Katherine Anne Keilty

Developing a Radiative Shock Experiment Relevant to Astrophysics

We report on the initial results of experiments being developed on the Falcon laser to simulate radiative astrophysical shocks. Cylindrically diverging blast waves were produced in low-density ( approximately 1018 cm-3), high-Z gas by laser-irradiating Xe gas jets containing atomic clusters. The blast-wave trajectory was measured by Michelson interferometry. The velocity for the blast wave is slightly less than the adiabatic Sedov-Taylor prediction, and an ionization precursor is observed ahead of the shock front. This suggests energy loss through radiative cooling and reduced compression due to preheat deposited ahead of the shock, both consistent with one-dimensional radiation hydrodynamics simulations.

An Analytic Approximation to Radiative Blast Wave Evolution

The evolution of the well-known Sedov-Taylor adiabatic blast waves can be derived analytically using simple physical arguments. However, radiative blast waves with energy loss concentrated at the shock front have only been studied numerically. Here we show that if the shock radius evolution can be expressed as R ~ (at)n, then using an approximation similar to that discussed by Zeldovich & Raizer for the adiabatic case, we can derive simple analytic relations for n and a as functions of the pre- and postshock adiabatic indices, the energy loss fraction e, the initial energy E0, and initial deposition size R0. These analytic results are in excellent agreement with previous numerical results. We also find that the results are much more sensitive to the postshock adiabatic index in the central cavity than the preshock adiabatic index.

MODELING OF LASER-GENERATED RADIATIVE BLAST WAVES

We simulate experiments performed with the Falcon laser at Lawrence Livermore National Laboratory to generate strong, cylindrically diverging blast waves of relevance to astrophysics. In particular, we are interested in producing and modeling radiative shocks. We compare numerical simulations with the data and with an analytic approximation to blast-wave propagation with a radiative-loss term included. Our goal is to develop a laboratory setting for studying radiative shocks of relevance to supernova remnants, gamma-ray burst afterglows, and other high-energy astrophysics phenomena. We will show that a good degree of agreement exists between the experimental data and the numerical simulations, demonstrating that it is indeed possible to generate radiative shocks in the laboratory using tabletop femtosecond lasers. In addition, we show how we can determine the energy-loss rate from the blast-wave evolution. This analytic method is independent of the exact mechanism of radiative cooling and is scalable to both the laboratory and astrophysical radiative blast waves.

Numerical Simulations of Blast Waves Generated by an Impulsive Temperature Source

We describe some of the results arriving from numerical simulations of blast waves using different volumes for the energy deposition region.

Modeling of radiative blast waves

We simulate experiments performed with the Falcon laser at Lawrence Livermore National Laboratory to generate strong blast waves expanding in cylindrical geometry of relevance to astrophysics. In particular, we are interested in producing and modeling radiative shocks. Our goal is to develop a laboratory setting for studying radiative shocks of relevance to supernova remnants (SNR). Although late-term supernovae are known for exhibiting radiative shocks, it is also likely that some young SNR are also radiative when they expand into a dense interstellar medium (ISM). In previous work we have demonstrated that it is possible to generate radiative shocks in the laboratory. In addition, we have shown how we can determine the energy-loss rate of the shock from the blast wave evolution using a simple analytic method that is independent of the details of radiative cooling, and is scalable to both the laboratory and astrophysical blast waves. Our future work deals with instabilities associated with radiative blas...