Sun in a Bottle - Charles Seife [42]
The enthusiasm surrounding the technology, though, hid a lot of difficulties—and some infighting. Spitzer and the Princeton Stellarator team thought their idea was the path to fusion energy, and tried to tear down the Perhapsatron idea championed by their rival, Los Alamos’s Tuck. In fact, Spitzer spent some of his AEC grant trying to prove that a Perhapsatron would not work. It was money well spent. Two of his team members, Princeton professors Martin Schwarzschild and Martin Kruskal, found a very disturbing flaw that threatened to disrupt the Perhapsatron research program altogether. A pinched plasma was unstable.
Perhaps the easiest way to understand stability and instability is to imagine a ball sitting at the bottom of a hill. This is a stable system. Give the ball a slight nudge and it will roll right back to where it started. The system resists change; it won’t be ruined by small perturbations. A ball perched on the top of a steep hill, on the other hand, is in a precarious position. Give it even the slightest nudge and it will roll down the slope, abandoning its previous place. This system doesn’t resist change—indeed, even a tiny disturbance will change it dramatically. This is an unstable system.
Kruskal and Schwarzschild had discovered that a pinched plasma was like a ball perched on a hill. The slightest disturbance would destroy it. Send a current through a cylinder of plasma and it indeed squashes itself into a dense little filament of hot matter. But the filament is unstable. If it is not perfectly straight, if it has even the tiniest kink, the magnetic fields generated by the pinching current immediately exaggerate and expand the kink. This makes the kink grow, getting more and more pronounced. Any little imperfection in the plasma filament rapidly becomes a huge imperfection. In a tiny fraction of a second, the plasma kinks, bends, and writhes out of control.
As soon as the Perhapsatron started up in 1953, the Princeton team’s calculations were proved correct. The Los Alamos experimenters found that as soon as they got a pinch, forming a nice, tight filament in the center of the Perhapsatron’s chamber, it went poof! The pinch would disappear, setting the whole chamber aglow. High-speed cameras revealed the filament buckling and writhing, quickly striking the walls of the chamber. The kink instability had claimed its first victim. The Perhapsatron, as built, was incapable of fusing anything at all. The Los Alamos scientists needed to figure out how to stabilize the filament if they were to progress. They tried using an external magnetic field to “stiffen” the plasma filament somewhat, but the essential instability remained. Pinches were in trouble.
KINK INSTABILITY: If a pinching plasma has even a tiny kink in it, that kink will grow; the plasma will writhe out of control and hit the walls of its container.
Soon, the other designs were as well. In 1954, Edward Teller figured out that a plasma held in place by magnetic fields was unstable under certain conditions. The magnetic fields behave somewhat like a collection of rubber bands: as the plasma pressure increases, they try to relieve the increasing tension by writhing. “They try to snap inward and let the plasma leak out between them,” Teller wrote. This system was also unstable. Even a tiny irregularity in the magnetic field would rapidly get worse, and scientists would lose control of the plasma. The so-called Teller instability affected the Stellarator as well as Livermore’s magnetic mirror approach. Instabilities were everywhere.
By the mid-1950s, all three groups had enormous difficulties to overcome. Their plasmas were unstable and their bottles were leaky. They spent ever-increasing amounts of money building bigger and more elaborate machines in attempts to get unstable plasmas under control. The few hundred thousand dollars spent on magnetic fusion in the early 1950s turned into nearly $5