hohlraum is the small metal cavity that converts laser light into X-rays in indirect-drive inertial fusion, bathing the fuel capsule suspended at its center in a symmetric X-ray drive.
The word is German for "hollow space," and it entered physics through the nineteenth-century study of cavity radiation — the blackbody problem — where a heated enclosed cavity serves as an ideal radiator. Indirect-drive 📝fusion puts that same physics to work: rather than aiming laser beams at the fuel capsule itself, as 📝direct drive does, the beams enter the cavity through holes in its ends and strike its inner walls, which re-emit the energy as a bath of soft X-rays that ablates the capsule's surface from all directions at once.
At the National Ignition Facility, the hohlraum is a gold or depleted-uranium cylinder roughly a centimeter long, and all 192 laser beams heat it rather than the capsule. The scheme trades efficiency for symmetry: only a modest fraction of the laser energy ultimately reaches the fuel, but the X-ray bath smooths out beam-to-beam nonuniformities that would otherwise seed instabilities during the implosion. That trade is what carried 📝inertial confinement fusion (ICF) to the first laboratory 📝ignition in December 2022 — an indirect-drive shot.