Ultrafast Optical Measurements of Shocked Materials
Much of the research discussed above was conducted in preparation for the studies that examined shock-induced chemistry in energetic materials. The knowledge we obtained regarding the influence of direct laser-drive upon the target on which the energetic polymer was coated, has led to our confidence that the reaction chemistry studies provide relevant information regarding the first decomposition steps in shock-loaded energetic materials. For example, the PMMA interferometric data were well-modeled using the bulk Hugoniots of both the PMMA and the aluminum; thus, the use of direct-drive did not perturb the thermodynamic state significantly from the Hugoniot, allowing us to make the assumption that on these timescales (hundreds of picoseconds), the states are well-approximated as the same relevant state obtained with gas-gun systems. We have also shown that caution must be exercised when attempting to use the interferometric data for the characterization of material motion; changes in the material properties can influence the interferometric data and must be deconvolved to yield the “true” surface motion. However, this also offers the possibility of using the changes in material properties as a measure of the shocked material’s thermodynamic state; discontinuities will exist in the index of refraction as the material crosses a phase boundary. These changes may then be used as a characterization tool if they are accurately measured using the interferometric techniques to measure phase boundaries under shock-loaded conditions. Finally, the culmination of these studies involved the first observation of reaction under shock loading conditions of an energetic material. We have shown that when an energetic polymer, PVN, is shocked to ∼200 kbar, we observe a disappearance of the NO2 vibration infrared absorption(s), indicating chemical reaction as the Shockwave traverses the film.
This work was performed at Los Alamos National Laboratory by the University of California under the auspices of the Department of Energy. under contract W-7405-ENG.
KeywordsLaser Ablation Particle Velocity Probe Pulse Energetic Material Surface Motion
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