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 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 (who were in turn inspired by an original idea of O.A. Lavrent'ev).

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 1000 eV in a tokamak device (1 electronvolt is equal to 11605 kelvins). 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

Ions and electrons in a fusion plasma are at very high temperatures, and correspondingly have very significant velocities. In order to produce continuous fusion reactions, a fusion device must somehow ensure that the hot plasma does not lose its particles (and therefore its heat) at too rapid a rate, a goal known as confinement. Magnetic confinement fusion devices exploit the fact that charged particles in a magnetic field feel a Lorentz force and fall into helical paths along the field lines.

In the early days of fusion research, the devices used were variations on the Z-pinch, which aimed to use a poloidal field to contain the plasma. (See figure; the center graphic shows the poloidal field.) Researchers discovered that such plasmas are prone to many instabilities and quickly lose confinement. The tokamak introduces a toroidal field (see figure, top) that effectively "stiffens" the plasma against instability. (In practice, however, numerous instabilities occur, some of which are not yet fully understood.)

An aside: 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 hairs cross over each other (or are not parallel to each other). A strand of hair that crosses another would be equivalent to an instability in the reactor. However, a hairy doughnut can be so combed by combing along the circumference and with a slight twist, and thus adjustments to the magnetic field can be made to correct the irregularities. This allows the magnetic field to better confine the plasma.

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, or instabilities prevent extended operation. Consequently, the devices operate in short pulses and the plasma must be heated afresh in every pulse.

Ohmic heating

Since the plasma is an electrical conductor, it is possible to heat the plasma by passing a current through it; in fact, the current that generates the poloidal field also heats the plasma. This 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 the 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 (neutral) atoms into the ohmically-heated, magnetically-confined plasma. The atoms are immediately ionized and are trapped by the magnetic field. The high-energy ions then transfer part of their energy to the plasma particles in repeated collisions, thus 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

High-frequency electromagnetic waves are generated by oscillators (specifically, often by gyrotrons or klystrons) outside the torus. If the waves have a particular frequency (or wavelength), 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. This technique is also called electron cyclotron resonance heating or ECRH.

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 ^*, 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 ^*; in operation since 1992
HT-7 in Hefei, China; in operation since 1995; and HT-7U (EAST) since 2006
FTU in Frascati, Italy; in operation since 1990
TCV, EPFL, Switzerland; in operation since 1992
Aditya , in Gujarat, India; in operation since 1989 Institute for Plasma Research, IPR
STOR-M, University of Saskatchewan; Canada in operation since 1987 first demonstration of alternating current in a tokamak.

Planned

ITER, in Cadarache, France; 500 MW; start of operation expected in 2016

References

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.

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