As seen in Figure 2.2, the fusion of light elements also can be used for energy pro­duction. These thermonuclear processes provide the energy in stars (see Sections

6.2.4 and 6.2.5) and in the hydrogen bomb (see Section 7.5).

The potential of controlled thermonuclear reactions has been studied for several decades. These processes should provide the energy requirements of the Earth for a million years by the fusion of deuterium in the oceans. In addition, the fusion reac­tions produce no nuclear waste.

The thermonuclear reactions have two basic requirements. First, the temperature must be about 108 K because the ignition temperature of the 2H—2H reaction and the 2H—3H reaction are 3 X 108 and 3 X 107 K, respectively (Section 6.2.4). Second, the пт value, the so-called Lawson limit, must be higher than 1021 parti­cles s/m for the H— H reaction and 10 particles s/m for the H— H reaction, where п is the particle density and т is the confinement time. The Lawson limit indicates the ability of the plasma to retain heat. The two conditions depend on each other; that is, a given temperature needs a certain пт value.

There are two approaches to achieving a controlled thermonuclear reaction. A part of the reactors is based on the magnetic confinement of the hot (>108K) plasma containing the isotopes of hydrogen (deuterium and tritium). The most successful results with this method have been obtained in the Tokamak instrument, in Moscow. In this instrument, the plasma is toroid shaped. The other type of con­trolled thermonuclear reactor operates in pulsed mode (inertia confinements) when small pellets of solid deuterium and/or tritium are injected into a chamber and irra­diated by an intense beam of photons from lasers. Recently, there are experiments with the combination of the magnetic and inertia confinement.

The controlled thermonuclear reactors are in the experimental stage. Some examples of important experimental fusion reactors are JET (Joint European Torus, United Kingdom), DIII-D (USA, San Diego), EAST (Experimental Advanced Superconducting Tokamak, China), TFTR (Tokamak Fusion Test Reakctor, USA, Princeton), K-Star (Korea Superconducting Tokamak Advanced Research, South Korea), JT-60 (Japan Torus 60, Japan), TCV (Tokamak a configuration variable, Switzerland), and T-15 (Russian). The International Thermonuclear Experimental Reactor (ITER) in France is under construction. This reactor is scheduled to be operational in 2018. Its objectives are to demonstrate the feasibility of fusion power and to prove that it can work without negative impact. This includes to ignite self­sustaining plasma for at least 8 min, and to produce more than enough energy to ignite the fusion. Commercial reactors may be produced in the second part of the twenty-first century at the earliest. There are still many technical problems to be solved. For example, when heating to a suitably high temperature, the fuel sepa­rates from the walls of the vessels (no substances are able to withstand this temper­ature). In addition, the injection of fuel (deuterium and tritium) and the withdrawal of the product (helium), and the control of the fusion are problematic at this time.