Sonofusion
Published on 17 Mar 2007 at 1:13 am.
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Filed under Acoustics, Inventions, Physics, Research.
I am the chair of our physics department’s weekly colloquium, and this week our guest speaker was Dr. Richard T. Lahey Jr., the Edward Hood Jr. Professor of Engineering at Rensselaer Polytechnic Institute. Dr. Lahey spoke to us about his recent successes in sonofusion. In short, sonofusion is the act of achieving nuclear fusion by focusing sound waves on bubbles in a liquid to compress them to the density at which nuclear fusion occurs. The most amazing feature is that this can be performed in a tabletop experimental setup costing about $3000!
Dr. Lahey performs this research in collaboration with Rusi P. Taleyarkhan, the Ardent Bement Jr. Professor of Nuclear Engineering at Purdue University and Robert Nigmatulin, of the Russian Academy of Sciences. Below is a photo of Dr. Taleyarkhan with the reactor.
The chamber itself is 65 mm (2.5 inches) in internal diameter and 100 mm (4 inches) tall. It is filled with a well-degassed deuterated liquid hydrocarbon, in this case D-acetone. The fact that it is degassed means that there are no air bubbles floating around, and the fact that it is deuterated means that the hydrogens in the acetone molecules have been replaced with hydrogen’s heavier isotope deuterium. Large amplitude ultrasonic waves are driven into the cylindrical chamber. These waves come to focus in the center of the cylinder creating pressure changes on the order of 15-40 bar (15-40 times atmospheric pressure), which locally superheats the liquid.
The next step is to get bubbles of acetone vapor in the solution. They have found that a pulsed neutron source, that shoots neutrons through the fluid leaving a trail of acetone bubbles works very well by creating large bubbles with radii about 10 times larger than those produced by other methods. The fact that they are using a neutron source makes some researchers a bit suspiscious, since to verify that the reaction is indeed thermonuclear, one needs to detect neutrons of a given energy. This raises the bar for the researchers; making their experimental results a bit more difficult to demonstrate. However, the neutrons they are looking for have very specific energies (speeds) from the nuclear reaction (2.45 MeV). Apparently, forming bubbles in hydrocarbons is extremely difficult and would take extremely low pressures. For this reason the pulse neutron source works well.
Bubbles near the focus of the ultrasonic waves, are rapidly compressed at rates of 1000s of kilometers per second. As the bubble compresses a nonlinear shock front forms, which races into the bubble. As this shock reaches the center of the bubble, the local temperature reaches 100 million degrees Kelvin (180 million degrees F) and a density of 1 trillion bar, which is approximately the density of a neutron star. This is all occurring on a nanometer scale. Out of the perhaps 1000 bubbles near the center of the chamber, about 15 of them reach the appropriate conditions to undergo nuclear fusion. These bubbles then rapidly expand and then bounce back into compression, experiencing fusion again. This bouncing occurs approximately 50 times! Each fusion event yields only about 10 neutrons (thus only about 10 pairs of deuterium actually fuse). But given that there are about 15 bubbles, each bubble will fuse about 50 times, and that they are being driven to implode at 50 times a second gives an overall average neutron production rate of almost 40,000 neutrons per second.
To the outside observer, a fusion event results in a flash of light (similar to that produced by the lower energy phenomenon called sonoluminescence), and then a shockwave that hits the walls of the cylinder like a hammer!
There are actualy two nuclear processes going on in this reaction:
D + D -> 3He + n (2.45 MeV) + 3.3 MeV (in gamma rays)
D + D -> T + H + 4 MeV
3He is a Helium atom with only one neutron and the normal two protons. T is the Hydrogen isotope Tritium with one proton and two neutrons. The reactions have approximately a 1:1 branching ratio, which means that each reaction is equally likely. Thus 50% of the time, a Tritium is produced, which can also be fused in other types of reactions in the chamber.
The team has experienced many political difficulties. First, while this is not cold fusion, there is a remaining stigma attached to tabletop physics. This is reinforced by the fact that researchers who have been working to perfect other fusion techniques, such as laser confinement and magnetic confinement, require enormous budgets. Certainly, the discovery of a tabletop method is potentially threatening. However, in reality, each of these methods should be supported as necessary until we finally have successfully created fusion power plants. Last, these researchers published their initial results in the journal Science, which competes with the journal Nature. Typically, Science focuses on biology, whereas Nature focuses on other area of science. Lahey and colleagues, being nuclear engineers, were not aware of these turf boundaries. The end result of the rivalry between the two journals and the rivalry between scientific competitors was an accusation of scientific misconduct, which have since been cleared.
The team continues to perform tests to improve the efficiency and sustainability of the process. Currently the energy produced by their test chamber is 10,000,000 times less than the energy that they have to put in to get the reaction. One says that they are 10,000,000 times below breakeven. The good news, is that they are just starting to refine the process, and they have many parameters to play with, each potentially yielding orders of magnitude increases in efficiency. To make the process self-sustaining, one idea is to place two cylinders side-by-side and to drive them 180 degrees out of phase. When one cylinder is fusing, it produces neutrons that make bubbles in the second cylinder, which will then fuse in the next cycle and so on. A great benefit is that there is absolutely no way to get a runaway reaction. The second benefit is that for power generation, the cylinders will be designed so that the neutrons are absorbed in the fluid heating it up to eventually run turbines for electricity. This results in no resdiual radioactivity.
Dr. Lahey and his colleagues are hoping that they can scale the process up to create 100 megawatt generators. These generators would run on deuterium extracted from seawater, which would last us for about 1 million years at our current rate of energy consumption. The prospect that we could be on the verge of having an energy source with no CO2 output, no reliance on oil, and no radioactivity is almost too good to be true. But at this point in history, we could use some good news and from some truly smart people deserving of praise.
Kevin Knuth
Albany NY
