Seeking aneutronic fusion.

Seeking aneutronic nuclear fusion 

"Aneutronic' is a word that has not yet made its way into the dictionaries. It refers to processes of thermonuclear fusion that produce few or no neutrons. In energy-producing fusion reactors, aneutronic processes would have advantages in both safety and in ease of gathering the energy released. However, this breed has had low priority in the fusion research program funded for the last 40 years by the Department of Energy (DOE) and its predecessors. Now something of a push toward them seems to be developing.

Last week, the Committee on Advanced Fusion Power of the National Research Council's Air Force Studies Board issued a report advising the Air Force that research on aneutronic fusion processes is worth supporting as a possible answer to Air Force requirements both for electric current and for propulsion. As the report was issued, many of the interested scientists were gathered at the International Symposium on Feasibility of Aneutronic Power, meeting at the Institute for Advanced Study in Princeton, N.J.

The report was generally well received, although some people, particularly Bogdan Maglich of AELabs in Princeton, thought it too pessimistic in predicting how many years it would take to bring about practical aneutronic reactors.

Conventional fusion requires confining atomic nuclei at high density and high temperature. The easiest conditions of confinement and temperature, and therefore the ones sought first by the mainstream fusion program, are those for fusion of deuterium and tritium. However, the energy released in such a fusion is carried away by neutrons --dangerous, penetrating particles, which will yield their energy only by the inefficient means of heating something.

But in an aneutronic reaction (for example, deuterium and helium-3), the energy comes off with protons. Protons can be converted directly into electric current, or they can generate power in the form of radio waves. Protons are not very damaging or dangerous and so minimal shielding is necessary. However, in the jargon of the DOE, these substances are called "advanced' fuels, because the confinement and temperature conditions necessary for them go beyond those for deuterium-tritium.

Proponents of aneutronic fusion say that to the DOE "advanced' means far in the future or even in the hereafter. But Bruno Coppi of the Massachusetts Institute of Technology argues that experimentation with deuterium and helium-3 could be done in some current mainstream experiments--MIT's Alcator, for example. "You could make with today's technology an experiment that burns deuterium and helium-3,' he says. However, it lacks funding. Quoting the Swedish physicist Hannes Alfven, one of the grand old men of this kind of physics, Coppi says that there seems to be "a conspiracy not to do fusion.'

Instead of depending on more or less random encounters of nuclei that have been heated to overcome their repulsion for one another, as the mainstream experiments do, aneutronic systems like Maglich's "migma' use the principle of colliding beams, directing the nuclei into intersecting orbits, where they are more likely to encounter each other. "Our position is that the whole concept of heating to achieve collisions is obsolete,' he says.

The most recent migma experiment, Migma III, achieved confinement conditions that rival those of conventional experiments, and did it without the disruptive instabilities that plague conventional experiments (SN: 3/9/85, p.151). Migma IV, to be built in Palatka, Fla., in collaboration with the University of Florida at Gainesville, will attempt to increase the density of nuclei in the center of the experiment to 300 billion, 10 times that of Migma III, reaching the "space-charge limit,' the point where electric repulsions will prevent further crowding. It will test whether neutralizing some of the charge by introducing electrons will permit higher densities, and it will also test predictions that the resulting plasma should be stable under these conditions.

If deuterium-helium-3 fusion works out as a source of power, it will require a continuing supply of helium-3. (Deuterium can be obtained from sea water.) Although helium-3 is rare on earth, George Miley of the University of Illinois in Urbana-Champaign notes that it is "one of the most plentiful fuels we can find in the universe.' But we will have to go off the earth to get it.

On earth, the immediate source is radioactive decay of tritium, a by-product of nuclear fission reactors. According to Miley and the National Research Council, by the year 2000 we can obtain about 600 kilograms of helium-3 from tritium decay. This would run a 200-megawatt power plant for 20 years, "not enough for an economy,' says Miley.

Scientists would have to go to the moon and mine helium-3, which the solar wind generates on the lunar surface. Ultimately, when space travel is sophisticated enough, says Miley, we could get it from Jupiter.