Sun in a Bottle - Charles Seife [40]
The only problem with Spitzer’s tube was that it bottles the plasma on the sides, but not at the front or the back of the tube. When the moving plasma reaches the end of the tube, it spills right out. So what to do? Spitzer’s next clever idea was to imagine a tube without end: a donut. Such a donut (or a torus, as physicists and mathematicians like to call it) is just a tube that circles back upon itself. The plasma would move around and around in a circle, never spilling out the end of the tube. With the right magnetic field, it would be a perfect bottle.
Unfortunately, the very curvature that gets rid of the end of the tube makes it very difficult to set up the right kind of magnetic field. The straight tube merely needed a wire curled around it. But when you bend that tube into a donut, the loops of wire on the inside of the donut get bunched up and those on the outside get stretched and spaced out. As current flows down the wire, the magnetic field is stronger where the loops of wire are close together—near the donut hole—and weaker at the edge where the loops are far apart. The nice, even magnetic field in the tube is destroyed; it becomes an uneven mess with a strong side and a weak side. This unevenness is a big problem: it causes the nuclei and electrons in the plasma to drift in opposite directions and into the walls of the container. The bottle quickly loses its contents. It leaks. A straight tube leaks out its ends; a curved tube leaks through its sides.
Even though a torus-shaped bottle would be leaky, Spitzer quickly came up with a design to minimize the leak. Instead of a simple donut, he reasoned, it would be better to have two half donuts. These half donuts would be connected by tubes that crossed each other: a figure eight. The half-donut sections have the same problem as the full-donut bottle: the electrons and nuclei drift in opposite directions. However, because the tubes cross each other, the plasma winds up going through one half donut clockwise and the other one counterclockwise. This means that the drift on one side should be cancelled by an equal and opposite drift when the plasma goes through the other half donut. It doesn’t quite work that way; the drifts don’t cancel exactly, and the plasma still leaks out a bit, but the leak isn’t quite as severe as it would be in a torus-shaped bottle.
The figure-eight bottle was a good enough design for Spitzer to begin experimenting. In July 1951—as the Huemul controversy continued raging in the press—Spitzer got a grant from the Atomic Energy Commission and set to work at Princeton, generating a serious design for a figure-eight-shaped fusion reactor: the Stellarator.
The Stellarator wasn’t the only fusion reactor in town. Shortly after the Livermore laboratory was founded, some of its scientists proposed a slightly different shape for a magnetic bottle. They would stick with a straight tube. Instead of wrestling with the problems caused by curving the plasma’s path, they would try to cap the ends of the tube by tweaking the magnetic fields slightly. Strong magnetic fields at the ends of the tube and slightly weaker magnetic fields at the center would create barriers that would behave almost like a mirror. Some—not all—of the plasma streaming to the end of the tube would be reflected back inside. This magnetic mirror was extremely porous, so it was clearly not a perfect bottle, but neither was the Stellarator. The Livermore scientists got down to work.
A third contender came from across the Atlantic. In the late 1940s, British scientists were also beginning to think about confining plasma, and their method relied on an entirely different phenomenon that they called the “pinch” effect.
A pinch starts with a cylinder of plasma. Since the electrons are free to move around inside the cloud, the plasma itself conducts electricity; it’s almost like a piece of copper. You can send an electric current along a plasma cylinder just as