We have made the first detailed measurements of a diffusive supersonic radiation wave in the laboratory. A $10\mathrm{mg}/{\mathrm{cm}}^3$ ${\mathrm{SiO}}_2$ foam is radiatively heated by the x-ray flux from a laser-irradiated hohlraum. The resulting radiation wave propagates axially through the optically thick foam and is measured via time-resolved x-ray imaging as it breaks out the far end. The data show that the radiation wave breaks out at the center prior to breaking out at the edges, indicating a significant curvature in the radiation front. This curvature is primarily due to energy loss into the walls surrounding the foam.

• Received 11 August 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.274