Supersonic-to-subsonic transition of a radiation wave observed at the LMJ

Abstract

We detail results of an experiment performed at the Laser Mégajoule facility aimed at studying transition from supersonic radiation front to shock front in a low density CHOBr foam enclosed in a plastic tube driven by thermal emission produced in a laser heated spherical gold cavity. Time resolved 2D hard x-ray radiography imaging using a Sc source (photon energy at ∼4.3 keV) is employed to measure the density perturbation front position, absorption, curvature, and shocked material compression (defined as the compressed foam density normalized to its nominal value) from the supersonic to the subsonic regimes of propagation. Between these two regimes where compression goes from 1 (limited hydrodynamics) to 4 (strong shock formed), a quick increase in the foam compression is observed at the transition time tHS = 6.75 ± 0.75 ns, corresponding to the transient transonic regime (HS means “hydrodynamically separated” and refers to the instant when the shock and the radiation front physically separate). This time is associated with a foam compression ratio of ∼2 and a Mach number of the slowing down front below M < 2. Experimental results are successfully compared to 3D hydrodynamics simulations; comparisons are never presented for that regime in similar past studies to our knowledge. Simulations show that the transition time tHS is sensitive to the radiation closure of the tube entrance. This closure, which occurs in 3D, affects the amount of x-ray energy coupled from the laser heated cavity to the CHOBr foam and consequently the transient transonic regime dynamics.