In radiation thermodynamics, a hohlraum (a non-specific German word for a "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
The indirect drive approach to inertial confinement fusion is as follows; the fusion fuel capsule is held inside a cylindrical hohlraum. The radiation source (e.g., laser) is pointed at the interior of the hohlraum, which absorbs and re-radiates the energy as X-rays, rather than on the capsule itself, a process known as indirect drive. The advantage to this approach is that the energy is re-radiated in a much more symmetric fashion than would be possible in the direct drive approach, resulting in a more uniform implosion.
The X-ray intensity around the capsule must be very symmetrical to avoid hydrodynamic instabilities during compression. Earlier designs had radiators at the ends of the hohlraum, but it proved difficult to maintain adequate X-ray symmetry with this geometry. By the end of the 1990s, target physicists developed a new family of designs in which the ion beams are absorbed in the hohlraum walls, so that X-rays are radiated from a large fraction of the solid angle surrounding the capsule. With a judicious choice of absorbing materials, this arrangement, referred to as a "distributed-radiator" target, gives better X-ray symmetry and target gain in simulations than earlier designs.
Nuclear weapon design
The term hohlraum is also used to describe the casing of a thermonuclear bomb following the Teller-Ulam design. The casing's purpose is to contain and focus the energy of the primary (fission) stage in order to implode the secondary (fusion) stage.
Notes and references
- NIF Hohlraum – High resolution picture at Lawrence Livermore National Laboratory.