Formhohlraums

Formhohlraums are a class of hohlraums—cavities used in inertial confinement fusion to convert laser energy into a bath of x-ray radiation—defined by engineered geometric shapes intended to shape the radiation drive. The term Formhohlraum (plural Formhohlräume) is used in German-language literature to indicate a “shaped” or “form-designed” hohlraum.

In typical ICF experiments a fuel capsule is placed inside the hohlraum and irradiated by laser beams entering through laser entrance holes. The inner surface's geometry and the wall material influence how the laser energy is re-emitted as x-rays, thereby controlling the symmetry and time profile of the drive on the capsule. Formhohlraums explore non-spherical geometries—such as elongated, rugby-ball, or other tailored shapes—to reduce low-order asymmetries and to tailor the temporal evolution of the x-ray flux. Multi-hole configurations may be used to distribute drive more uniformly.

Benefits and challenges accompany these designs. Formhohlraums can improve drive uniformity and robustness against certain imbalances, but they require complex design, precise manufacturing, and sophisticated simulations to predict radiation transport and the hydrodynamic response of the capsule. They remain a topic of ongoing research in high-energy-density physics and inertial confinement fusion, contributing to the broader understanding of radiation hydrodynamics and implosion physics.