Passage VI

Nuclear fusion is the combining of the nuclei (central cores) of atoms and can be used to produce energy. Differ­ent nuclei contain various numbers of protons (particles with a single positive electrical charge) and neutrons (uncharged particles).

A containment vessel holds a mixture of deuterium nuclei (containing 1 proton and 1 neutron) and tritium nuclei (containing 1 proton and 2 neutrons). (The density, n, is the number of nuclei/cm³; higher densities increase the frequency of collisions between nuclei.) The nuclei col­lide, fuse, and emit high amounts of energy. Because like­-charged particles repel each other, getting two nuclei close enough to each other to fuse requires that the nuclei be accelerated to high speeds, which consumes energy. Moreover, energy is consumed in preventing the nuclei from striking the walls of the containment vessel, where the nuclei would lose speed.

To produce more energy from fusion than is consumed, the temperature must be maintained above 4 x 10⁷ K for a minimum time, τ, and the product of n and τ must exceed 10¹⁴ sec/cm³ (a requirement called Lawson's criterion).

Two methods for confinement (preventing nuclei from colliding with vessel walls) and for producing nuclear fusion are discussed below.

The Magnetic Confinement Method

A magnetic force field compresses a gas of tritium and deuterium nuclei. Because excessive gas densities will cause the magnetic field to buckle, allowing the gas to escape confinement and to strike the walls of the containment vessel, a maximum n of about 10¹⁴ nuclei/cm³ is allowed. Therefore, in order to satisfy Lawson's criterion, a temperature of approximately 10⁸ K is maintained for about 1 sec.

The Laser Confinement Method

A pellet containing deuterium and tritium atoms is subjected to pulses of laser Iight, producing a gas with a density of about 10²³ nuclei/cm³. The laser pulses are shaped to compress and heat the gas to approximately 10⁸ K for about 10^-9 sec. Unfortunately, some of the laser energy is reflected rather than absorbed by the gas.

At present, the ratio of power (energy per unit time) produced to the power consumed by the Magnetic Confine­ment Method is higher than that by the Laser Confinement Method. However, because of the very high density of nuclei around the pellet, the power produced by the Laser Confinement Method can be higher than that of magnetic confinement.