K. K. Dannenberg

Observation of collapsing radiative shocks in laboratory experiments

This article reports the observation of the dense, collapsed layer produced by a radiative shock in a laboratory experiment. The experiment uses laser irradiation to accelerate a thin layer of solid-density material to above 100km∕s, the first to probe such high velocities in a radiative shock. The layer in turn drives a shock wave through a cylindrical volume of Xe gas (at ∼6mg∕cm3). Radiation from the shocked Xe removes enough energy that the shocked layer increases in density and collapses spatially. This type of system is relevant to a number of astrophysical contexts, providing the potential to observe phenomena of interest to astrophysics and to test astrophysical computer codes.

Thomson scattering from a shock front

We have obtained a Thomson scattering spectrum in the collective regime by scattering a probe beam from a shock front, in an experiment conducted at the Omega laser at the Laboratory for Laser Energetics. The probe beam was created by frequency converting a beamline at Omega to a 2 ns pulse of 0.263 m light, focused with a dedicated optical focusing system. The diagnostic system included collecting optics, spectrometer, and streak camera, with a scattering angle of 101°. The target included a primary shock tube, a 20-m-thick beryllium drive disk, 0.3-m-thick polyimide windows mounted on a secondary tube, and a gas fill tube. Detected acoustic waves propagated parallel to the target axis. Ten laser beams irradiated the beryllium disk with 0.351 m light at 5 10 14 W/c m 2 for 1 ns starting at to, driving a strong shock through argon gas at o = 1 mg/ cc. The 200 J probe beam fired at t = 19 ns for 2 ns, and at t = 20.1 ns a 0.3 ns signal was detected. We attribute this signal to scattering from the shocked argon, before the density increased above critical due to radiative collapse.

PREHEAT ISSUES IN HYDRODYNAMIC HEDLA EXPERIMENTS

Hydrodynamic experiments have become a very active area within High Energy Density Laboratory Astrophysics. In such experiments, preheat of an interior surface due to heating prior to shock arrival can alter the initial conditions for further evolution and can change the nature of the experiment (Olson et al., 2003). Unfortunately, preheat cannot typically be detected without undertaking dedicated experiments for this purpose. We have designed such experiments, relevant to hydrodynamic instability experiments using Omega Laser at intensities of ~1015 W/cm2. Simulations using the HYADES code suggest that radiative preheat alone causes the interface to move approximately 2 μm before the blast wave reaches it. Hot-electron preheat could cause much larger motions. These experiments will use VISAR to examine the motion of an aluminum sample layer at the rear interface of a standard hydrodynamic target during the period before the shock reaches it (Allen and Burton, 1993).