One of the best-known pieces of apparatus used in nuclear-fusion research and in efforts to utilize this phenomenon for purposes of practical energy production is Zeta. It operates on the principle of the transformer. The primary winding is of the usual type; a condenser bank is discharged through it. The secondary winding is formed by plasma which is produced in an annular tube (torus). Before the condensers are discharged, the gas in the torus (e.g., deuterium) at a pressure of l0 mm is slightly ionized that is, made electrically conductive by the action of high-frequency electromagnetic waves. As soon as the discharge through the primary winding commences, a powerful current (up to 200,000 amps.) is induced in this plasma. The charge carriers move in circular paths parallel to the wall of the torus. These “current filaments,” like all electric currents flowing in parallel paths, attract one another: The ring of plasma, which initially fills the entire space within the torus, contracts and becomes detached from the wall. This phenomenon is called “pinch effect.” The resulting compression of the plasma is attended with a considerable rise in temperature. At the same time, the degree of ionization increases so greatly that the plasma becomes completely ionized. In this way the conditions in which nuclear-fusion processes can occur are established.
With the aid of Zeta it has proved possible to keep the compressed plasma tube, the so-called “pinch,” stable for periods of a few milliseconds and to reach temperatures of 5 million degrees centigrade. It has not yet, however, proved possible to make the fusion process self-sustaining.
Another type of apparatus is the Stellarator. In this device the containment and the heating of the plasma take place independently of each other. In a torus of double-loop shape, a magnetic field is produced by an electric current flowing through a winding. The magnetic-field strength increases with the distance from the axis to the wall of the torus. The plasma is thereby kept away from the wall. Heating is affected on the transformer principle, just as in Zeta (electrical resistance produces heat in the plasma functioning as the secondary winding). However, this phenomenon generates a temperature of only about I million degrees centigrade, because at elevated temperatures the electrical conductivity of the plasma increases and the resistance therefore decreases. Yet another method of raising the temperature to very high values is based on periodic variation of a magnetic field by means of a booster coil. An alternating current flowing through this coil produces periodic increases and decreases in the density of the magnetic lines of force in the torus (this effect is called “magnetic pumping”). By choosing an appropriate frequency it is thus possible to supply energy more particularly to the nuclei (as distinct from the electrons), so that the bremsstrahlung losses due to the electrons are kept low.
In a third type of apparatus, the containment of the plasma is achieved by a magnetic field which is stronger at the (externally open) ends than in the middle. The regions of higher field strength act as “magnetic mirrors”: They are able to reflect plasma particles. This type of configuration for the containment of plasma in controlled-thermonuclear-reaction experiments is referred to as a “magnetic bottle.” The initially cold plasma is compressed and heated by rapid intensification of the magnetic field. Temperatures exceeding 10 million degrees centigrade are thus attained in very small regions of the plasma.