Note that a simple mirror has unfavorable curvature (Fig. 7.10) and is unstable to the basic Rayleigh-Taylor interchange instability (Fig. 5.5). This problem was solved by M. S. Ioffe [21] by adding what are now known as Ioffe bars, shown in Fig. 10.21. These are four conductors parallel to the axis adding a poloidal field to the mirror field. The plasma is squeezed into a peppermint-candy shape. The strength of the magnetic field now increases outward in every perpendicular direction, so it is ener­getically impossible for a Rayleigh-Taylor instability to develop and push the plasma out. This is called a minimum-B configuration, since the plasma sits in a minimum of the B-field. Of course, the plasma can still leak out the ends.

Now imagine how to combine the Ioffe bars with the circular coils into a single coil. This proceeded in two steps. First, one can combine them into two identical coils, called yin-yang coils, shown in Fig. 10.22. This was such an attractive shape that an artist made a sculpture of it (Fig. 10.23). Finally, all the necessary currents can be combined into a single coil called a baseball coil because it resembles the seam on a baseball. This is shown in Fig. 10.24.