Challenges to understanding radiative shocks

Shock waves driven above a threshold velocity near 100 km/s become strongly radiative, converting most of the incoming energy flux into radiation. We produce such shock waves in Xe or Ar by using a laser to shock, ionize, and accelerate a Be plate into a gas-filled shock tube. Structure develops in these systems due to both radiative energy transfer and hydrodynamic instability. We are conducting such experiments, implementing a code to model them, and implementing software to assess the predictive capability of the code in our Center for Radiative Shock Hydrodynamics. The experiments typically use a 20 micron thick Be slab, which caps a polyimide tube (~600 microns dia.) filled with nominally 1.1 atm. Xe or Ar. Laser irradiation by 0.35-micron light at about 10¹⁵ W/cm² for 1 ns initiates the experiment. The diagnostics have included X-ray radiography, X-ray Thomson scattering, optical pyrometry, UV Thomson scattering, and VISAR. Radiography is the workhorse. The code combines a hyrodynamic solver implemented in the 3D, adaptive MHD code BATSRUS with radiation solvers in BATSRUS (diffusion) or PDT (transport). Material tracking uses a level sets approach. Equation of state and opacity are being implemented to use either inline models or tables. The approach to assessing predictive capability involves a combination of dimension reduction, to reduce the input space to be sampled, with sensitivity studies to assess which experimental or computational variables are important individually or in combination. Bayesian and other methods will be used to improve understanding of the modeling methods and parameters.