTOR_tokamak|TEXTOR]], in [[Jülich]], [[Germany]]; in operation since 1978 * [[Joint European Torus]] (JET), in [[Culham]], [[United Kingdom]]; 16 MW; in operation since 1983 * [[JT-60]], in [[Naka, Ibaraki|Naka]], [[Ibaraki Prefecture]], [[Japan]]; in operation since 1985 * [[Plasma Physics Laboratory (Saskatchewan)|STOR-M]], [[University of Saskatchewan]]; [[Canada]] in operation since 1987; first demonstration of alternating current in a tokamak. * [[Tore Supra]],<ref name=ToreSupra>[http://www-drfc.cea.fr/gb/cea/ts/ts.htm Tore Supra]</ref> at the [[Commissariat à l'Énergie Atomique|CEA]], [[Cadarache]], [[France]]; in operation since 1988 * [[Aditya (tokamak)|Aditya]], at [[Institute for Plasma Research]] (IPR) in [[Gujarat]], [[India]]; in operation since 1989 * [[DIII-D]],<ref name=DIII>[http://www.educatedearth.net/video.php?id=2763 DIII-D] (video)</ref> in [[San Diego]], [[United States|USA]]; operated by [[General Atomics]] since the late 1980s * [[COMPASS-D]],<ref name=cas/>, in [[Prague]], [[Czech Republic]]; in operation from 2008, previously operated from 1989 to 1999 in Culham, United Kingdom * [[Frascati Tokamak Upgrade|FTU]], in [[Frascati]], [[Italy]]; in operation since 1990 * [[Tokamak ISTTOK]], <ref name=Isttok>[http://www.cfn.ist.utl.pt/eng/Prj_Tokamak_main_1.html#intro ISTTOK]</ref> at the [[IPFN - Instituto Superior Técnico]], [[Lisbon]], [[Portugal]]; in operation since 1991 * [[ASDEX Upgrade]], in [[Garching bei München|Garching]], [[Germany]]; in operation since 1991 * [[Alcator C-Mod]],<ref name=Alcator>[http://www.psfc.mit.edu/research/alcator/ Alcator C-Mod]</ref> [[Massachusetts Institute of Technology|MIT]], [[Cambridge, Massachusetts|Cambridge]], USA; in operation since 1992 * [[Tokamak à configuration variable]] (TCV), at the [[EPFL]], [[Switzerland]]; in operation since 1992 *[[Tokamak Chauffage Alfvén Brésiliene|TCABR]], at the [[University of Sao Paulo]], [[Sao Paulo]], [[Brazil]]; this tokamak was transferred from [[Centre des Recherches en Physique des Plasmas]] in [[Switzerland]]; in operation since 1994. * [[HT-7]], in [[Hefei]], [[China]]; in operation since 1995 * [[Mega Ampere Spherical Tokamak|MAST]], in [[Culham]], [[United Kingdom]]; in operation since 1999 * [[National Spherical Torus Experiment|NSTX]] in [[Princeton, New Jersey]]; in operation since 1999 * [[Experimental Advanced Superconducting Tokamak|EAST]] (HT-7U), in [[Hefei]], [[China]]; in operation since 2006 ===Previously operated=== * 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 (tokamak)|CASTOR]],<ref name=cas>[http://www.ipp.cas.cz/Tokamak TOKAMAK DEPARTMENT]</ref> 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 * [[Electric Tokamak|UCLA Electric Tokamak]], in [[Los Angeles]], [[United States]]; in operation from 1999 to 2005 * Tokamak de Varennes; [[Varennes, Québec|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) * [[Small Tight Aspect Ratio Tokamak|START]] in Culham, United Kingdom; in operation from 1991 until 1998 ===Planned=== * [[KSTAR]], in [[Daejon]], [[South Korea]]; start of operation expected in 2008 * [[ITER]], in [[Cadarache]], [[France]]; 500 MW; start of operation expected in 2016 * [[SST-1]], in [[Institute for Plasma Research]] [[Gandhinagar]], [[India]]; 1000 seconds operation; currently being assembled<ref>[http://www.ipr.res.in/sst1/SST-1.html The SST-1 Tokamak Page<!-- Bot generated title -->]</ref> == See also == * The section on [[Plasma scaling#Dimensionless parameters in tokamaks|Dimensionless parameters in tokamaks]] in the article on [[Plasma scaling]] * [[Edge-Localized Mode]] ==Notes== {{reflist}} ==References== *{{cite book | last = Braams, C.M., Stott, P.E. | year = 2002 | title = Nuclear Fusion: Half a Century of Magnetic Confinement Research | publisher = Institute of Physics Publishing | id = ISBN 0-7503-0705-6 }} *{{cite book | last = Dolan | first = Thomas J. | year = 1982 | title = Fusion Research, Volume 1 - Principles | publisher = Pergamon Press | id = {{LCC|QC791.D64}} }} *{{cite book | last = Nishikawa, K., Wakatani, M. | year = 2000 | title = Plasma Physics | publisher = Springer-Verlag | id = ISBN 3-540-65285-X }} *{{cite book | last = Raeder | first = J. | coauthors = et al | year = 1986 | title = Controlled Nuclear Fusion | publisher = John Wiley & Sons | id = ISBN 0-471-10312-8 }} *{{cite book | last = Wesson | first = John | year = 2000 | title = The Science of JET | url = http://www.jet.efda.org/documents/wesson/wesson.html }} *{{cite book | last = Wesson | first = John | coauthors = et al | year = 2004 | title = Tokamaks | publisher = Oxford University Press | id = ISBN 0-19-850922-7 }} == External links== *[http://www-fusion-magnetique.cea.fr/gb/fusion/physique/sommaire.htm Plasma Science] - site on tokamaks from the French [[Commissariat à l'Énergie Atomique|CEA]]. *[http://fusion.gat.com/ Fusion programs] at [[General Atomics]], including the DIII-D National Fusion Facility, an experimental tokamak. *[http://ocw.mit.edu/OcwWeb/Nuclear-Engineering/22-012Spring-2006/CourseHome/index.htm Fusion and Plasma Physics Seminar] at MIT OCW *[http://www.iterfan.org Unofficial ITER fan club], Club for fans of the biggest tokamak planned to be built in near future. *[http://www.tokamak.info www.tokamak.info] Extensive list of current and historic tokamaks from around the world. *[http://www.youtube.com/watch?v=Pj-CR6TVABg] Overview video of a small scale tokamak concept. *[http://www.youtube.com/watch?v=VBkIikDfWb8] Section View Video of a small scale tokamak concept. *[http://www.youtube.com/watch?v=E2-Y8bYtvX4] Fly Through Video of a small scale tokamak concept. {{fusion methods}} [[Category:Fusion reactors]] [[Category:Russian loanwords]] [[Category:Science and technology in the Soviet Union]] [[Category:Russian inventions]] [[bg:Токамак]] [[cs:Tokamak]] [[de:Tokamak]] [[es:Tokamak]] [[fr:Tokamak]] [[it:Tokamak]] [[lt:Tokamakas]] [[hu:Tokamak]] [[nl:Tokamak]] [[ja:トカマク型]] [[no:Tokamak]] [[pl:Tokamak]] [[pt:Tokamak]] [[ru:Токамак]] [[sk:Tokamak]] [[sl:Tokamak]] [[tr:Tokamak]] [[zh:托卡马克]]