If you’re having a baby scan, rest assured there’s nothing to worry about here, this piece of research is about high-energy ultrasound. The difference is like comparing a research laser beam to the light from a cheap flashlight…

Anyway, according to Ken Suslick, when high-intensity ultrasound passes through a liquid, the expansion wave of the sound can create isolated bubbles, which can be driven into implosive collapse. Under the right conditions, the collapse of the bubble is so extreme that light is emitted in a process called sonoluminescence. The origin of the light comes from the intense heating that occurs during the collapse, which can be hot enough to break molecular bonds (sonochemistry) and trigger sonoluminescence.

Recent experiments have shown that the gas atoms and molecules inside these bubbles are ionized (they form a plasma), and that the temperature can be several times that of the surface of the sun and the pressure greater than that at the bottom of the Mariana Trench.

By studying sonoluminescence from liquids, such as concentrated sulfuric acid, Suslick’s team at the University of Illinois at Urbana-Champaign working with David Flannigan, now at Caltech, have quantified the properties of the plasma inside these imploding bubbles. They could also determine how these properties are affected by the ferocity with which the bubble collapses, and they found that the plasma conditions generated may indeed be extreme. Astoundingly, the maximum plasma electron density that they are able to experimentally measure (more than 1021 per cubic centimeter) is comparable to the densities generated by the Nova Inertial Confinement Laser Fusion installation at Lawrence Livermore National Laboratory. Such laser installations cost billions of dollars, says Suslick, “the apparatus we put together was just a few thousand.”

The effective plasma temperatures ranging beyond five-times the temperature of the Sun’s surface (more than 16000 Kelvin). The results of this work are reported in the journal Nature Physics.

In 2005, Suslick and Flannigan reported in the journal Nature (PDF) their observation of light emission from atomic excited states during sonoluminescence from sulfuric acid solutions. The energies of the populated atomic levels suggested a plasma, but at that time there was no estimate of the density of the plasma, a crucial parameter to understanding the conditions created at the core of the collapsing bubble. The new report makes use of the same system, but now with a detailed analysis of the shape of the observed spectrum, which contains information pertaining to the conditions of the region around the atoms.

The plasma properties extracted from these analyses show a strong dependence on the violence of bubble implosion, and the observable physical conditions suggest the limits of energy focusing during cavitation may approach those achievable only by much more expensive means.

It is evident from these results that the upper bounds of the conditions generated during bubble implosion have yet to be established.

Flannigan and Suslick have not, however, observed evidence for thermonuclear fusion during bubble collapse. By way of background, in 2002, Rusi Taleyarkhan (then at Oak Ridge National Laboratory and now at Purdue University) reported controversial evidence that suggested acoustic cavitation in deuterated acetone induces thermonuclear fusion, calling this “sonofusion”.

The results of this work was never reproduced outside of Taleyarkhan’s labs, however, and the validity of his claims is still in doubt. In 2008, Purdue found Taleyarkhan guilty of research misconduct, stripped him of his chaired position, and forbade his supervision of graduate students; in 2009, the Federal government debarred Taleyarkhan from future research funding for two years.

Flannigan, D., & Suslick, K. (2010). Inertially confined plasma in an imploding bubble Nature Physics DOI: 10.1038/nphys1701

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