Migma: an approach to neutron-free fusion.

Nuclear fusion, according to its proponents, will be the ultimate cheap-fuel energy source, an answer to the world's energy problems -- if they can make it work. Although significant progress has been made in recent years, development has been much slower than the first proponents of fusion hoped when they began 40 years ago.

About 14 years ago, at a meeting of the American Physical Society, physicist Bogdan Maglich presented an unorthodox method of approaching fusion. At the time, other physicists were quite skeptical. Now, in the Feb. 25 PHYSICAL REVIEW LETTERS, Maglich and co-workers report a significant achievement in what they call "aneutronic fusion--"a word so new it is not yet in any dictionary." Other physicists are still somewhat reserved.

In principle, "conventional" magnetic fusion experiments involve the formation of a plasma (consisting of atomic nuclei and electrons) by ionizing a gas. This plasma is then confined by a suitably shaped magnetic field and heated to a temperature at which significant numbers of fusions occur. In practice, magnetic fields do not confine very well. Instabilities in the plasma's behavior tend to build up until they enable the plasma to break out of confinement. So the race is to hold of plasma at least long enough for a useful number of fusions to occur.

In conventional experiments the plasma is heated so that the nuclei gain enough energy to overcome the electrical repulsion between them and so are able to fuse. In Maglich's scheme, which he calls a migma (from the Greek word for mixture), the nuclei gain energy not by heating but by being accelerated in a linear accelerator. In the current experiment, deuterons -- the nuclei of deuterium, an isotope of hydrogen--come out of the accelerator with 0.7 million electron-volts energy, the equivalent of heating to 7 billion kelvins. They also have a directed motion rather than the random motions of a thermally energized plasma. Therefore, a magnetic field can be set up in the migma cell, as they call the vessel they use, that forces the nuclei into self-intersecting orbits that form a kind of rosette around the center of the field. Orbits of this kind provide many opportunities for nuclei to encounter each other and fuse.

The confinement time in this experiment, 20 seconds, was less than the 60 seconds of the DCX-1 device, a conventional experiment chosen for comparison. But the triple product of energy, confinement time and density--the three critical parameters -- is 10 to 20 times that of DCX-1, and none of the typical instabilities formed.

The word "aneutronic" comes from the reaction they ultimately hope to use, in which hydrogen and lithium fuse to helium with two protons left over. The easiest reaction (and the goal of most conventional experiments) is deuterium and tritium yield helium plus a leftover neutron. The neutron is a penetrating and potentially damaging particle. The protons from the lithium reaction, being electrically charged, are easy to capture and not damaging. Energy is harvested from these leftover particles and that, too, is easier with charged particles.

According to James Nering of United Sciences, Inc., in Princeton, N.J., the organization Maglich and co-workers formed to do these migma experiments, the present experiment used deuterons because they make measuring and following the action easier.

In 1973, when Maglich first presented the idea, the physics community reacted so skeptically that he could not get funds from the Department of Energy, which funds most of the U.S. fusion program. He did, however, obtain $20 million in private funds from sources in Japan, Switzerland, Saudi Arabia and the United States. Now, according to an announcement by United Sciences, money is included in the Defense Appropriation Bill for fiscal year 1985 for a study of migma's space applications.