tokamak

An increasingly common term for a fusion engine, particularly in science fiction. This latter may give away the fact that fusion power is not yet a practical proposition. However, the tokamak reactor is the most popular design in recent avenues of research.

Strictly speaking, tokamak is actually a description of the chamber which holds the plasma. The word tokamak is derived from the Russian words "TOroidalnaya KAmera ee MAgnitnaya Katushka," meaning "Toroidal Chamber and Magnetic Coil". The hot, charged plasma is held in place by a magnetic field, keeping it away from the physical container. The toroidal design avoids the difficulties inherent in older designs. These tended to be a simple cylinder, requiring stronger magnetic fields at the ends to "pinch" the plasma to keep it inside the tube.

Of course, for a practical fusion power source, there are still the questions of initiating the reaction and of drawing the power.

See http://library.thinkquest.org/17940/texts/magnetic_confinement/magnetic_confinement.html for pictures.

A tokamak fusion reactor uses a toroidal coil array to create a magnetic field capable of containing plasma at the temperatures required for fusion. As well as a primary toroidal field, transformers are used to induce a current in the plasma itself, creating a poloidal magnetic field (at right angles to the toroidal magnetic field) the two fields combine to create a total helical magnetic field, which spirals round the toriod plasma.Heating is acheived by passing a current through the plasma, neutral beam heating, or radio frequency heating.

The original reason for the use of the toroidal reactor shape was because it was the most efficient way for the plasma to be contained, as it did not require "end" fields. Tokamak fusion reactors include JET and the planned future energy-producing ITER.