Even though a suitable driver has not yet been found, reactor studies can still be made, especially for the simpler direct drive case [42]. The energy released from each cap­sule is equivalent to tens of kilograms of TNT, but the blast waves of TNT cannot be produced because there is so little material involved [43]. The only ions are from the tiny capsule and the DT fuel, plus the helium that is produced. Much of the energy comes out as radiation, and the first wall has to withstand that. The neutrons, as usual, go through the first wall into breeding blankets. The first wall has to withstand the radiation, mostly in the form of X-rays. Inertial fusion has the advantage over toka — maks in that there is a much larger distance between the energy source and the wall. The main candidates for the first wall are (1) a solid material like the SiC/SiC com­pounds proposed for tokamaks, (2) a wall thinly wetted with a liquid, and (3) a water­fall of liquid FliBe (Chap. 9) covering a solid wall. In laser fusion, solid walls would suffer from repeated thermal cycling, which greatly shortens their life.

In direct drive, 71% of the fusion energy comes out as neutrons, 27% as ions, and only 1.4% as X-rays. In indirect drive, 69% comes out as neutrons, 5.8% as

ions, and a whopping 25% as X-rays since the hohlraums are designed to produce X-rays [47]. The ions and X-rays deposit their energy in a very thin layer on a dry wall [48], which must be well cooled to take the heat. A more serious problem is the deposition of the fast alpha particles into the wall, forming helium bubbles that cause the wall to exfoliate. A method to avoid this is to impose a cusp magnetic field (Fig. 7.8) to protect the wall and lead the ions into divertors. However, this requires strong superconducting coils as in magnetic fusion.

A wetted wall can be a thin layer of FliBe injected through small holes in the first wall and protecting the wall from ions and X-rays. The liquid is collected at the bottom, re-processed, and re-injected at the top of the chamber. A thick liquid wall [43] is a cylindrical waterfall of FliBe or PbLi between the target and the solid wall. The waterfall intercepts the fusion products, goes into a tank below the cham­ber, and is re-processed and re-injected at the top. In this case, the target has to enter from the top or bottom. Sethian et al. [42] have compared direct-drive reactors based on diode-pumped glass lasers and KrF lasers. Both kinds have been shown to withstand repeated pulsing at 5-10 Hz at low power. They have similar wall-plug efficiencies: 10% and 7%, respectively; they are compared in case high-power pulsing can be developed.

In inertial fusion, there is the problem of restoring the vacuum in the 100 ms between shots. The remaining gas must not be ionized by the laser. The laser beams have to strike the target with 20-pm accuracy from 10 to 20 m away, and a small amount of gas will deflect the beams. A “glint” system has been tested to overcome this [42]. As the cap­sule nears the center of the chamber, a small laser is fired to illuminate it. The direction of the reflection is detected, and mirrors are moved to keep the beam on target. To do this with 48 beams, however, is a daunting task, and only spherical targets in direct drive can be used. There is no clear path to fusion energy with lasers.