So far, we have encountered no insuperable problems. We can build a torus with a helical magnetic field and nested magnetic surfaces which should contain a plasma. We know how to make coils that will generate the 1-T magnetic field to hold the plasma pressure. Even if the plasma pressure is higher than 3-4 atm, a mere dou­bling of the field strength to 2 T will hold four times as much plasma because the field pressure increases as the square of the field strength. That toroidal fields which hold single particles for millions of traverses around a torus was shown very early in the game [3]. As we shall see, the Lawson criterion on the nt product would be easy to attain if a plasma behaved like a normal gas. The problem is that plasma is a special kind of gas, and an ornery one at that.

We said before that “plasma” is a misnomer because a plasma is not easily shaped or formed. Nature abhors a vacuum. A magnetic field is a vacuum. Plasma will try to cross the field and expand to fill the material container. Although the magnetic field keeps each ion and electron spiraling in a Larmor orbit so that each particle by itself cannot cross the field lines, the ensemble of particles can form ways to escape. This is because the particles are charged and can clump together to create electric fields, and these electric fields can take plasma across field lines. The plasma behaves more like a fluid (air or water, for instance) than like a collection of parti­cles, each acting by itself. Since the particles are charged, a plasma can pull Houdini tricks that air or water cannot. Like an ant colony, the community can accomplish more than the individuals. Metaphors aside, these escape mechanisms are called instabilities, which are responsible for the slow progress in fusion up to now, and which are the subject of most of the technical literature on plasma physics. Before we can describe instabilities, we have to tell more about how a plasma behaves.