Experimental Evidence of Radiation Trapping in Plastic Shell Implosions on OMEGA

In systems at extreme energy densities, the radiation field may couple to the hydrodynamic motion of the system, leading to interesting emergent phenomena. A quintessential manifestation of this is the formation of a Marshak wave, in which a steep temperature front is driven into an optically thick medium by an intense radiation source. These radiatively-driven heat waves have been observed in numerous laboratory and astrophysical plasmas and are predicted to occur during the stagnation phase of certain laser-driven implosion experiments. In these implosions, a large fraction of the emitted radiation is absorbed by the shell and re-radiated, reducing radiative losses in a process known "radiation trapping." This process is key to many volume ignition schemes for inertial confinement fusion but is difficult to measure experimentally. This work presents a set of nuclear and x-ray self-emission data from a series of implosion experiments at the OMEGA laser facility using deuterium-filled plastic (CH) shells that is consistent with the formation and burnthrough of a Marshak wave during stagnation, providing the first experimental evidence that radiation trapping may play a role in the dynamics of these systems and providing insight into the interaction of matter and radiation at extreme conditions.