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Abstract:

We have previously reported the experimental discovery of a second shock forming ahead of a radiative shock propagating in Xe. The initial shock is spherical, radiative, with a high Mach number, and it sends a supersonic radiative heat wave far ahead of itself. The heat wave rapidly slows to a transonic regime and when its Mach number drops to two with respect to the downstream plasma, the heat wave drives a second shock ahead of itself to satisfy mass and momentum conservation in the heat wave reference frame. We now show experimental data from a range of mixtures of Xe and N-2, gradually changing the properties of the initial shock and the environment into which the shock moves and radiates (the radiative conductivity and the heat capacity). We have successfully observed second shock formation over the entire range from 100% Xe mass fraction to 100% N-2. The formation radius of the second shock as a function of Xe mass fraction is consistent with an analytical estimate.

Abstract:

In situ picosecond X-ray diffraction has proved to be a useful tool in furthering our understanding of the response of shocked crystals at the lattice level. To date the vast majority of this work has used single crystals as the shocked samples, owing to their diffraction efficiency, although the study of the response of polycrystalline samples is clearly of interest for many applications. We present here the results of experiments to develop sub-nanosecond powder/polycrystalline diffraction using a cylindrical pinhole camera. By allowing the incident X-ray beam to impinge on the sample at non-normal angles, the response of grains making a variety of angles to the shock propagation direction can potentially be interrogated. © 2007 American Institute of Physics.

Abstract:

We report on large-scale non-equilibrium atomistic simulations of shock-induced transformations in poly crystalline iron samples. These simulations show that, depending on the crystallographic orientation of the body-centered-cubic (bcc) parent phase grains with respect to the shock direction, a significant fraction of the product phase can be face-centered-cubic (fcc) instead of the expected hexagonal-close-packed (hcp) structure. This observation is explained by the existence of different transformation mechanisms for shocks along different crystallographic directions. We conclude that the observation of different product phases can be explained by simple geometric considerations of the involved transformation mechanisms between the parent bcc structure and the product hcp and fcc structures. Ultrafast high-energy laser-based experiments are underway to further investigate this subject. © 2007 American Institute of Physics.