Sun in a Bottle - Charles Seife [58]
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.
RAYLEIGH-TAYLOR INSTABILITY IN INERTIAL CONFINEMENT FUSION: Use lasers or particles to bombard a pellet of fuel and small imperfections on the surface of the pellet quickly become large fingers that cool the fuel and prevent it from fusing properly.
It was almost as if the laser scientists were trying to invert a glass so carefully that the surface of the water inside wouldn’t ripple at all. This is an extraordinarily difficult task. Even the twenty-armed Shiva machine, heating the plasma from twenty different directions at once, wasn’t uniform enough to keep the Rayleigh-Taylor instabilities in check. The twenty pinpricks of laser light were far enough apart from one another that they would create hot spots in the target rather than heating it uniformly. The pellet would compress, getting hot and dense enough to induce a little bit of fusion, but before the reaction really got going, the Rayleigh-Taylor instability would take over. Tendrils would form. Instead of getting denser and hotter, the deuterium would squirt out.
The Livermore scientists tried everything they could to get the Rayleigh-Taylor problem under control. One method mimicked the Teller-Ulam design for the hydrogen bomb. Instead of using the lasers to push directly onto a dollop of deuterium, the new method did it indirectly. The pellet was ensconced at the center of a hollow cylinder known as a hohlraum. Instead of striking the pellet, the lasers struck the insides of the hohlraum. The hohlraum then radiated x-rays toward the pellet. This setup is known as indirect drive, and it helped ameliorate the problems with the instabilities.
DIRECT DRIVE VERSUS INDIRECT DRIVE: In direct drive (left), laser beams shine directly on a pellet of fuel. Indirect drive (right), on the other hand, has laser light shining on a hohlraum, which evaporates and shines x-rays on the pellet.
But it didn’t do enough. Shiva, which had cost $25 million to build, only performed a fraction as well as its designers had hoped. It didn’t come close to producing