Bubble Fusion

Bubble Fusion

Sonoluminesence is the production of light from sound. It was first obversed in 1934, yet comparatively little is known about the process. Ultrasound in water can lead to the expansion and contraction of small bubbles dissolved in the water. In the reifaction, the bubble expands while during the compression, the bubbles collapse rapidly (1.4 km s-1 at the point of light emission) leading to a compression of the gas inside. The compression halts when the van der Vaals forces between molecules will not allow them to get any closer. The mechanism for the light emission is proposed by the shock-wave model.

The shock-wave model of sonoluminescence has the collapsing bubble generate an imploding shock-wave which focuses at a point. As the shock-wave propagates, it heats and intensifies the gas it passes through. At the focus, the shock-wave bounces back and makes the gas even hotter. Light is emitted because the shock-wave heats the gas enough to become ionised. The electrons emit light when they collide with the ions which results in the observed continuous emission spectra.

Experiments measuring the emission wavelength, show it to correspond to an energy of around 6 eV, which in turn, corresponds to a temperature inside a collapsing bubble of 70,000 K. There may be photons of even higher energy which would mean higher temperatures, but these wavelengths are absorbed by the water so it is not known what the maximum temperature inside the collapsing bubble is. It is at this point we turn our attention to fusion.

While 70,000 K is far below the 100 million K required for fusion, the temperature of the gas in the centre depends on the minimum size of the of the collapsing bubble. This is another unknown. The so-called shock radius, the radius of the bubble at the point of sonoluminescence, is around 0.1 μm but its minimum size could be smaller for an emission energy of 6 eV. If the radius of the bubble reaches 10 nm (just 10 times smaller) and the gas inside where deuterium, fusion could be ignited. Experiments producing sonoluminescence in deuterated acetone have claimed to produce nuclear fusion with the tell-tale emission of neutrons. Once again, the reproducibility of these results is at issue since it is very difficult to tell whether the neutrons were produced by nuclear processes or part of the background neutron count.