So we have found that the best way to produce fusion reactions in a continuous manner is to make a very hot plasma, so hot that it cannot be held in place by any material container. We also decided that of all the forces that we can use to make a wall-less container, only the magnetic force would work. What would a magnetic bottle look like? Actually, it looks like a bagel; but before we get to this, we have to review what we know about magnetic fields. Most people know that the earth has a magnetic field, as shown in Fig. 4.6. The lines with arrows show the direction of the field. A compass needle aligns itself with the field line that passes through it on the earth’s surface, and therefore points toward the magnetic pole, which is close to the geographic pole. The earth’s field is already a magnetic bottle, but an imperfect one. Protons and electrons coming from the sun in the solar wind3 get trapped in this field because charged particles tend to move along field lines, not across them. But the trap has large leaks at the north and south poles where the field lines run into the ionosphere, bringing the particles with them. When electrons strike oxygen atoms in our atmosphere, visible light is emitted which we call the Aurora Borealis. Since the plasma particles can travel in either direction, the same thing happens in the southern hemisphere. The Aurora Australis is not as well known because few people stay out on a winter’s night in Antarctica to watch it, and penguins have other agenda.

Magnetic field lines are, of course, only a mathematical construct. Electric or magnetic fields can be detected only by the forces that they exert. It was the great Scottish physicist James Clerk Maxwell4 who invented the concept of a “field” to describe action at a distance. Once a field at a given position is known, one can calculate the forces which that field would exert on an object there. To depict the shape of a field, one can draw any number of lines. A visual display of magnetic field lines is commonly given in textbooks, where the pattern of iron filings traces the field lines around a horseshoe magnet, as in Fig. 4.7.

Magnetic field lines are sometimes called “lines of force,” but this is a misnomer. The magnetic force is actually perpendicular to the lines! A compass needle points north-south because, when it is not aligned, the north pole of the needle is pushed one way by the magnetic field of the earth, and the south pole the other way, until the needle is aligned with it. Similarly, each elongated iron filing in the horseshoe demonstration acts like a miniature compass needle and points in the direction of the field at its location. It is important to understand what a field line represents, because how a magnetic bottle works depends critically on how these lines are shaped.

The problem with permanent magnets is that the strongest magnetic field it generates is inside the iron of the magnet, where we cannot put any plasma. Fortunately, we can create magnetic fields with electromagnets. In Fig. 4.8a, we show the field around a bar magnet, which is a magnetized iron cylinder; it has basically the same shape as the earth’s field. In Fig. 4.8b, we have replaced the iron bar with a glass tube of the same length and diameter, and we have wound many turns of wire around the tube. When we hook the wire up to a DC voltage source, such as a battery, the current in the wire generates a magnetic field of the same shape as that of the bar magnet! But now we can put plasma inside the glass tube, where the field is much stronger, as you can tell because the lines are closer together.

Fig. 4.8 The magnetic field around (a) a bar magnet and (b) an electromagnet of the same size

Now we can move on to see how to make a leak-proof magnetic bottle for plasma using cleverly shaped wire coils to produce field shapes that will plug all the leaks.