In radiation thermodynamics, a hohlraum (a German loanword, originally a non-specific word for "hollow area" or "cavity") is a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity. This idealized cavity can be approximated in practice by making a small perforation in the wall of a hollow container of any opaque material. The radiation escaping through such a perforation will be a good approximation to blackbody radiation at the temperature of the interior of the container.
Inertial Confinement Fusion
In the "indirect drive" approach to inertial confinement fusion, the fusion capsule is held inside a cylindrical hohlraum. The radiation source (e.g., laser) is pointed at the interior of the hohlraum, rather than on the capsule itself (a process known as "direct drive"), which absorbs and reradiates the energy as X-rays. The advantage to this approach is that the energy is reradiated in a much more symmetric fashion than would be possible in the direct drive approach, resulting in a more uniform implosion.
Nuclear weapon design
The Teller-Ulam design for thermonuclear weapons supposedly utilizes a radiation case or hohlraum to contain the energy of the first fission stage (primary) and implode a second fusion stage (secondary).
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