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Wednesday, March 6, 2002
Scientists claim to achieve tabletop fusion reaction
By ROWAN HOOPER
Russian and U.S. researchers announced Monday that they had reproduced the source of the sun's energy -- nuclear fusion -- on a tabletop in a laboratory.
Like many major scientific finds, the results were immediately contested.
But if they can be replicated, nuclear physics will have taken a giant step forward, and the door could be open to an easy and safe source of nuclear energy.
The results will be published in full in the journal Science on Friday, but the discovery was announced early because of the extreme interest surrounding it.
On Aug. 6, 1945, when U.S. President Harry Truman announced the first military use of an atomic bomb, he said: "Sixteen hours ago an American airplane dropped one bomb on Hiroshima. . . . The force from which the sun draws its power has been loosed against those who brought war to the Far East."
Truman wasn't strictly correct, but who was going to argue with him? The sun, and all stars, produce energy by fusing atoms. Atomic bombs, and nuclear power stations, run on nuclear fission, the splitting of atoms.
Nuclear fission is dirty, producing nasty radioactive byproducts. Fusion is clean, producing no greenhouse gases and few radioactive materials. It's also cheap, running on a plentiful energy source -- sea water. Lots of energy is needed to start a fusion reaction, but far more is produced once it gets going.
For this reason, fusion is seen as the energy source of the future, supplying us with unlimited amounts of clean energy.
Nuclear fission splits the atoms of heavy elements like uranium into the atoms of lighter elements (converting some of the uranium atom's mass into energy). Nuclear fusion joins together the light atoms of hydrogen, forming a heavier helium atom, and converts some of the original mass into energy.
There's just one problem. A hydrogen atom will only fuse with another hydrogen atom at immense temperatures: around 10 million degrees Kelvin (or Celsius). That's as hot as the center of the sun, where hydrogen has been fused to make helium for 4.5 billion years. Producing those sorts of conditions on Earth is tricky, to say the least, and hence the joke among physicists: "Nuclear fusion is 40 years away . . . and always will be."
But Rusi Taleyarkhan and colleagues, of the Oak Ridge National Laboratory in Tennessee, in collaboration with the Russian Academy of Sciences, used "acoustic catavation" to create conditions mimicking those inside the sun. Acoustic catavation is a process whereby ultrasonic waves are focused in a liquid to produce tiny bubbles, each no bigger than the period at the end of this sentence. The pressure of the sound wave creates and collapses the bubbles.
The shock waves of the collapse of the bubbles create ultra-high temperatures and pressures in the bubble's gas, and then a burst of energy, a brilliant but extremely brief flash of light called "sonoluminesence." Temperatures inside the bubbles can be as high as 5,000 to 7,000 degrees Kelvin, about as hot as the surface of the sun. That's hot, but not enough for nuclear fusion to take place.
So Taleyarkhan and colleagues turned up the heat. They created conditions under which very small bubbles are allowed to grow rapidly before they collapse. The super-hot bubbles are trapped in a special type of acetone (nail polish remover with its hydrogen atoms replaced by deuterium, an isotope of hydrogen). If deuterated hydrogen undergoes fusion then it leaves telltale signs: tritium (another hydrogen isotope) and neutrons in a special energy state.
The experiment appears to have worked. The scientists say temperatures inside the bubbles reached up to 10 million Kelvin, and that in samples with extensive bubble implosion, they found tritium. The researchers also detected the emission of neutrons with the energy levels characteristic of fusion.
Even if fusion is confirmed in further tests, there is a long way to go before these tiny bubbles could be considered as an energy source, said Richard Lahey, coauthor of the Science paper. First, the bubble reaction would need to produce more energy than is needed to drive the reaction, and second, the reaction must perpetuate itself.
"If the results are confirmed, this new, compact apparatus will be a unique tool for studying nuclear fusion reactions in the laboratory," wrote Fred Becchetti of the University of Michigan in a commentary in Science. "But scientists will -- and should -- remain skeptical until the experiments are reproduced by others. Many, including the author, could not reproduce past claims made for table-top fusion devices," Becchetti added.
The claim has already been challenged by researchers from the physics department of Oak Ridge, who said they could not detect the all-important neutrons when they repeated the experiment. Taleyarkhan said that their instruments had been improperly calibrated, but agreed that more work needed to be done.
One hundred years ago we were only just starting to understand that the atoms of our bodies were made in distant, long-dead stars. D.H. Lawrence wrote, "I am part of the sun as my eye is part of me." Before then, nuclei and subatomic particles, the source of the sun's energy, were unknown. Now we are re-creating the conditions of the center of the sun on tabletops in laboratories.
Nuclear fusion may still be a long way off, but it's suddenly looking like an attainable dream.