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The nuclear fusion that takes place in the core of the Sun, where temperatures reach 15 million degrees Celsius, liberates enormous amounts of energy. We see the result of this energy liberation in the Sun’s glare. The elementary particle known as a muon, however, provides a means of achieving nuclear fusion at sub-zero temperatures. “Using muons, we can achieve nuclear fusion in a comparatively small facility at reasonable cost,” says Teiichiro Matsuzaki, director of the RIKEN-RAL Muon Facility. Matsuzaki and scientists at the facility have been conducting unique experiments as part of fundamental research into the use of muons to develop industrially viable nuclear fusion technology.

Learn more about muon fusion at this link

An astonishingly simple demonstration of nuclear fusion in a tabletop device has been performed, involving heating an ordinary crystal soaked in deuterium gas.

While the technique is unlikely to lead to power generation, such a device could act as a portable source of neutrons for analysing materials and medical imaging, and perhaps even spacecraft propulsion.

The key to the system is a crystal made of lithium tantalate. The crystal is asymmetric and, as a result, heating the material causes positive and negative charges to migrate to opposite ends of the crystal, setting up an electric field. The phenomenon is known as the pyroelectric effect.

In 1992, James Brownridge at the State University of New York in Binghamton, US, used crystals of lithium tantalate to generate X-rays by heating the crystals to about 100ºC in a dilute gas. The resultant electric field strips electrons from the gas molecules and accelerates them to huge energies. The electrons then collide with stationary nuclei in the crystal and generate X-rays.

When Seth Putterman at the University of California, Los Angeles, US, heard of the phenomenon a few years ago, he immediately realised that the electric fields were powerful enough for nuclear fusion to occur, specifically to fuse nuclei of an isotope of hydrogen called deuterium.

Strong case
To test whether these fields could indeed cause nuclear fusion, Putterman and UCLA colleagues Brian Naranjo and James Gimzewski first bathed a crystal of lithium tantalate in deuterium gas. The setup was then cooled to -33ºC and then heated to about 7 ºC over three and a half minutes.

The resultant electric field accelerated deuterium nuclei over a distance of 1 centimetre to energies in excess of 100 kiloelectronvolts. The accelerated nuclei then collided and fused with deuterium nuclei that had permeated the surface of the crystal lattice. The fusion produced 400 times more neutrons than found in background measurements.

Fusion science is littered with hype and over-optimistic claims, but Putterman has convinced his peers that something interesting is going on. "They make a very strong case for having seen fusion," says Nigel Hawkes, a nuclear physicist at the National Physical Laboratory in Teddington, UK.

But he is cautious about the potential for desktop neutron machines: "It's too early to say where this might lead."

Microthrusters
One problem is the small number of neutrons the experiment produces - a few hundred per second. A commercial neutron generator would need to produce at least tens of millions of neutrons per second.

Today, neutrons are created in nuclear reactors or particle accelerators which can cost millions of dollars to build and maintain. The prospect of a desktop alternative is a powerful incentive to continue the research and Putterman's team hopes to increase the yield by operating at lower temperatures and by using an array of crystals.

Putterman also suggests the crystals could be used as microthrusters for tiny spacecraft. By accelerating deuterium in one direction, the spacecraft would be propelled in the opposite direction.

18:00 27 April 2005

The effect was first discovered at the University of Cologne in 1934 as a result of work on sonar. H. Frenzel and H. Schultes put an ultrasound transducer in a tank of photographic developer fluid. They hoped to speed up the development process. Instead, they noticed tiny dots on the film after developing and realized that the bubbles in the fluid were emitting light with the ultrasound turned on. It was too difficult to analyze the effect in early experiments because of the complex environment of a large number of short-lived bubbles.

In 1989 a major advancement was introduced by Felipe Gaitan and Lawrence Crum, who produced stable single-bubble sonoluminescence (SBSL). In SBSL, a single bubble, trapped in an acoustic standing wave, emits a pulse of light with each compression of the bubble within the standing wave. This technique allowed a more systematic study of the phenomenon, because it isolated the complex effects into one stable, predictable bubble. It was realized that the temperature inside the bubble was hot enough to melt steel. Interest in sonoluminescence was renewed when an inner temperature of such a bubble well above one million Kelvin was postulated. This temperature is thus far not conclusively proven, though recent experiments conducted by the University of Illinois at Urbana-Champaign indicate temperatures around 20,000 Kelvin. Research has also been carried out by Dr. Klaus Fritsch of John Carroll University, University Heights, Ohio.

The US Navy studied propeller-induced sonoluminescence during the Cold War.


Theory of Operation

Single bubble sonoluminescence (SL) is the spontaneous emission of picosecond pulses of broadband light from a micron-size gas bubble levitated in water by the application of an external sound field.The bubble expands and contracts in phase with the oscillating pressure field.

Much of the recent work on single bubble sonoluminescence has been concerned with the dynamics of the bubble motion and the detailed spectrum in the 200 to 700 nm range using a variety of gas mixtures as the contents of the sonoluminescing bubble. Recent theoretical work with shock wave focusing in the bubble has given peak temperatures up to 10^9 K, while other estimates place the peak temperature in the range of 10^4 to 10^6 K.