Perfecting the Magnetic Bottle

Abstract

The last chapter had a lot of information in it, so let us recapitulate. To get energy from the fusion of hydrogen into helium as occurs in the sun and other stars, we have to make a plasma of ionized hydrogen and electrons and hold it in a magnetic bottle, since the plasma will be too much hot to be held by any solid material. The way a magnetic field holds plasma particles is to make them turn in tight circles, called Larmor orbits, so that they cannot move sideways across magnetic field lines. However, the ions and electrons can move along the field lines in their thermal motions without restraint. Consequently, the magnetic container has to be shaped like a doughnut, a torus, so that the field lines can go around and around without ever running into the walls. The field lines also have to be twisted into helices to avoid a vertical drift of the particles that occurs in a torus but not in a straight cylinder. Ideally, each field line will trace out a magnetic surface as it goes around many times without ever coming back exactly on itself. The plasma is then confined on nested magnetic surfaces which never touch the wall. This ideal picture will be modified in this and later chapters as we understand more about the nature of these invisible, nonmaterial containers.

Numbers in superscripts indicate Notes and square brackets [] indicate References at the end of this chapter.