Sun in a Bottle (17 page)

The laser measurements of Artsimovich’s plasma sparked a fusion energy renaissance in the United States. But the laser was about to change the landscape even more dramatically by providing an alternative to the magnetic bottle.

Lasers produce particularly intense and yet easily controlled light beams. You can point a laser with great precision and make it dump an enormous amount of energy in a very tiny space. To Andrei Sakharov, this suggested that laser beams could be used to heat and contain a plasma of hydrogen. If it worked, laser fusion would be an even more straightforward method than that using magnets. One could simply shine laser light on a pellet of deuterium fuel from all directions: the beams would heat and compress the pellet, creating a tiny fusion reaction—a miniature sun girdled on all sides by light. The plasma would be compressed not by magnetic fields but by particles of light (or by atoms that had been heated by the beams of light).
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This was the birth of
inertial confinement fusion.
The Americans, too, immediately saw the potential of lasers for inducing fusion. At Teller’s Livermore laboratory, physicists like Ray Kidder, John Nuckolls, and Stirling Colgate set to work designing laser fusion schemes soon after the first laser was built.
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Their calculations seemed to show not only that laser fusion was possible, but also that it might be relatively easy to achieve breakeven. Livermore scientists began building laser bottles intended to ignite and contain fusing plasma.
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The first big one, built in 1974, was known as Janus. Two-faced like the god it was named after, Janus had two laser beams that shot at a tiny pellet of deuterium and tritium from opposite directions. It was more a test of the laser system than a concerted attempt to initiate fusion reactions. A true laser-based bottle would require laser beams to hit the target from all sides at once to fully confine it, but Janus’s lasers only struck from two sides, allowing the plasma to squirt out in various directions. Nevertheless, the Livermore scientists were soon detecting tens of thousands of neutrons coming from the pellet. They had achieved thermonuclear fusion, even though it was on a tiny scale. It was a success, but it was not the first.

The Russians and French had already detected neutrons from pellets hit by lasers, but the American press, skeptical of the foreigners’ claims, did not give them much attention. The press did have a field day, though, with the curious tale of a rogue company—KMS Industries, Inc.—that had built its own laser system. By May 1974, KMS, named after its physicist founder and president, Keeve M. Siegel, reported that it was producing neutrons from laser fusion.

Within two weeks, the story was plastered all over the newspapers. The
New York Times
touted KMS’s achievement as “a significant step toward the long-range goal of nuclear fusion as a source of almost limitless energy.” The Atomic Energy Commission was less thrilled, because a private firm was doing an end run around the government. If the KMS claims were true, an AEC statement read, it would be “a small but significant initial step toward the achievement of fusion power.” Siegel was making the AEC look bad—and fusion energy look good.

Not only was Siegel using lasers to ignite fusion, but he was doing it as the head of a private company, not as a scientist in a government laboratory. The public took this as a sign that private industry was embracing fusion reactors as a viable source of energy. Siegel, the entrepreneur, exuded confidence in public. He was sure, he said, that he could turn lasers into “efficient fusion power” within “the next few years.” After false starts and two decades of struggle with magnetic bottles, the era of fusion finally seemed at hand.

The timing could scarcely have been better. The United States was just getting through its first oil crisis. Because of American support for Israel during the 1973 Yom Kippur War, the Arab members of the Organization of the Petroleum Exporting Countries (OPEC) cut off oil supplies to the U.S. Gas prices skyrocketed. It was becoming painfully clear that the country had to find another source of energy—anything other than petroleum—if it was to avoid being held hostage to OPEC’s interests. It was scarcely two months after the embargo was lifted that a jittery nation learned about Siegel and KMS. It seemed that fusion would be the way to get out from under OPEC’s thumb. The dream of unlimited power once more beckoned. Fusion energy seemed possible again, and it was more important than ever.

Congress immediately seized upon it and started pouring money into fusion research. Laser fusion saw a dramatic increase in funding, growing from almost nothing to $200 million per year by decade’s end.
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Livermore and some other laboratories around the country, particularly those at Los Alamos and at the University of Rochester in New York, began to plan massive laser projects with an eye toward creating a viable fusion reactor. Magnetic fusion, too, benefited from the renewed interest in fusion energy. After stagnating for a decade at around $30 million per year, magnetic fusion budgets doubled and doubled and doubled again. In 1975, more than $100 million went to magnetic fusion; by 1977, more than $300 million; and by 1982, almost $400 million.

