The attainment of a sufficiently high reaction-driven energy density is a requirement of all energy systems. For fusion it is essential that the reactant nuclei attain a sufficiently high kinetic energy of relative motion in order to achieve substantial rates of exothermic reactions. These conditions must then be retained for a sufficiently long time in a specified reaction domain. Confining the interacting fuel particles at an appropriate high temperature is thus a most basic consideration of fusion energy systems.

3.2 Necessity of Confinement

Unlike fission reactions which involve a neutral reactant and thus do not experience repulsive effects, fusion reactants are positively charged and must overcome their electrostatic repulsion in order to get close enough for the strong nuclear forces of attraction to dominate. Hence, the essential condition for fusion is the requirement for a sufficiently high kinetic temperature of the reacting species in order to facilitate the penetration of the Coulomb barrier.

The attainment of ion energies in excess of this Coulomb barrier, which is about 370 keV for d-t fusion, poses little technical difficulty. For example, readily available medium-energy accelerators could be used to inject deuterons, of say Ed = 500 keV, into a tritiated target; surrounding neutron and alpha detectors could then be used to identify the reaction products as evidence of whether the reaction d + t—>n + a+17.6 MeV had taken place. Obviously, if each injected deuteron were to lead to d-t fusion, then the energy multiplication would be EoW / Ein = 17.6 / 0.5 = 35 and thus adequate for fusion energy utility purposes.

Theory suggests and experiment has confirmed that such a beam-target concept is totally inadequate for the following reasons: as beam deuterons enter a target they lose energy through the processes of ionization and heating the target. As discussed in the preceding chapter and displayed in Fig. 3.5, they are far more likely to scatter-rather than fuse-with an additional attendant energy loss by bremsstrahlung radiation. Thus, very quickly, the projectiles will have slowed down to energies far below the Coulomb barrier rendering further fusion reactions most unlikely. Thus, the overall fusion energy release can not exceed the energy required for beam acceleration. The futility of this approach was

recognized early in fusion energy research.

A more promising approach, however, soon emerged. One begins with a population of deuterium and tritium atoms in some confined space, and by heating one causes both ionization and the attainment of high temperature of the fuel ions. The resulting ensemble of positive and negative charges thus forms a plasma which is expected to attain thermodynamic equilibrium as a result of random collisions. The resultant spectrum of particle energies is then well described by a Maxwell-Boltzmann distribution with the high energy part of this distribution providing for most of the desired fusion reactions. Because the reaction activation occurs here due to random thermal motion of the reacting nuclei, this process is therefore called thermonuclear fusion. The critical technical requirement is the sustainment of a sufficiently stable high temperature (~108 K) plasma in a practical reaction volume and for a sufficiently long period of time to render the entire process energetically viable. Confinement of the fuel ions by some means is thus crucial to maintain these conditions within the required reaction volume.

We add that in contrast to this high-temperature approach to fusion, there exists also a low-temperature approach free of the above type of confinement problems. As we will show in Ch. 12, confinement by atomic-molecular effects may also be exploited.