Hohlraum

A small metal can, about a centimetre long and usually made of gold or depleted uranium, used in indirect-drive laser fusion. Lasers heat the inside of the hohlraum until it glows with X-rays, and those X-rays, rather than the laser light itself, implode the fuel capsule held at its centre. The National Ignition Facility uses this method. Converting laser light to X-rays and then into the capsule costs energy: roughly a tenth of the laser energy delivered to a hohlraum ends up absorbed by the capsule inside it. That is why capsule gain is so much larger than target gain. Direct drive avoids the conversion but must light the capsule far more evenly.

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