SPACE WIRE
Japanese-British team sketch new path to dream of nuclear fusion
PARIS (AFP) Aug 22, 2001
Japanese and British physicists reported Thursday they had carried out experiments pointing to a new way of achieving nuclear fusion, the elusive goal of cheap, abundant and safe energy.

Fusion entails the release of energy from combining two heavy forms of hydrogen -- deuterium and tritium, which can be garnered from plentiful, naturally-occurring resources.

Harnessing this power has obsessed scientists for more than a quarter century, a period in which oil shocks, the Chernobyl disaster and global warming have driven home the problems of fossil fuels and nuclear fission.

But the vision has always been clouded by technical problems. So far, fusion has only been achieved for a maximum of one second in laboratory conditions, and with a disappointingly low energy return.

The phenomenon is sparked at temperatures beyond 100 million C.million F.), which means a lot of energy is needed to get it going.

But in research published in Nature, the British science weekly, a 20-member team led by Ryosuke Kodama of Japan's Osaka University report on an innovation that, they hope, may get round some of the biggest problems.

Their approach enhances a technique called inertial confinement fusion (ICF).

Under conventional ICF, a solid sphere of deuterium and tritium just a few millimetres in diameter is compressed and ignited by a laser beam, and the reaction of the two hydrogen isotopes causes a burst of energy.

The idea is that the energy chamber is continuously fed with these tiny fuel balls to produce repeated micro-explosions, rather like a car's internal combustion engine.

But the little spheres have to be extremely smooth and accurately machined for this to work; fusion is only possible using ultra-powerful and precise lasers; and the energy yield is disappointingly low.

Kodama's so-called "fast ignition" technique uses a hollow fuel ball placed next to a hollow metal cone.

The first step is to direct laser beams at the ball for one nanosecond, causing it to implode into a cloud of charged particles called plasma.

As the compression peaks, another, more powerful laser sends a pulse inside the metal cone, directed towards its apex.

Its energy focused by the cone, the beam ignites the plasma and sets off the fusion reaction.

Because the sphere is hollow, much less energy is needed to ignite it compared with the traditional ICF method.

In theory, the yield -- the energy output from the fusion compared with the energy needed to cause it -- could be as much as 300 to one.

That compares with hopes of achieving a yield of 15 to one when stadium-sized facilities, called tokamaks, are completed in France and the United States.

Kodama's experiment comprised a polystyrene shell coated with deuterium as the fuel, and gold for the hollow cone.

"Our approach... permits efficient compression and heating to be carried out simultaneously, providing a route to efficient fusion energy production," his team said.

Although his is a small-scale test and many more experiments are needed, the results are "promising," commented Michael Key, a fusion expert at Lawrence Livermore National Laboratory in California.

"Stepping up to megajoule laser drivers, it might even lead to a 300-fold energy gain, and could initiate serious efforts worldwide to produce fusion energy by fast ignition," Key told Nature. He cautioned, however, that the history of fusion energy was littered with false dawns.

Deuterium is a hydrogen isotope found in water, and is thus an almost limitless resource, while tritium is derived from lithium, a light metal which is abundant in the Earth's crust.

Besides ICF, the other fusion method being explored entails confining deuterium-tritium plasma within a magnetic field and adding fresh fuel to it, as within a furnace.

Even though fusion generates very high temperatures, in controlled circumstances it would be far safer than nuclear fission, its proponents claim.

The reaction has to take place in a protective chamber that would become radioactive because it is bombarded by neutrons.

However, the radioactivity decays quickly and the fuel itself has none of the long-lasting pollution risks of conventional fissile material.

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