Investigation of shock wave formation in radiative plasmas*

Shock waves developing in high-energy density laboratory or astrophysical conditions often involve the presence of a radiative field that needs to be properly modeled. Past theoretical efforts focused on the structure of stationary radiative shocks. A systematic classification proved to be complex as regions of optically thin and thick layers intertwine to form the precursor and relaxation regions, between which the hydrodynamic shock is embedded. In this talk, we present an alternative analysis where we consider the temporal evolution of weak shocks (nonlinear waves) in a radiative media. Applying a reductive perturbative method allows to derive a Korteveg-de Vries-Burgers equation (KdVB) that governs the evolution of the perturbed variables taking into account the radiation field. The method is used to investigate shock wave formation with applications to Z-Pinch implosions. It is found that the transition between optically thin and thick regimes is dynamic, and scaling laws for characteristic times and lengths are provided for each regime. The existence of the optically thin regime is related to the presence of an over-dense layer in the compressed material. Identifying the conditions for which this over-compression occurs has been fundamental in the design of staged Z-Pinch implosions. The theoretical analysis is supported by FLASH simulations, and a set of runs has been performed to systematically explore this effect in shocks of arbitrary intensity.