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Dream may come true
For more than fifty years now, scientists have been trying to control and contain the power of nuclear fusion. Nuclear fusion — also known as thermonuclear reaction — is responsible for the destruction caused by hydrogen bombs. In the fifties, it was said that a power reactor harnessing fusion energy to produce electricity would be ready in fifty years. Scientists still believe they will construct a thermonuclear reactor by 2050.
Fusion attracted unprecedented media coverage in 1989, when Martin Fleichmann and Stanley Pons, two American chemists, claimed to achieve fusion reaction at room temperature, naming it cold fusion. However, they failed to reproduce the results when they tried to demonstrate it at a press conference.
Later, other nuclear scientists from various laboratories unsuccessfully tried to simulate the experiment. Finally, cold fusion became an “urban legend.” Then, in 2002, Rusi Taleyarkhan and his colleagues from Oak Ridge National Laboratory, Tennessee, reported the achievement of fusion inside an acetone-filled beaker, fired with neutrons. The acetone used in this experiment was almost similar to that used in the production of nail polish removers.
However, there was one difference. Instead of hydrogen, it contained deuterium, a heavier isotope of hydrogen, with an extra neutron in its nucleus. Miniscule bubbles started to form when the so-called “heavy acetone” was bombarded with neutrons. Those bubbles expanded and collapsed when hit with sound waves.
Interestingly, Taleyarkhan and his colleagues observed the creation of neutron and tritium (hydrogen with two extra neutrons) in small quantities. Taken separately, they point towards a nuclear reaction. However, when observed together, neutron and tritium are considered the evidence of nuclear fusion. Published in Science (vol. 295, p 1868), these results created quite a stir.
These results showed that bubble temperature could have been as high as one million Kelvin. Later, this was known as “sono-fusion” (fusion caused by sound), “desktop fusion” (it required little space), and “bubble fusion” (it took place inside bubbles.
Some skeptics promptly challenged the validity of these experiments, noting that Taleyarkhan may have observed reflected neutrons that he originally used to form bubbles with. Others claimed that tritium in this experiment was a laboratory contamination, which had managed to reach the beaker.
To appease critics, Taleyarkhan’s team ran another series of experiments. In these experiments, they monitored the release of neutrons. They also managed to sufficiently delay the burst of neutrons from the creation of bubbles. This time, the results were published in Physical Review Letters E (vol. 69, p 036109) in March 2004. These results were so conclusive that several critics were forced to change their mind.
Another of group of scientists, led by Ken Suslick and David Flannigan from the University of Illinois at Urbana-Champaign, recently tried similar experiments and arrived upon some remarkable conclusions. The results of their experiments, published in Nature (vol. 434, p 52), are the first detailed measurements of the phenomenon that involves the creation and collapse of bubbles to produce a tremendous amount of energy. The process, which lies at the heart of sono-fusion, is also called “sono-luminescence” (light emission caused by sound).
Instead of fusion, Suslick and Flannigan have found compelling evidence of the creation of plasma — the energetic soup of free electrons and ions that serve as raw material for fusion. Their results can neither confirm, nor deny claims of sono-fusion made by Taleyarkhan’s team. But, according to Suslick, any controlled and contained fusion reaction undoubtedly requires plasma.
In this experiment, researchers showered a sample of concentrated sulphuric acid, containing traces of argon gas, with ultrasonic waves (20-40kHz). These waves produced areas of high and low densities within the acid, creating pressure at any one point, oscillating between two extremes. Bubbles of gas in the liquid swell rapidly at lower pressures before being squeezed by the resultant high pressure.
A frequency between 20-40kHz results in such a rapid pressure change that the bubble implodes with enough force to generate heat in a process called “acoustic cavitation.” Thermal energy separates electrons from their atoms, but soon they join back atoms to release excess energy as bursts of light.
The presence of ionic oxygen molecules (O2+) serves as proof of plasma in this experiment. Right now, no one really knows how to remove an electron from the molecule without breaking the chemical bond holding the two atoms together. Suslick and Flannigan think the molecule was ionized after it collided with high-energy electrons or other ions.
In previous measurements, scientists looked at the acoustic cavitation in water bubbles and were unable to observe the energy produced by sono-luminescence. Sulphuric acid is also less volatile than water, so bubbles in recent experiments had a small number of sulphuric acid molecules, consisting almost entirely of argon. And because argon is an atom rather than a molecule, it contains no chemical bonds to vibrate or break, so the chances of energy being absorbed are minimum.
The collapsing bubbles released about 2,700 times more light than equivalent bubbles in water, making it significantly easier to measure the temperature accurately. It turned out that bubbles in sulphuric acid reached a temperature of more than 15,000ºC, four times hotter than the surface of the sun.
Though the experiments of Suslick and Flannigan did not aim for sono-fusion, but they have validated basic concepts associated with it. The fusion reactor is still another fifty years away, but this time its existence seems possible.
The writer is a science journalist and editor of the monthly magazine, Global Science, Karachi. Email: globalscience@yahoo.com
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