At the University of California at Berkeley, the Department of Nuclear Engineering operates a specialized machine that can accelerate deuterium ions up to four percent of the speed of light. These ions are created in the accelerator by removing the electrons from atoms of deuterium, the heavy isotope of hydrogen pictured below in Fig. 1. With its electron gone, each of the deuterium ions has a positive charge, equal and opposite to the charge of a single electron. The ions are created in a special vessel, maintained at a high vacuum and mounted on top of a stack of ceramic insulators like those used to isolate high-voltage power lines. This electrical insulation system is vital, because large transformers keep the vacuum vessel charged to a positive potential of 400,000 volts, thousands of times greater than the voltage of home electrical systems.

A vacuum tube comes out horizontally from the vacuum vessel and disappears into a small room with heavy concrete walls ten feet away. What then happens to the deuterium ions makes takeoff from an aircraft carrier seem trivial--in a distance of a few feet, the ions are accelerated violently to a velocity of 4,400 kilometers per second (25,000,000 mph). Upon reaching the end of the vacuum tube, each ion has a kinetic energy of 400,000 electron volts (eV). This is a tiny amount of energy (only 6 x 10^-14 joules), but when concentrated in the tiny mass of a single deuterium ion (3.3 x 10^-27 kilograms), it becomes enough energy to do extraordinary things.

Fig. 1 - The isotopes of interest for deuterium-tritium (D-T) fusion, consisting of various combinations of neutrons and protons .

At the end of the vacuum tube the beam of high-energy deuterium ions hits a rotating metal disk, coated with a thin layer of titanium, that holds a small amount of tritium in tight chemical bonds. At 4,400 kilometers per second, the deuterium nuclei can slam into tritium atoms, and with their very high velocity, the deuterium nuclei can approach very close to the nuclei of the tritium atoms, even though the positive electrical charges of the deuterium and tritium nuclei repel each other strongly. Here the unique characteristic of the deuterium-tritium fusion reaction, and its sister reaction of ³He and deuterium, allows fusion to occur. Unlike the majority of the fusion reactions that power the sun, like fusion of hydrogen with hydrogen and deuterium with deuterium, a quantum-mechanical quirk called tunneling allows deuterium and tritium to fuse together at a much larger separation distance than most fusion reactions. Thus only 110,000 eV of kinetic energy is needed to get deuterium and tritium nuclei close enough together to allow fusion, a factor of ten less than the other fusion reactions. With 400,000-eV deuterium ions, this D-T fusion reaction occurs six-trillion times per second in the basement of Etcheverry Hall, when the Rotating Target Neutron Source (RTNS) is operating.

Fig. 2 - The D-T fusion reaction in motion.

When deuterium and tritium fuse, for a brief instant they form ⁵He, and then burst apart as seen above in Fig. 2, with a ⁴He helium nucleus with 3.5 MeV (1 MeV = 1 million eV) of kinetic energy traveling off in one direction, and a neutron with four times as much energy, 14.1 MeV, going off in the other. With its positive charge the helium ⁴He nucleus, more commonly called an alpha particle, interacts strongly with surrounding material and stops rapidly, depositing 3.5 MeV of heat close to the site of the fusion reaction. The neutron, which has no charge, can only slow down by colliding with other nuclei, transferring small amounts of kinetic energy to each just as a cue ball when it hits a pool ball, until finally the neutron is absorbed by some atom's nucleus, potentially several meters from the original fusion reaction. In an IFE power plant, the neutrons stop in a thick blanket of material that contains ⁶Li lithium, the material that breeds the tritium that provides the fusion fuel. Nuclear processes in the blanket generate another 3.2 MeV of energy for each fusion reaction.

Even at six-trillion fusion reactions per second, the RTNS makes less than 2 watts of fusion power, while consuming thousands of times as much electricity. How then to sustain fusion reactions on the Earth?

High temperatures provide the key to driving fusion. In gases, temperature provides a measure of the kinetic energy stored in the random motion of atoms, and the higher the temperature, the higher the velocities that the atoms reach. Heating gases above a few thousand degrees generates collisions strong enough to strip electrons from nuclei, transforming the gas into an ionized plasma. At extremely high temperatures, tens of millions of degrees, some of the nuclei can reach speeds as high as the deuterium nuclei in the RTNS, and collisions of deuterium and tritium nuclei can begin to cause fusion reactions. Further increases in plasma temperature can increase the fraction of the collisions with energies greater than 110,000 eV, and the fraction of collisions resulting in D-T fusion. Fusion reactions in turn create high-velocity alpha particles, which stop and deposit their kinetic energy, heating the surrounding plasma. If the rate of alpha-particle heating equals the rate at which energy leaks from the plasma then ignition has been achieved and a self-sustaining fusion burn occurs.

While a campfire emits most of its radiant energy in the infrared spectrum, which we can't see but can feel as a warm glow on our skin, and while the tungsten filament of a light bulb reaches temperatures high enough to emit considerable radiant energy in the visible spectrum, a fusion plasma is so hot that it emits x rays. This energy loss makes it challenging just to heat a plasma to a temperature high enough to cause fusion, much less to achieve fusion reaction rates high enough to keep the plasma hot after outside heating stops. The fusion reaction rate depends not only on the temperature, which controls the probability that collisions can cause fusion, but also on the plasma density. High density--more nuclei in a given volume--means a shorter average travel distance between collisions and therefore more collisions for each nuclei. Thus the goal in fusion energy production is to generate plasmas with sufficiently high densities and sufficiently high temperatures to achieve ignition. In inertial confinement fusion, this goal is achieved by compressing a very small mass of fusion fuel, a few millionths of a kilogram, to densities 30 times the density of lead, while simultaneously heating the center of the fuel to the hundred-million degree Celsius temperature needed to ignite a propagating fusion burn.