A few years back, bubble fusion was all the rage and sonoluminescence could do everything from generating free energy to curing baldness. Alas, the stigma of research fraud has cooled much of the interest in sonoluminescence, while several research findings have suggested that the bubbles are never hot enough to initiate fusion anyway.
This has left sonoluminescence as an interesting curiosity, lacking the impetus of applied research to drive a major effort into understanding it. Nevertheless, progress is being made, with a new paper published in the New Journal of Physics that proposes a model that explains much of the heating process. In doing so, the researchers have discovered a possible route towards heating the interior of the bubble further. Could bubble fusion be back on the table?
Sonoluminescence is a phenomena observed when bubbles of noble gases are driven by ultrasonic waves. The pressure increase from the ultrasonic wave causes the bubbles to rapidly collapse, strongly heating the internal gas to the point where it becomes a tiny, dense plasma that emits a flash of light. As the peak of the ultrasonic wave passes, the pressure drops again until the bubble reforms, allowing the process to be repeated.
Notably, the temperature is typically in the range of 1000-10,000 Kelvin. A few experiments had even reported temperatures up to 106 Kelvin, bringing sonoluminescence tantalizing close to the temperature-pressure regime required for fusion. Unfortunately, no one really knows how the temperature gets that high that quickly.
Now, a group of researchers from Leeds University has proposed a heating mechanism that takes a trick or two from physicists who typically deal with ions that have temperatures of just a few milliKelvin. In these experiments, ions are trapped using electric fields so they cannot move freely; instead, they vibrate at sets of frequencies that are dictated by the shape of the trap. This vibration couples the ions' internal excitation state to their motion through the absorption and emission of light. Heating and cooling is then achieved by choosing light colors that either damp or accelerate the ions' motion.
Noting that the ions in a collapsing bubble are similarly trapped, the researchers assumed that their motion would also be coupled to the trap, and only certain modes of motion would be allowed. They then went looking for sources of energy that could heat the ions in the trap. What they found was that motion in the trap was strongly influenced by the gradients of electric fields. That is, ion heating doesn't require a large electric field—which is good, because as far as we know, none are present—instead, weak fields that are not constant across the bubble can strongly accelerate the motion of the ion in the bubble trap.
They show that this can drive rapid heating on time scales that are consistent with those observed in sonoluminescence. More interestingly, they also find that this coupling among the ions, gradient fields, and the trap drives some of the ions into an electronically excited state. These should then emit radiation but, unlike the flash of radiation typically observed, it should be at a color specific to the type of ion involved. Furthermore, it should occur before the main flash of light and be much weaker. An excellent prediction to test in experiments.
Finally, because of the similarity between sonoluminescence and ultracold ions, the researchers propose a way to control the temperature of the gas in the bubbles—shine some laser light on them. As with all ions, those in the bubbles will have a set of characteristic emission/absorption colors, with the color changing depending on the motion of the ion.
If a laser has a color that is tuned to a color that is just right to influence ions moving toward the laser beam, it will act to cool them. On the other hand, if the color is tuned to ions moving away from the laser, it will act to heat them.
There are, of course, a million experimental details that would need to be sorted out to get the heating and cooling system right. For instance, the ion must always return to the same lower state, but it could easily return to—and stay—in some other state, meaning we'd need another laser to get it out of that state. Nevertheless, there are some useful predictions here and good prospects for getting some really hot plasmas inside bubbles.
So, is bubble fusion back on the table? If this model proves to be accurate, and experimentalists can find a suitably clean system to work with, then yes, it might well be. Will bubble fusion ever be a replacement for the likes of ITER? No.
New Journal of Physics, 2009, DOI: 10.1088/1367-2630/11/5/053001
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