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.

Energy that is released as a result of fission of heavy atom nuclei (e.g., uranium) in atomic reactors has been utilized already for a good many years. Another possible method of nuclear energy production is by the fusion of the nuclei of the lightest chemical elements. Fusion of this kind called nuclear (or thermonuclear) fusion may occur, for example, when in a mixture of the two gases deuterium and tritium (which are heavy hydrogen isotopes indicated by the symbols ²H and ³H respectively) the atom nuclei collide with one another with sufficiently high energy (i.e., with sufficiently high relative velocity) In such circumstances the electrostatic repulsion operating between the nuclei (which are positively charged) can be overcome, so that the colliding nuclei “fuse” together. This phenomenon is accompanied by the release of individual nuclear components with high kinetic energy.

The principal processes that may occur in a mixture of deuterium and tritium are:
D+D    =     ³He+n+3.25 MeV                  ³He+D     =      ⁴He+p+ 18.3 MeV
D+D     =     T+p+4MeV                           T+D        =    ⁴He+n+17.6MeV

D denotes a deuterium nucleus (deuteron), T a tritium nucleus (triton), p a proton and n a neutron, while 3He and He denote helium nuclei with mass numbers equal to 3 and 4, respectively. The energy that is released is expressed in mega-electron volts (1 MeV = 4.45 > 10-20 kWh).

There are many other possible ways of achieving the fusion of light atom nuclei. In the sun and other stars there occur, besides the fusion of hydrogen nuclei, complex fusion processes involving the participation of heavy nuclei. It is these processes that produce the vast amounts of energy that arc constantly radiated into space by the stars. The devastating effect of a hydrogen-bomb explosion is also due to the energy released by fusion processes of this type: with an atomic bomb as a detonator, an explosive or uncontrolled chain reaction of nuclear fusion processes is initiated. “Taming the hydrogen bomb” in the sense of bringing these reactions under control in a so-called fusion reactor would place a tremendous new source of energy at man’s disposal. Using pure deuterium as “fuel,” it would be possible to produce about 1025 kWh of energy from the quantity of heavy water (deuterium oxide) estimated to be present in the natural waters of the earth.

As already stated, fusion processes take place only when nuclei of atoms collide at high velocities. To achieve these, the gas serving as fuel must be heated to such extremely high temperatures T that the average energy kT of the particles (k denotes the Boltzmann constant) is of the order of magnitude of the potential wall U0. Since the potential wall is to a certain extent “permeable” to the particles whose energy is less than U_o (quantum-mechanical tunnel effect), nuclear reactions already take place at kinetic-energy values of 10 to 100 keV. Hence, sufficiently large numbers of nuclei can react with one another in “thermal collisions,’ if kT= 100 keV i.e., T =100 million degrees centigrade. Nuclear reactions caused by thermal collisions are generally called thermonuclear reactions.

Since each fusion results in the release of several MeV of energy, it is possible in principle to achieve a positive energy balance even when only a small proportion of the nuclei react with one another. If the energy that is released can be kept together for some length of time, spontaneous heating of the gas may be initiated. At these extremely high temperatures the gas employed is completely ionized: i.e., all the atoms have been split up into freely moving electrons and “naked” nuclei. The gas has thus become completely ionized plasma. Because of this, it can be compressed into a relatively small space and be contained there by the action of a sufficiently strong magnetic field (approx. 100 kilogauss);  (It is, of course, not possible to contain the plasma in any sort of material receptacle, as this would be vaporized at these high temperatures.) The escape of neutrons and radiation (more particularly the so-called “bremsstrahlung,”) inevitably causes losses of energy. A positive energy balance and therefore a continuation of the fusion processes can be achieved only if the energy of the electrically charged reaction products that are formed is able to make up for these losses. This is the case with a mixture comprising 50% deuterium and 50% tritium at temperatures of more than 50 million degrees centigrade (for pure deuterium it would require a temperature of 400 million degrees).