Facts of Tokamaks [part 1]

Tokamaks

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The tokamak is the most successful device developed so far to attain the conditions for fusion. It is a toroidal device (shaped like a car tire) in which a vacuum vessel contains a plasma ring confined by twisting magnetic fields.

Note - the word tokamak is an acronym for the Russian words toroidal'naya kamera magnitnoi katushki, meaning toroidal chamber and magnetic coil.

Tokamak configuration - The transient electric current that circulates in the primary coil of a tokamak induces a current in the plasma ring, which both heats the plasma and produces the poloidal magnetic field. The other important component is the toroidal magnetic field, which is generated by electric currents circulating in the toroidal field coil rings around the torus. In addition, the currents circulating in the position control coils generate auxiliary magnetic field components that modify the poloidal field, equilibrating the plasma ring and controlling its position. It is the combination of toroidal and poloidal magnetic fields that leads to the improved confinement of tokamak plasmas.

Main components of the tokamak type magnetic confinement system.

Plasma heating - The most efficient way to heat a tokamak plasma is by passing through it a current induced by the primary coil. This coil is the primary circuit of a transformer in which the plasma ring constitutes the secondary circuit. It works like an electric heater, the amount of heat generated depending on the current and the resistance of the plasma. Unfortunately, the plasma resistivity decreases as the temperature rises and the heating process becomes less effective. The maximum temperature that can be achieved in tokamaks by the resistive heating (or ohmic heating) method is about 3×10⁷ K, twice the temperature in the center of the sun but less than needed to startup a reactor, about 10⁸ K. In tokamak experiments auxiliary heating is used to reach temperatures currently as high as 5×10⁸ K (more than 30 times the temperature at the sun-center). The two main methods of additional heating is by the injection of high-energy neutral particle beams and radiofrequency waves of various types.