Mimicking the nuclear reactions of the sun is no easy feat. Confining and controlling matter at high temperatures involves numerous scientific and engineering problems that are a long way from being solved. “We've made substantial progress, but we are nowhere near the Promised Land,” says Herbert Woodson, dean of the Center for Energy Studies at the University of Texasat Austin.
The first challenge in “controlled” fusion is to create a plasma, a super-hot state of matter in which orbiting electrons are stripped from atoms, leaving a gas composed of free electrons and atomic nuclei. The problem is that these nuclei have a positive charge, making them avoid each other. However, if the plasma is very hot and dense, light nuclei like those of deuterium, a heavy form of hydrogen, will smash into one another and fuse to form larger nuclei like helium, releasing a burst of energy in the process. The energy of the neutrons emitted can be captured as heat, which in turn could be used to generate electricity.
Only a small amount of plasma must be “burned” this way to produce large amounts of energy, in accordance with Albert Einstein's famous mass-to-energy formula, E = MC
Most often, controlled fusion seeks to join the nuclei of deuterium and tritium, another isotope of hydrogen. In a fusion reactor, the temperature must be considerably higher than that of the sun, in excess of 100 million degrees centigrade, since the fusion fuel undergoes far less compression than materials at the center of the sun.
The second, greater challenge is to confine this super-hot plasma, a task that fusion pioneer Edward Teller once described as “like trying to confine jelly with rubber bands.” Clearly, no earthly vessel is up to the task. Any kind of metal, ceramic or glass would quickly melt. So scientists at the outset of the quest turned to magnetism, creating increasingly powerful magnetic “bottles.”
Over the years, several magnetic confinement devices have been developed. The most successful method centers on the doughnut-shaped tokamaks, invented in the late 1960s by Andrei Sakharov and other Soviet scientists. These set up two magnetic fields, an external one and an internal one, that flow through charged particles to help keep them in place. The magnetic fields force the particles to circle around the hole in the doughnut.
Elaborated and modified, that is the approach still being taken at such state-of-the-art facilities as Princeton's Plasma Physics Laboratory (the Tokamak Fusion Test Reactor), the European Community's Joint European Torus (JET), and Japan's JT-60.
The immediate goal is to reach “scientific breakeven,” the point at which the amount of energy released in fusion reactions matches the amount that is pumped into the machine for plasma heating. All of the mentioned machines are on the verge of accomplishing that task. But the real work lies ahead for the next generation of fusion machines. Even though breakeven represents a historic scientific milestone, experts say it is still far short of “ignition,” the point at which sufficient heat is generated by fusion reactions to make them self- sustaining.
There is another less well-developed approach to heating and confining deuterium that some scientists feel might work much better than magnetic confinement. The method, called inertial confinement, uses lasers or particle beams (called drivers) to bombard tiny fuel pellets for a split second, heating and squeezing them to the point of ignition. There are no complex magnets. Instead, the fuel pellet is held together by its own inertia for a billionth of a second before being consumed and flying apart in a burst of energy.
Over the years, a series of increasingly large lasers have been built at Lawrence Livermore Laboratory to explore the feasibility of the method. Today its $200 million Nova laser, the size of a football field, has 10 beams to bombard tiny fuel pellets. But a more powerful laser is needed to spark a useful reaction.
The problems that must be solved in order to make inertial confinement work are no less formidable than the problems raised by tokamaks. Numerous lasers have to be able to focus energy on precisely the same point at precisely the same time, in pulses lasting a billionth of a second or less. The chamber in which the reactions take place has to withstand explosions equivalent to about 20 kilograms of high explosive, and as many as 20 of these explosions would be taking place per second.
Both approaches to fusion energy, inertial confinement and magnetic confinement, require technology that is not yet available: more efficient but less expensive lasers on the one hand, and more powerful but less energy-intensive magnets on the other. It's too soon to say which technology will mature faster, or whether either approach to fusion will be economically feasible in the future. The one thing that is clear, scientists say, is that if commercial fusion reactors ever come on line, they will probably be the most complex machines ever built.