Siegel’s 1974 announcement helped ignite public enthusiasm (and governmental largesse) for fusion research, but his story had a tragic ending. In 1975, he keeled over while testifying about his work in front of Congress. Though he was rushed to the nearby George Washington University Hospital, he died shortly thereafter, the victim of a stroke. He was fifty-two years old. Siegel didn’t survive to benefit from the surge of optimism he generated. He also didn’t survive to see the worsening problems laser fusion scientists faced as their lasers grew more powerful.

Livermore’s Janus was already in 1975 suffering from a major snag. Its lasers were extremely powerful for their day, pouring an unprecedented amount of laser light into very tiny spaces. Livermore’s scientists managed to get this level of power by taking enormous slabs of glass made of neodymium and silicon and exciting them with a flash lamp. This glass was the heart of Livermore’s laser. The slabs were what produced an enormous number of infrared photons in lockstep. The resulting beam exited the glass and was bounced around, guided by lenses and mirrors to the target chamber. However, the beam was so intense that it would heat whatever material it touched. This heat changed the properties of lenses, mirrors, and even the air itself. When heat changes the properties of a lens or a mirror, it alters the way the device focuses the beam. These little changes in focus would start creating imperfections in the beam, such as hot and cold spots. These could be disastrous. The hot spots in the beam would pit lenses, destroying them in a tiny fraction of a second. Every time they fired the Janus laser, the machine tore itself to shreds.

Luckily, the Livermore scientists were already working toward a fix. Their next-generation fusion machine, Argus, used a clever technique to eliminate those troublesome hot spots. By shooting the beam down a long tube and carefully removing everything but the light at the very center of the beam, the scientists would be assured of getting light that was uniform and pure—and free of hot spots. This meant that the laser had to be housed in a very large building to accommodate the tubes, which were more than a hundred feet long. In addition, since they were tossing out some of the beam because of its imperfections, they were sacrificing some of the laser’s power. This was a minor inconvenience; the technique worked, and the hot spots disappeared for the time being.

More serious was the problem with electrons. Magnetic fusion researchers had trouble heating the plasma evenly; the lightweight electrons would get hot faster than the heavyweight nuclei, making for a very messy plasma soup. This problem was worse with lasers: light that is shined on a hunk of matter tends to heat the electrons first. This was a huge issue. The electrons in a laser target would get so hot that the target would explode before the nuclei got warmed up. Hot electrons and cold nuclei were no good for fusion—it was the nuclei that scientists really wanted to heat up.

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.
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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, becoming enormous tendrils of water drooping down from the surface; the troughs also grow, and large fingers of air prod deep into the glass. The tendrils break, the fingers bubble off, and the entire glass of water rains down onto the floor. This is the Rayleigh-Taylor instability in action. Even though the air exerts an enormous amount of pressure on the water, the less-dense air is unable to keep the denser water contained in the glass because of these growing tendrils and fingers. Get rid of those instabilities and the air can keep the water contained. (The cardboard is not susceptible to Rayleigh-Taylor instabilities because it is a solid, so the air-pushing-on-cardboard-pushing-on-water system is stable.) But if Rayleigh-Taylor instabilities are present, then they will wreak havoc on your attempt to keep the denser fluid contained where you want it.

Laser fusion is the equivalent of keeping water trapped in an upside-down glass. As you compress a pellet of deuterium, it becomes denser and denser. Long before you get it hot and dense enough to fuse, it will be much denser than whatever substance you are using to compress it, whether it is particles of light or a collection of hot atoms. You are using a less-dense substance to squash and contain a much denser one, and that means you will get Rayleigh-Taylor instabilities. Any tiny imperfections on the interface between the plasma and the stuff that is pushing on the plasma will immediately grow. Even an almost perfectly round sphere of deuterium will quickly become distorted, squirting tendrils in all directions. Just as this ruins any attempt to keep water in an inverted glass by means of air pressure, it seriously damages a machine’s ability to compress and contain a plasma by means of light. The only way around this was to make sure there were almost no imperfections. The target had to be perfectly smooth, and the compressing lasers had to illuminate the target completely uniformly, without any hot or cold spots that would lead to ever-growing Rayleigh-Taylor tendrils.