Keep cool with cold nuclear fusion.

Keep cool with cold nuclear fusion

Cold fusion, or muon-catalyzed fusion,is an unconventional approach to nuclear-fusion power that suddenly looks promising due to recent experimental surprises. On his way to a related experiment at the Rutherford Laboratory in Oxford, England, Steven E. Jones of Brigham Young University (BYU) in Provo, Utah, last week discussed the latest results at the National Bureau of Standards in Gaithersburg, Md.

Back in the 1940s, Russian physicistsAndrei Sakharov and F. C. Frank suggested that the subatomic particles called muons might be able to catalyze nuclear fusions, but it seemed at the time that the efficiency of the reaction, the number of fusions that an individual muon could accomplish, was too low to be practically interesting.

However, one of the recent surprisesshows that if the hydrogen isotopes deuterium and tritium are used as fuel, a resonance occurs that greatly raises the fusion efficiency of the muons. Experiments at the Los Alamos (N.M.) National Laboratory (LANL) have confirmed the existence of the resonance, as have others at the Swiss Institute for Nuclear Research (SIN) at Villigen and at the Japanese KEK laboratory. The latest results --achieved by a group from LANL, BYU and the Idaho National Engineering Laboratory at Idaho Falls--show an average of 150 fusions of deuterium and tritium nuclei catalyzed by a single muon rather than the one-per-muon or so that earlier predictions expected. This is still some way from the 1,200 fusions per muon that Jones calls "energetically interesting.'

Because atomic nuclei are allpositively charged, they repel each other. To make them fuse, that repulsion has to be overcome. Conventional methods heat the nuclei to millions of degrees or crush them together by implosion.

If muons are introduced into a gaseousmixture of deuterium and tritium at room temperature (300|K), they will replace electrons in the atoms. The resonance means that the energy balance is favorable for such a muonic-tritium atom to invade a molecule consisting of two deuterium atoms and replace one of the deuteriums. This makes a molecular ion of deuterium and tritium bound together by the muon orbiting them both. The large mass of the muon makes this molecular ion so small that the deuterium and tritium nuclei encounter each other and fuse. The fusion produces a helium nucleus and a free neutron that carries away energy.

Raising the temperature and manipulatingthe density and proportions of the fuel mix seem to be able to enhance the efficiency, but above a certain temperature the resonance disappears, and so, says Jones, "You can't make a bomb with muon-catalyzed fusion.'

Whether you can make an energy reactorwith it depends on yet a second question: What fraction of the muons stick to the helium nuclei that come out of the fusions? Those that do are out of the game and can't repeat the fusion cycle. Swiss results show 0.4 percent of the muons sticking, but U.S. results give fractions as low as 0.1 percent. A U.S. experiment just completed counted both bare helium nuclei and those with muons attached, and found a lot of bare ones but almost none with muons attached. If accurate, that argues for a very low sticking fraction. The Rutherford Laboratory experiment is trying to confirm this.

If sticking proves a surmountable problem,the third, and possibly last, question is the cost of the muons. They have to be made by particle accelerators, and a questioner wanted to know whether "to light up the Texas panhandle you would have to cover New Mexico with accelerators.' Probably not. In fact things look promising enough from that angle that Marshall Rosenbluth of the University of Texas at Austin has been looking at possible designs for a cold fusion reactor.