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 t_HS = 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 t_HS 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.

中文翻译：

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在 LMJ 观测到的辐射波从超音速到亚音速的转变

我们详细介绍了在 Laser Mégajoule 设施中进行的一项实验的结果，该实验旨在研究低密度 CHOBr 泡沫塑料管中从超音速辐射前沿到激波前沿的转变，该泡沫塑料管由激光加热球形金腔中产生的热发射驱动。使用 Sc 源（光子能量约为 4.3 keV）的时间分辨 2D 硬 X 射线摄影成像用于测量密度扰动前沿位置、吸收、曲率和冲击材料压缩（定义为压缩泡沫密度归一化为其标称值）值）从超音速传播到亚音速传播机制。在压缩从 1（有限流体动力学）到 4（形成强冲击）的这两种状态之间，在过渡时间 t _HS观察到泡沫压缩的快速增加= 6.75 ± 0.75 ns，对应于瞬态跨音速状态（HS 的意思是“流体动力分离”，指的是激波和辐射前沿物理分离的瞬间）。这个时间与~2 的泡沫压缩比和低于 M < 2 的减速前沿的马赫数有关。实验结果成功地与 3D 流体动力学模拟进行了比较；据我们所知，在过去的类似研究中，从未对该制度进行比较。模拟表明过渡时间t _HS对管入口的辐射闭合敏感。这种在 3D 中发生的闭合会影响从激光加热腔耦合到 CHOBr 泡沫的 X 射线能量量，从而影响瞬态跨音速状态动力学。