INDIRECT DRIVE

The basic principle of compressing a deuterium-tritium mixture is the same as for direct drive. (Deuterium and tritium are hydrogen isotopes with atomic numbers 2 and 3; they require the least energy to fuse.) However, the original light does not come from a laser, but from an (underground) nuclear explosion; and the real difference is that this pulse of radiation, with most of its energy in the light range, enters an underground Hohlraum or black-body cavity, presumably via some corridor through a small opening, where it sloshes around as in the figure for a picosecond or so, raising the temperature to some enormous effective value, and this effective temperature determines to what wavelength the radiation will be changed¾or rather, at what wavelength the spectrum will peak. It will be seen from the Planck curves in the figure that the higher the temperature, the more pronounced the peak, i.e., the wavelength around which the rest of the spectrum is crowded. Also, as the temperature increases, this peak moves toward shorter wavelengths. In the present case of an effective temperature of perhaps a million degrees (it no longer matters much whether Centigrade or Kelvin), this narrow spectrum falls into the band of X-rays, and they will now hit the pellet in the cavity from all sides¾not with the Nova microradian surgical knives from exact directions, but with a club from all directions. This is less efficient (a significant fraction is absorbed by the walls), but there is no arguing with success. If you are willing to explode one nuclear bomb per pellet, scientific breakeven has been achieved.