E.S. Myra

CRASH: A BLOCK-ADAPTIVE-MESH CODE FOR RADIATIVE SHOCK HYDRODYNAMICS-IMPLEMENTATION AND VERIFICATION

We describe the Center for Radiative Shock Hydrodynamics (CRASH) code, a block-adaptive-mesh code for multi-material radiation hydrodynamics. The implementation solves the radiation diffusion model with a gray or multi-group method and uses a flux-limited diffusion approximation to recover the free-streaming limit. Electrons and ions are allowed to have different temperatures and we include flux-limited electron heat conduction. The radiation hydrodynamic equations are solved in the Eulerian frame by means of a conservative finite-volume discretization in either one-, two-, or three-dimensional slab geometry or in two-dimensional cylindrical symmetry. An operator-split method is used to solve these equations in three substeps: (1) an explicit step of a shock-capturing hydrodynamic solver; (2) a linear advection of the radiation in frequency-logarithm space; and (3) an implicit solution of the stiff radiation diffusion, heat conduction, and energy exchange. We present a suite of verification test problems to demonstrate the accuracy and performance of the algorithms. The applications are for astrophysics and laboratory astrophysics. The CRASH code is an extension of the Block-Adaptive Tree Solarwind Roe Upwind Scheme (BATS-R-US) code with a new radiation transfer and heat conduction library and equation-of-state and multi-group opacity solvers. Both CRASH and BATS-R-US are part of the publicly available Space Weather Modeling Framework.

Oblique radiative shocks, including their interactions with nonradiative polytropic shocks

A theory of shocks dominated by radiation energy flux in optically mixed thin-upstream thick-downstream systems, in which the temperature immediately ahead and some short distance behind the shock front are equilibrated by radiation transport, is presented. This theory is applied to determine properties of the normal and oblique radiative shock, followed by applications to interactions when radiative and polytropic shocks are present in the same system. Comparison with experimental data is presented.

Experiments and simulations of radiative shocks

When shocks propagate at a sufficiently high velocity, radiation can significantly alter their structure. The radiation rapidly cools the shocked material, and the overall compression produced by the shock, which is limited to a factor of 4 for a simple hydrodynamic shock in a monatomic gas, can easily reach a factor of 25 or more. In addition, the radiation pre-heats the material ahead of the shock. This paper provides an overview of efforts at the Center for Radiative Shock Hydrodynamics at the University of Michigan to study the physics of radiative shocks propagating through plasma. This study is motivated by a desire to understand radiative shocks in astrophysical environments, particularly shocks emerging from supernova explosions. This paper will describe both laboratory experiments with high-power lasers and numerical simulations designed to study radiative shocks.

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.

Using high power lasers to create radiative shock waves

The paper discusses experiments, modeling, and uncertainty assessment radiative shocks produced by using high-power lasers to launch a thin Be plate into Xe gas at > 100 km/s.