A machine used in nuclear fusion research. A helical system of magnetic fields confines the plasma of reactive charged particles in a hollow doughnut-ring-shaped chamber, where it is then heated by passing an electric current through it, and additional methods using radio-frequency fields and ion beams, to temperatures in excess of 108°C.
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 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. 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.
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).
Since this performance was far superior to any obtained in their existing devices, most fusion research programs quickly switched to using tokamaks.
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 a too rapid 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. 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. The hair is analogous to the magnetic field lines needed in a fusion reactor. 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. 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.
Neutral-beam injection
Neutral-beam injection involves the introduction of high-energy (neutral) atoms into the ohmically-heated, magnetically-confined plasma. 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.
Experimental tokamaks
Currently in operation
(in chronological order of start of operations)
T-10, in Russia; in operation since 1988 Aditya, at Institute for Plasma Research (IPR) in Gujarat, India; in operation since 2006
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