Aneutronic fusion

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From this weeks economist

LIKE conquistadors seeking El Dorado, physicists cannot leave the idea of fusion power alone. Some spend billions of dollars of taxpayers’ money on the huge machines they believe are the best way to generate the temperatures and pressures needed to persuade atomic nuclei to merge with one another. Others still think there is something to the idea of “cold” fusion, and tinker hopefully with desktop apparatus full of electrodes made from exotic metals and electrolytes containing obscure isotopes of hydrogen.

Eric Lerner, however, believes there is a third way. His experimental device does not quite fit on a desktop (its sides are a couple of metres long) but nor does it cost billions (a few hundred thousand is closer to the mark). Nor, in truth, does it do fusion yet. But on October 20th he announced it had reached what might be seen as base camp on the climb to that goal.

Mr Lerner’s machine is called a dense plasma focus fusion device. It works by storing charge in capacitors and then discharging the accumulated electricity rapidly through electrodes bathed in a gas held at low pressure. The electrodes are arranged as a central positively charged anode surrounded by smaller negatively charged cathodes.

When the capacitors are discharged, electrons flow through the gas, knocking the electrons away from the atomic nuclei and thus transforming it into a plasma. By compressing this plasma using electromagnetic forces, Mr Lerner and his colleagues at Lawrenceville Plasma Physics, in New Jersey (the firm he started in order to pursue this research) have created a plasmoid. This is a tiny bubble of plasma that might be made so hot that it could initiate certain sorts of fusion. The nuclei in the plasmoid, so the theory goes, would be moving so fast that when they hit each other they would overcome their mutual electrostatic repulsion and merge. If, of course, they were the right type of nuclei.

For the test run, Mr Lerner used deuterium, a heavy isotope of hydrogen, as the gas. This is the proposed fuel for big fusion reactors, such as the $12 billion International Thermonuclear Experimental Reactor being built at Cadarache in France and the $4 billion National Ignition Facility at Livermore, California. It is not, however, what he proposes to use in the end. In fact his trick (and the reason why it might be possible to produce a nuclear reaction in such a small piece of apparatus) is that what he does propose is not really fusion at all. Rather, it is a very unusual form of nuclear fission. Normal fission involves breaking uranium or plutonium atoms up by hitting them with neutrons. The reaction Mr Lerner proposes would break up boron atoms by hitting them with protons (the nuclei of normal hydrogen atoms). This process is known technically, and somewhat perversely, as aneutronic fusion. The reason is that the boron and hydrogen nuclei do, indeed, fuse. But the whole thing then breaks up into three helium nuclei, releasing a lot of energy at the same time. Unlike the sort of fusion done in big machines, which squeeze heavy hydrogen nuclei together, no neutrons are released in this reaction.

From an energy-generation point of view, that is good. Because neutrons have no electric charge they tend to escape from the apparatus, taking energy with them. Helium nuclei are positively charged and thus easier to rein in using an electric field, in order to strip them of their energy. That also means they cannot damage the walls of the apparatus, since they do not fly through them, and makes the whole operation less radioactive, and thus safer.

The plasmoids Mr Lerner has come up with are not yet hot enough to sustain even aneutronic fusion. But he has proved the principle. If he can get his machine to the point where it is busting up boron atoms, he might have something that could be converted into a viable technology—and the search for El Dorado would be over.
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