Tokamak


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tokamak

[′täk·ə‚mak]
(plasma physics)
A device for confining a plasma within a toroidal chamber, which produces plasma temperatures, densities, and confinement times greater than that of any other such device; confinement is effected by a very strong externally applied toroidal field, plus a weaker poloidal field produced by a toroidally directed plasma current, and this current causes ohmic heating of the plasma.

Tokamak

 

a closed magnetic trap, or magnetic bottle, of toroidal shape that is used for the generation and confinement of a high-temperature plasma. The name “Tokamak” is an acronym formed from the Russian words for “toroidal chamber with an axial magnetic field.” Such a device was first proposed in 1950 by I. E. Tamm and A. D. Sakharov as a means of achieving controlled thermonuclear fusion. Fundamental contributions to the development and study of Tokamak-type systems have been made by a group of Soviet scientists headed by L. A. Artsimovich, which in 1956 instituted a series of experimental investigations of such systems at the I. V. Kurchatov Institute of Atomic Energy.

The magnetic field that confines and stabilizes the plasma in a Tokamak is the sum of three fields: the field Hω generated by a current I induced along the plasma column; the much stronger toroidal field Hφ, which is parallel to the current; and the relatively weak transverse field H, which is directed parallel to the major axis of the torus. The field Hφ is produced by coils wound on the torus, and the field H is generated by conductors located along the torus. The lines of force of the overall magnetic field have the form of helices, which in running numerous times around the torus form a system of nested closed magnetic surfaces.

The plasma in a Tokamak is magnetohydrodynamically stable if the Kruskal-Shafranov condition is satisfied: Hφa/HωR > 1, where R is the major radius of the torus and a is the radius of the cross section of the plasma column. The transverse field HHωa/R is required to keep the plasma in equilibrium. The plasma is heated by the current that flows through it. Alternating magnetic fields and the injection of energetic neutral atoms are used to provide additional heating of the plasma.

The first quasi–steady-state thermonuclear reaction was obtained in 1968 with the T-4 Tokamak, which was built at the Institute of Atomic Energy. The parameters of the T-4 were as follows: a = 17 cm, R = 90 cm, Hφ = 3.5 × 104ergs, I = 1.5 × 105 amperes. The maximum attained plasma parameters were the following: temperature of deuterium ions, ~8 × 106°K; density of the ions, ~ 1014 cm–3; and time of plasma confinement, ~0.02 sec. During the early 1970’s the Tokamak systems took the world lead in research on controlled thermonulear fusion. A number of Tokamaks much larger than the T-4 had been constructed by 1976; examples are the T-10 in the USSR, the PLT and Alcator in the USA, and the TFR in France. A number of designs for thermonuclear reactors are based on Tokamak systems; the designs are scheduled for implementation at the end of the 20th century.

V. S. MUKHOVATOV

References in periodicals archive ?
Hirsch mentions, for example, superconducting magnets failing in accelerators, causing long downtimes for repair, and plasma-disruptive shutdown in tokamaks. This argument ignores the learning curve of technology improvement.
Prieur, Safety Factor Profile Control in a Tokamak, Springer, New York, NY, USA, 2014.
where [D.sub.max] and [D.sub.min], respectively, denote the diffusion coefficients of H-mode and L-mode, the parameters [v.sub.A], c, [B.sub.p], B, [alpha], [beta], and [g.sub.0] are constants, and [v.sub.A], c, [B.sub.p], and B are the Alfven velocity, the light velocity, the magnetic field which is parallel to the poloidal direction in Tokamak, and the characteristic magnetic field, respectively.
Along with NSTX--a complementary experiment at Princeton University--MAST is one of the world's two leading spherical tokamaks (STs).
Experiments using this technology to heat a high-density plasma in a Tokamak supplied by M.I.T.
At Tokamak Energy, we're creating a way to do it quicker by being a private, agile firm.
"Compact, high-field tokamaks provide another exciting opportunity for accelerating fusion energy development, so that it's available soon enough to make a difference to problems like climate change and the future of clean energy - goals I think we all share," Dennis Whyte, the head of the department of nuclear science and engineering at MIT, said in the statement.
As this analysis will show, tokamak fusion power will almost certainly be a commercial failure, which is a tragedy in light of the time, funds, and effort so far expended.
A group of scientists and engineers at the MIT Plasma Science and Fusion Center (PSFC) is about to publish a conceptual design paper of a small fusion pilot plant, using high temperature superconducting magnets, that would produce 270 Megawatts of electricity in a tokamak with a major radius about half the size of ITER, the fusion engineering test reactor currently under construction in France.
MAST on the other hand, pioneered a more spherical shaped Tokamak, which is more compact and requires much less magnetic field to maintain the stability of the plasma than the previous experiments.
High temperature superconductors have remarkable properties: they conduct electricity with zero resistance, even with simple cooling by liquid nitrogen; they can withstand high magnetic fields and huge current densities - and they continue to be superconducting throughout the plasma pulse in a tokamak.