The selection of a method to protect an ICF chamber wall depends on a number of factors including the energy yield per pulse, the pulse repetition rate, and chamber vacuum requirements. In general, as the pulse rate increases, more protection is needed-such as thick liquid metal wall chambers; relatively large radius dry wall chambers could be considered for pulse energies less than 200 MJ.

Laser and heavy ion beam drivers require a high vacuum for good beam transmission; for example, lasers are limited to less than a few Torr (1 Torr ~

133.3 Pa) to prevent excessive scattering. In cases such as light ion beams, a higher pressure of the order of 5 to 50 Torr may be acceptable. If higher pressures are allowed, additional protection can be obtained by using a gas-fill in the chamber. The gas absorbs much of the radiation energy which is re-emitted and hits the chamber wall; this process significantly spreads out the time-width of the energy pulse thus reducing the shock effect.

Another important consideration for chambers with wetted walls, is the time required to clean-that is to purge-the chamber between pulses. This sets a limit for a maximum pulse rate. For example, it appears that about 1 s will be required to pump vapourized lithium out through an exhaust nozzle for a lithium-waterfall system. For such designs, a practical operating region could be a 1 Hz repetition rate with 100 MJ micro-explosions giving an average power of ~ 100 MW per chamber. Multiple chambers employing sequential switching of the laser beam from one to another might be used with a laser operating at a higher pulse rate.