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Abstract The response of forsterite, Mg2SiO4, under dynamic compression is of fundamental importance for understanding its phase transformations and high‐pressure behavior. Here, we have carried out an in situ X‐ray diffraction study of laser‐shocked polycrystalline and single‐crystal forsterite (a‐,b‐, andc‐orientations) from 19 to 122 GPa using the Matter in Extreme Conditions end‐station of the Linac Coherent Light Source. Under laser‐based shock loading, forsterite does not transform to the high‐pressure equilibrium assemblage of MgSiO3bridgmanite and MgO periclase, as has been suggested previously. Instead, we observe forsterite and forsterite III, a metastable polymorph of Mg2SiO4, coexisting in a mixed‐phase region from 33 to 75 GPa for both polycrystalline and single‐crystal samples. Densities inferred from X‐ray diffraction data are consistent with earlier gas‐gun shock data. At higher stress, the response is sample‐dependent. Polycrystalline samples undergo amorphization above 79 GPa. For [010]‐ and [001]‐oriented crystals, a mixture of crystalline and amorphous material is observed to 108 GPa, whereas the [100]‐oriented forsterite adopts an unknown phase at 122 GPa. The first two sharp diffraction peaks of amorphous Mg2SiO4show a similar trend with compression as those observed for MgSiO3in both recent static‐ and laser‐driven shock experiments. Upon release to ambient pressure, all samples retain or revert to forsterite with evidence for amorphous material also present in some cases. This study demonstrates the utility of femtosecond free‐electron laser X‐ray sources for probing the temporal evolution of high‐pressure silicate structures through the nanosecond‐scale events of shock compression and release.
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Rippled shock propagation in a laser-driven target at multimegabar pressures
The evolution of non-uniform shocks produced by modulated laser irradiation or surface perturbations is relevant to studies of inertial confinement fusion and material properties at high-energy-density conditions. We present results from an experiment conducted at the OMEGA EP laser facility, where a 300 GPa shock was driven into a fused silica sample with pre-fabricated single-mode surface modulations. Using time-resolved optical velocimetry, we captured the continuous evolution of rippled shock motion, enabling a comprehensive mapping of the spatial amplitude history from formation to phase reversal in a single experiment. Initially, the ablation-driven shock inherits a fraction of the surface modulation amplitude from the sample, which subsequently grows before decaying, ultimately leading to the flattening of the rippled shock and a phase reversal. We find that two-dimensional inviscid hydrodynamic simulation of the experiment is able to qualitatively capture many aspects of the rippled shock evolution but over-predicts the initial amplitude growth. This experimental platform, capable of accommodating varying ripple wavelengths, lays the groundwork for a potential viscometry method at extreme pressures, where viscous effects manifest as differences in shock flattening times between rippled shocks of two distinct wavelengths propagating through the sample.
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- PAR ID:
- 10577712
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- Journal of Applied Physics
- Volume:
- 137
- Issue:
- 11
- ISSN:
- 0021-8979
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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