Formhohlraum

Formhohlraum, or shaped hohlraum, is a type of hohlraum used in inertial confinement fusion experiments. Its inner geometry is deliberately non-spherical to control how the laser-produced X-ray radiation fills the cavity and is delivered to the fusion capsule. By shaping the cavity and the arrangement of laser entrance holes, researchers seek a more uniform radiation drive, which is crucial for symmetric capsule compression.

Typical formhohlraum designs employ ellipsoidal or rugby-ball shapes (and other three-dimensional geometries) with high-Z inner walls, commonly gold or a gold-coated material, to efficiently convert laser energy into X-rays. The lasers are arranged around the hohlraum so that the resulting X-ray bath reaches the capsule with minimal asymmetry. The entrance holes and wall coatings are tuned to adjust spectral content and drive timing.

The intended benefit is improved implosion symmetry and reduced hydrodynamic instabilities, enabling more reliable compression and potentially higher fusion yield. Shaped hohlraums also enable tailored radiation pulses, allowing researchers to sculpt the drive in time to match the capsule dynamics. However, manufacturing tolerances, laser-plasma instabilities, and energy losses at the laser entrance holes pose ongoing challenges.

Formhohlraums are part of the broader study of hohlraums in inertial confinement fusion, including spherical and cylindrical variants. They have been tested in facilities such as the National Ignition Facility and the OMEGA Laser Facility as part of drive-symmetry experiments and spectroscopic studies of x-ray production and coupling to the capsule.