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Picosecond Diffraction from Shocked CrystalsJustin Wark, Oxford University, UK In recent years multi-million atom molecular dynamics (MD) simulations of shocked crystals have opened up new avenues of understanding of the fundamental physics underlying the shock compression process at the lattice level. [1] Much of this work has been pioneered at LANL. In parallel with these computational advances, experimental technique such as in situ picosecond X-ray diffraction have been developed that also yield lattice level information on time-scales that are now not too dissimilar to those pertinent to the MD simulations. We report on experiments where we have used laser-plasma generated X-rays to study how single crystals of metals (copper and iron) react to uniaxial shock compression. The iron work is of interest, as it is well-known that iron undergoes the £\-£` transition under compression, where the initially body centred cubic iron (bcc) is thought to transform to a hexagonal close-packed (hcp) structure. This transition is of particular historical importance, as, indeed, it was the first to be inferred in a shock-wave measurement (again at LANL) over half a century ago (via the observation of a multiple-wave structure). However, in situ direct evidence for the structure has not previously been reported. We present nanosecond diffraction data that show atomic rearrangement which is remarkably consistent with MD simulations,[2] as well as new work that indicates that the transition dynamics are highly dependent on crystal orientation.[3][1] B. L. Holian and P. S. Lomdahl, Science, L2085¡V2088 (1998). [2] D. H. Kalantar, J. F. Belak, G. W. Collins, J. D. Colvin, H. M. Davies, J. H. Eggert, T. C. Germann, J. Hawreliak, B. Holian, K. Kadau, P. S. Lomdahl, H. E. Lorenzana, M. A. Meyers, K. Rosolankova, M.S. Schneider, J. Sheppard, J. S. Stolken, and J. S. Wark, Phys. Rev. Lett., 95, 075502 (2005). [3] K. Kadau, T. C. Germann, P. S. Lomdahl, and B. L. Holian, Science, 296, 1681¡V1684 (2002).
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