Hohlraum. (Source: Wikipedia.org)

A metallic chamber into which a target of frozen fusion fuel is inserted and compressed to fusion temperatures by X-rays generated when laser beams hit the inner walls of the hohlraum. Hohlraum means “cavity” in German and it is usually a hollow cylinder made of lead, gold, or gold-coated uranium. In the middle of it is a pellet of frozen hydrogen suspended on plastic filaments. On both ends of the hohlraum are openings through which lasers are aimed at the inner walls of the hohlraum. Heated by an intense laser pulse, walls start to generate strong X-rays. As the hohlraum is effectively a resonant cavity, those X-rays will homogeneously fill its interior and evenly compress the pellet. The hohlraum is a disposable device that is destroyed during the experiment.