A tokamak is a machine producing a toroidal 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 is an acronym made from the Russian words: "тороидальная камера в магнитных катушках" (toroidal'naya kamera v magnitnykh katushkakh) — toroidal chamber in magnetic coils (Tochamac)). It was invented in the 1950s by 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 rotational symmetry and in which all of the confining magnetic fields are produced by external coils with a negligible current flowing through the plasma.
Ions and electrons in the center 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 a poloidal field to contain the plasma . Researchers discovered that such plasmas are prone to rapid instabilities and quickly lose confinement. The tokamak introduces a toroidal field that makes the plasma stable enough that sustained fusion burn is feasible. The particles can stream parallel (but not perpendicular) to this magnetic field, confining them on a toroidal surface.
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).
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.
High-frequency electromagnetic waves are generated by oscillators (often by gyrotrons or klystrons) 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.
Tokamak Fusion Generator 2016-10-05
Tokamak Fusion Generator