A Z-pinch, called a Zed-pinch in England, is the simplest of all fusion approaches. It involves no more than pulsing a large current between two electrodes immersed in deuterium or DT. A column of plasma is ionized by the current and is confined by the magnetic field generated by the current itself, with no need for external coils. The B-field is very strong, since it surrounds the plasma closely, and it compresses the plasma until its density and temperature are high enough for fusion. It is very unstable, of course, because of the kink instability (Fig. 6.2). Since Z-pinches are so easy to make, much effort had been spent in trying to stabilize the pinch or to make it so fast that fusion occurs before the instability breaks it apart. These unsuc­cessful efforts became outmoded with the invention of wire arrays.

Starting with a current through a tungsten wire was found to improve Z-pinches because of the initial straight path and the slower motion of the heavy ions. A ring of tungsten wires, as shown in Fig. 10.36, was found to be qualitatively better because of the blending of their magnetic fields. If the wires are close enough together, the B-fields outside the circle form an overall azimuthal field that com­presses all the plasma into the center without kink instabilities because the current through each wire is comparatively small.

Figure 10.37a is a photograph of a 4-cm diameter array of 240 tungsten wires, each 7.5 pm (0.0075 mm) in diameter, at Sandia National Laboratories in New Mexico. In the Z-machine (described later), some 20 MA of current was pushed through the wires, forming a dense Z-pinch in the center. The aim was to generate X-rays, and about 200 TW (200 trillion watts) of these were generated [36]. This is a spectacular result, since the total electrical generating capacity of the USA is only about 1 TW. Of course, the X-ray pulse lasts only a nanosecond or so. Figure 10.37b is the same pinch fitted with an inner array of 120 wires. The plasma from the inner wires creates a plasma that smoothes out the instabilities that start to develop in the outer plasma, and the X-ray power is increased to 280 TW [37]. However, these pinches cannot produce continuous energy for primary power.

The Magpie Project of Imperial College, London, conducts innovative research on wire-array Z-pinches in which the wires point radially outward from the center [38]. External magnetic fields can also be imbedded in the pinch. Though the work is interesting, it is done for other purposes than energy production.