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.