Sun in a Bottle - Charles Seife [39]
The electrons are still nearby, unattached to any particular nucleus. Unbound electrons and nuclei roam in one big blob, unattached to each other. At extremely high temperatures a hunk of hot matter becomes an undifferentiated soup of unconnected negatively charged electrons and positively charged nuclei.
This is a plasma. Pour enough energy into a piece of matter—heat it enough—and atoms lose their individuality. The positively charged nuclei are still attracted to the negatively charged electrons, but they are not bound together. And this gives a plasma some unusual properties. Unlike most kinds of ordinary matter—unlike most solids, liquids, and gases—the free-floating electrons and protons of a plasma are strongly affected by electric and magnetic fields.
To Lyman Spitzer, this suggested a design of a bottle that could hold a miniature sun. Spitzer’s bottle would not be made of steel or stone or diamonds. It would not be made of any kind of material at all; after all, nothing would be able to stand up to the immense heat of a fusion reaction. Spitzer’s bottle would be made of invisible lines of force: it would be made of magnetic fields.
By the twentieth century, these fields were extremely well understood. Physicists had long been amazed by the intricate interplay of electric fields, charged particles, and magnetic fields, but in the nineteenth century, physicists figured out that these interactions are governed by only a handful of relatively simple rules. Nonetheless, even simple rules can have seemingly complicated consequences.
For example, the laws of electromagnetism dictate that moving charges are affected by magnetic fields, while stationary ones are not. It’s a quirky-sounding rule, but it’s what the equations dictate: if you put a stationary charged particle (like a proton) in a magnetic field, it won’t feel the field at all. A charged particle that is moving, on the other hand, is tugged and deflected by a magnetic field. More specifically, a moving charged particle feels a magnetic pull perpendicular to its motion. This force makes the particle change course. Instead of moving in a straight line, the particle moves in a circle, and the stronger the magnetic field, the tighter the circle. Conversely, the equations of electromagnetism dictate that a moving electric charge (like an electron moving down a wire) will generate a magnetic field. A stationary electric charge won’t. In mathematical terms, these are pretty simple rules to describe. But just these rules can give you a hint of how complicated a plasma must be.
In a plasma, you have a large number of charged particles—electrons and nuclei—moving about at relatively high speeds. These moving particles generate magnetic fields. These magnetic fields change the motion of the moving particles. When the motion of the moving particles change, so do the magnetic fields that they are generating—which changes the motion of the particles, changing the magnetic fields, and so on. Add to that the electric attraction that the electrons and nuclei feel for each other and you’ve got an incredibly complex soup.
Nevertheless, to Spitzer, the mere fact that the plasma responds to magnetic fields suggested a way to bottle it up. He realized that if you had a plasma moving through a tube and you subjected that tube to a nice, strong magnetic field in the proper orientation, the charged particles in the soup would be forced to move in little circles. They would spiral down the tube in tight little helices, confined by the magnetic field, never even getting close to the walls of the cylinder. The plasma would be confined. In theory, even an extremely hot plasma could be trapped in such a bottle. Furthermore, it was fairly easy to generate the right sort of magnetic field: just wrap a coil of wire around the tube and put a strong current