WC Archive

IEEE Spectrum is aimed at an audience comfortable with technical and scientific jargon, but I found the piece gave an extremely readable account of just how the sonofusion technique works. In short, a flask filled with a liquid -- in the original experiments, acetone -- in which 99.9 of the hydrogen are of the more-readily-fused "deuterium" isotope is hooked up to a piezoelectric crystal used to produce acoustic waves in the liquid. At a certain frequency of oscillation, the liquid creates a standing wave which concentrates sound energy. A 6 microsecond burst of neutrons into the liquid knocks some of the molecules around, which in turn creates a small number (~1000) of tiny bubbles with a radius of a few tens of nanometers. But the pulsing oscillation is rapidly changing the pressure in the liquid, and during the low pressure part of the cycle, the bubbles swell by 100,000 times; when the pressure goes back up, the bubbles collapse with such force and energy that the small amount of "deuterated acetone vapor" undergoes thermonuclear fusion, producing temperatures (for a split second, in a tiny space) of 100 million °C. (A graphic of how all of this works can be found here -- PDF.)

Sounds impressive. But the amount of actual energy produced by the split second, tiny fusion events is actually rather small. And they aren't sustained without an ongoing neutron flow. Neither of these are insurmountable problems (and the Lahey, et al, piece in IEEE Spectrum includes some intriguing suggestions of just how they will be overcome), but they are significant barriers. Additional challenges arise from the difficulty of getting sonofusion to work. Although Lahey, et al, claim to have done so repeatedly (and have proven their case sufficiently to pass extra-heavy peer-review criticism prior to the 2004 article), no other lab has managed to duplicate the Purdue results completely. Plenty are trying, both in academia and in commercial ventures, but until sonofusion can be regularly triggered in the lab, it will carry a bit of "cold fusion" flavor for some observers.

If acoustic inertial containment fusion doesn't actually work, then it will go on the pile of failed efforts and will soon be forgotten. If it does work, but can't (for some reason) scale up to usability, it would still be a useful tool for fusion research -- perhaps even a good stepping stone to figuring out large scale "traditional" fusion. But if it works, and the technical challenges to getting it to produce usable amounts of energy are overcome, then it would be worldchanging news.

As Lahey, et al, put it:

...the Holy Grail of all fusion research is the development of a new, safe, environmentally friendly way to produce electrical energy. Fusion produces no greenhouse gases and, unlike conventional nuclear fission reactors, it produces no noxious radioactive wastes that last for thousands of years. With the steady growth of world population and with economic progress in developing countries, average electricity consumption per person will increase significantly. Therefore, seeking new sources of energy isn't just important, it is necessary. Much more research is required before it is clear whether sonofusion can become a new energy source. But then there is just one way we can find out—we will continue making bubbles.

Fusion is appealing in principle because, as mentioned above, it produces no substantive radioactive wastes (years of neutron flux eventually make the containment vessel mildly radioactive, but nowhere near as dangerous as plutonium) and there's no way to turn the fusion reactor into a weapon. There's no radioactive fuel to leak, and pulling the plug just makes it drop below fusion temperatures, ending the reaction. There's a philosophical appeal, as well: fusion is the primal energy of the universe, the ultimate source of all other energy (solar power is just using the results of stellar fusion, and wind power is the result of solar energy heating the atmosphere).

Despite the ongoing developments in wind, solar, tidal and biofuels, it would be good to add fusion -- especially in a form that doesn't require massive, multi-story reactors -- to our toolkit of power sources. Every method has its drawbacks, and the greater diversity of energy options we have, the better we'll be able to handle unexpected demands and problems. Sonofusion's biggest drawback, for now at least, is that it's entirely unproven.

If sonofusion works -- and we're probably still another couple of years from having solid confirmation -- it will take a decade or two at least before we could see any real world applications from it. It's not going to save us from having to do the hard work of moving away from fossil fuels. But another decade or two would mean it would be starting to come online in the early part of the 2020s, just when the conversion from gasoline and coal will start to really hit high gear -- an ideal moment, then, for another clean source of power to step onto the stage.

By 2050, we could be living in a world powered by wind, the Sun, and stars in a jar.

(Thanks to Eric Boyd for pointing me to the articles!)