Sun in a Bottle - Charles Seife [57]
For technical reasons, the bluer the laser beam, the smaller this effect. So the Livermore scientists shined the laser light through crystals that would make the infrared beam green or even ultraviolet.51 The color conversion worked well to reduce the heating of the electrons, but the process was inefficient. The beam lost some of its energy becauseof the color change. It also made the laser more expensive, as big, high-quality color-change crystals were not cheap. Nevertheless, the results—and the number of neutrons—from Argus led Livermore’s physicists to push for a full-size machine, Shiva, that would use twenty beams to zap a pellet of deuterium from all directions. It would ignite the pellet, creating a fusion reaction that would generate as much energy as the laser poured in. Or so the scientists hoped. They were wrong by a factor of ten thousand. Laser fusion scientists, like the magnetic fusion advocates that preceded them, were about to come face-to-face with a nasty instability—one so fundamental that you often encounter it in your kitchen.
It is hard to imagine an instability in the kitchen, but ask yourself the following question: When you invert a glass of water, why doesn’t the water stay in the glass? This seems like a silly thing to ask: gravity pulls the water down and onto the floor. But if you look a little more deeply, the answer is not quite so obvious. Atmospheric pressure makes the question more complicated than you might expect.
Every surface that is exposed to air is under pressure. The very weight of the atmosphere is squashing us from all directions. Every square inch of our skin is subjected to 14.7 pounds of pressure from the air pushing against us. We don’t notice it because our bodies are used to it, but this is an enormous force, easily enough to crush a steel can under the right conditions. It is also more than enough to support a glassful of water and prevent the liquid from falling to the ground. Try it yourself (over a sink, of course). Fill a glass to the rim with water. Hold a smooth, rigid piece of cardboard over the mouth of the glass and invert the whole thing. Gently let go of the cardboard. If you do it carefully enough, you will see that the water stays in the glass. The cardboard isn’t holding the water in. It’s not stuck tightly to the glass; even a gentle touch will dislodge the cardboard and cause the water to run out. And the water isn’t miraculously defying gravity. It is being supported by air pressure. The atmosphere’s upward push of 14.7 pounds per square inch is much, much stronger than the three or four ounces per square inch downward push of the water in the glass. When the two pressures go head to head, the upward push of the atmosphere wins and the water stays put. Believe it or not, the forces are so mismatched that you would need an enormously tall glass of water—about thirty feet high—if you wanted the downward-pushing weight of the water to equal the upward-pushing atmospheric pressure. With such vastly mismatched forces, the question seems a lot less stupid: Why doesn’t water stay in a glass when you turn it upside down?
RAYLEIGH-TAYLOR INSTABILITY IN A GLASS OF WATER: Invert a glass quickly and little ripples on the surface of the water will grow, becoming large blobs. The blobs break off and the water rains down out of the glass.
The water falls out because of an effect known as the Rayleigh-Taylor instability. Whenever a not-very-dense fluid (like air) pushes on a denser fluid (like water), it is an inherently unstable situation. If the interface between the two fluids has any imperfections—any bumps or divots—then those imperfections immediately get bigger and bigger.
An inverted glass of water, no matter how carefully it is inverted, has a few crests and troughs on the surface of the liquid. In a tiny fraction of a second, the crests grow,