Hohlraum

In blackbody radiation at the temperature of the interior of the container.

Inertial Confinement Fusion

In the "indirect drive" approach to inertial confinement fusion, the 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 utilizes a radiation case or Hohlraum to contain the energy of the first fission stage (primary) and implode a second fusion stage (secondary).