Researchers at Purdue University are reporting new evidence supporting their earlier discovery of an inexpensive "tabletop" device that uses sound waves to produce nuclear fusion reactions.
The researchers believe the new evidence shows that "sonofusion" generates nuclear reactions by creating tiny bubbles that implode with tremendous force. Nuclear fusion reactors have historically required large, multibillion-dollar machines, but sonofusion devices might be built for a fraction of that cost.
"What we are doing, in effect, is producing nuclear emissions in a simple desktop apparatus," said Rusi Taleyarkhan, the principal investigator and a professor of nuclear engineering at Purdue University. "That really is the magnitude of the discovery – the ability to use simple mechanical force for the first time in history to initiate conditions comparable to the interior of stars."
The technology might one day result in a new class of low-cost, compact detectors for security applications that use neutrons to probe the contents of suitcases; devices for research that use neutrons to analyze the molecular structures of materials; machines that cheaply manufacture new synthetic materials and efficiently produce tritium, which is used for numerous applications ranging from medical imaging to watch dials; and a new technique to study various phenomena in cosmology, including the workings of neutron stars and black holes.
The device is a clear glass canister about the height of two coffee mugs stacked on top of one another. Inside the canister is a liquid called deuterated acetone. The acetone contains a form of hydrogen called deuterium, or heavy hydrogen, which contains one proton and one neutron in its nucleus. Normal hydrogen contains only one proton in its nucleus.
The researchers expose the clear canister of liquid to pulses of neutrons every five milliseconds, or thousandths of a second, causing tiny cavities to form. At the same time, the liquid is bombarded with a specific frequency of ultrasound, which causes the cavities to form into bubbles that are about 60 nanometers – or billionths of a meter – in diameter. The bubbles then expand to a much larger size, about 6,000 microns, or millionths of a meter – large enough to be seen with the unaided eye.
"The process is analogous to stretching a slingshot from Earth to the nearest star, our sun, thereby building up a huge amount of energy when released," Taleyarkhan said.
Within nanoseconds these large bubbles contract with tremendous force, returning to roughly their original size, and release flashes of light in a well-known phenomenon known as sonoluminescence. Because the bubbles grow to such a relatively large size before they implode, their contraction causes extreme temperatures and pressures comparable to those found in the interiors of stars. Researches estimate that temperatures inside the imploding bubbles reach 10 million degrees Celsius and pressures comparable to 1,000 million earth atmospheres at sea level.
At that point, deuterium atoms fuse together, the same way hydrogen atoms fuse in stars, releasing neutrons and energy in the process. The process also releases a type of radiation called gamma rays and a radioactive material called tritium, all of which have been recorded and measured by the team. In future versions of the experiment, the tritium produced might then be used as a fuel to drive energy-producing reactions in which it fuses with deuterium.
Whereas conventional nuclear fission reactors produce waste products that take thousands of years to decay, the waste products from fusion plants are short-lived, decaying to non-dangerous levels in a decade or two. The desktop experiment is safe because, although the reactions generate extremely high pressures and temperatures, those extreme conditions exist only in small regions of the liquid in the container – within the collapsing bubbles.
One key to the process is the large difference between the original size of the bubbles and their expanded size. Going from 60 nanometers to 6,000 microns is about 100,000 times larger, compared to the bubbles usually formed in sonoluminescence, which grow only about 10 times larger before they implode.
Future work will focus on studying ways to scale up the device, which is needed before it could be used in practical applications, and creating portable devices that operate without the need for the expensive equipment now used to bombard the canister with pulses of neutrons.
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