Originally, glass microballoons containing DT gas were used as targets. They were like the glass beads used to coat projector screens but had to be perfectly round and smooth.

One is shown in Fig. 10.45a, and a number of them are shown on a coin in Fig. 10.45b. When hit with lasers, the glass exploded, half going out and half going in, compressing the gas. This is how the first fusion neutrons were observed.

Later targets used low-Z ablators to have a more controlled compression (Z is the atomic number). Examples of target designs are shown in Fig. 10.46. All of them have a shell of frozen DT as the fuel. In panel (a), there is also a bit of DT at the center, confined by a heavy pusher. This is supposed to ignite first, giving energy to help ignite the main fuel. In panel (b), the ablator is polystyrene foam, which allows DT gas to be permeated into the capsule without using a fine tube, as in Fig. 10.45a. The DT is frozen at cryogenic temperatures, and is melted and smoothed by the little bit of heat from the decay of the tritium. In panel (c), a beryllium

ablator is used in a design to optimize shock heating. To improve compression, multiple shocks can be created by shaping the laser pulse into increasingly strong steps. Since strong shocks travel faster than weak ones, multiple shocks can be timed to catch up with one another just when they reach the center.

Target design is very computation-intensive, since the progression of the implo­sion has to be predicted. Designs differ depending on their purpose and the driver. Making just one of these targets takes great skill and cost. In a reactor, each pellet can cost no more than $0.50. Surprisingly, it is predicted that in mass production, these targets can be made for only $0.16 each [45]. Tens of thousands can be made at once in fluidized beds, and the infusion of DT into the spheres and the freezing of a layer at 18 K can be done to a whole batch at once since injection of DT through individual micron-size tubes is no longer necessary.