Tokamak

A split image of the largest tokamak in the world, the JET, showing hot plasma in the right image during a shot.

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 comes from the Russian words: "тороидальная камера в магнитных катушках" (toroidal chamber in magnetic coils). It was invented in the 1950s by Igor Yevgenyevich Tamm and Andrei Sakharov.

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 was conducted after World War II, it was done under classified programs. 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 scientists from different countries allowed to collaborate.

In 1968, at the third IAEA International Conference on Plasma Physics and Controlled Nuclear Fusion Research at Novosibirsk, Russian scientists announced that they had achieved electron temperatures of over 1 keV (1 electron volt is equal to 11605 Kelvin) 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 of their previous 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.

Why doughnut shaped?

The distinctive shape of the fusion reactor is necessary in order to produce a magnetic field with as few irregularities as possible. The doughnut has a particular topological property that a sphere (for example) does not have. The problem is referred to as the hairy ball theorem. Imagine a sphere with hair growing out of it. The hair is analogous to the magnetic field lines needed in a fusion reactor. It turns out that it is impossible to comb hair on a sphere so that no hair sticks up. A strand of hair that is standing on end would be equivalent to an instability in the reactor. However, a hairy doughnut can be so combed, and thus adjustments to the magnetic field can be made to correct the irregularities. This allows the magnetic field to better confine the plasma.

Experimental tokamaks

In operation

• TFTR, Princeton University, USA; in operation from 1982 until 1997
• Joint European Torus, in Culham, United Kingdom; 16 MW; in operation since 1983
• JT-60, in Naka, Ibaraki Prefecture, Japan; in operation since 1985
• T-15, in Russia; 10 MW; in operation from 1988 until 2005
• Tore Supra [1], at the CEA, Cadarache, France; in operation since 1988
• D3D, in San Diego, USA; operated by General Atomics since the late 1980s
• START and MAST in Culham, United Kingdom; START in operation from 1991 until 1998, MAST in operation since 1999
• Alcator C-Mod, MIT; USA [2]; in operation since 1992
• HT-7 in Hefei, China; in operation since 1995; and HT-7U since 2006

Planned

• ITER, in Cadarache, France; 500 MW; start of operation expected in 2016
• EAST, in Hefei, China

See also

• The section on Dimensionless parameters in tokamaks in the article on Plasma scaling

External links

• Plasma Science - site on tokamaks from the French CEA.
• Fusion programs at General Atomics, including the DIII-D National Fusion Facility, an experimental tokamak.