This term describes systems which can deposit huge amounts of energy in a short time, but directly, without lasers. Alan Kolb, one of the earliest fusion researchers, left that program to start the field of pulsed power by founding Maxwell Laboratories in San Diego, California, to develop large, fast capacitors for storing energy. They were the first to put “a megajoule in a can.” A megajoule is not an incomprehensible amount of energy. It is the heat energy of a pot (3 L) of water at boiling temperature. A 50 ampere-hour car battery contains 2 MJ. What matters is how fast the energy can be released to get power. Power is the rate of energy delivery. While a car battery can be drained in an hours, capacitors can release their energy in nanoseconds. Capacitors can store over 2 J/cm3. A megajoule can be crammed into 500,000 cm3, which is the size of a cube 80 cm (30 in.) on a side. A pulsed power installation has hundreds of these.

To get high voltage, the capacitors are hooked up in a Marx bank, shown in Fig. 10.52. In this arrangement, a DC power supply is connected to each capacitor as shown in the top half of the figure. After charging, the switches between the

capacitors are opened, as shown in the bottom half, and the diagonal switches are closed, connecting the capacitors in series. A much higher voltage is then produced than a single power supply can generate.

The current is then carried to the machine in a Blumlein. This is a big, specially designed transmission line that can handle the huge currents and voltages that the Marx bank can provide. The Blumlein uses water as the insulator, and also has magnetic insulation by the B-field generated by the current. The pulses can also be made shorter in the process. The spark-gap switches are perhaps the most important high-tech elements in the system.

Figure 10.53 is a diagram of the Z-machine at Sandia National Laboratories in Albuquerque, New Mexico, and Fig. 10.54 shows the actual machine. The capaci­tors surround the machine, and the cylinders are the Blumleins carrying the energy pulses into the vacuum chamber at the center. The capacitors in Z store 11.4 MJ,

of which 5 MJ is delivered by the Blumleins to the load. A 100-ns pulse can carry 20 MA of current and 60 TW of power. For military applications, the machine can produce 2 MJ of X-rays per pulse at a power of 200 TW.

For fusion applications, the Z-machine can produce heavy — or light-ion beams to transport to a capsule larger than those in laser fusion because of the higher energy here. The problem is in the transport. Ion beams are hard to keep in focus across the large distance to the pellet. When the beam becomes narrow near the target, its space charge will tend to expand it unless the charge is neutralized. The best way to do that is to send the ions through a preformed plasma, whose electrons can neutralize the space charge. This is a perfect setup for a beam-plasma instability. Ion-beam drivers have not been successfully developed. The plans now are to use the intense X-rays from pulsed power to fill a hohlraum. Even if this works, it cannot work at 10 Hz. Pulsed power is not a promising source for an inertial fusion driver.