The first attempts at fusion were carried out with a simple device called a “pinch,” which we will describe first. This was a tube filled with a low-pressure gas in which a large pulsed current was driven by a voltage applied to electrodes at either end. As shown in Fig. 7.19, the current first ionizes the gas into a plasma and then gener­ates a magnetic field surrounding the plasma. If the cylinder were turned into a torus, the current would be in the toroidal direction, and the field in the poloidal direction. This is like the current in a tokamak, but in a pinch there is no toroidal

field from external coils. The magnetic pressure of the “poloidal” field in Fig. 7.19 then compresses the plasma to a smaller diameter, whereupon the magnetic field gets stronger, compressing the plasma even more. Since compression heats the plasma, the hope was that the heating would reach fusion temperatures. Of course, the system suffered from the kink instability described in Chap. 6, and the kinks drove the plasma into the walls.

The Ware pinch [6] is a more subtle effect occurring in tokamaks and affecting mostly particles which move in banana orbits. The mechanism is illustrated in Fig. 7.20. In tokamaks in which the toroidal current J is driven by a toroidal electric field E, the poloidal-field component Bp which J produces is in the direction shown in diagram. This is the field that gives the necessary twist to the magnetic lines. Crossed electric and magnetic fields give rise to a perpendicular E x B drift of the guiding centers, as shown in Fig. 5.4. This drift is always toward the center of the cross section regardless of where the particle is in its banana orbit, and the drift has the same direction and magnitude for both ions and directions. Note that Bp is small compared to the toroidal component Bt, but Bt is parallel to E, not crossed with it, so it does not give an E x B drift. Thus, the principal fields in a tokamak generate a drift that counteracts the outward diffusion of the plasma, at least for particles trapped in bananas. The Ware pinch effect was invented to explain observations of oscillations occurring when the pinching reached its limits and started over again. This effect has been observed in other tokamaks and is not an artifact of neoclassical theory. It is another of Mother Nature’s gifts.