Suzanne Ali

Heterogeneous flow and brittle failure in shock-compressed silicon

We combine a recently developed high-resolution two-dimensional (2D) imaging velocimetry technique (velocity interferometer system for any reflector (VISAR)) with 1D VISAR measurements to construct a moving picture of heterogeneous deformation in shock-compressed single crystal silicon. The 2D VISAR takes an intensity snapshot of target velocity and reflectivity over a mm field-of-view while the compression history is simultaneously recorded by the 1D VISAR. Our data show particle velocity surface roughening due to the anisotropic onset of plasticity and, above ∼13 GPa, a structural phase transformation. Shock arrival at the Si free-surface is characterized by the formation of fracture networks and incipient velocity jetting.

Probing the seeding of hydrodynamic instabilities from nonuniformities in ablator materials using 2D velocimetry

Despite the extensive work done to characterize and improve the smoothness of ablator materials used in inertial confinement fusion (ICF), features indicative of seeded instability growth in these materials are still observed. A two-dimensional imaging velocimetry technique has been used on Omega to measure the velocity non-uniformities of shock fronts launched by indirect drive in the three ablator materials of current interest, glow-discharge polymer, beryllium, and high-density carbon ablators. Observed features are deviations from shock front planarity with amplitudes of a few tens of nanometers, local velocity variations of a few tens of m/s, and transverse spatial scales ranging from 5 to 200 μm. These data will help develop a full understanding of the effects of surface topography, dynamic material response, and internal heterogeneities on the stability of ICF capsules. For all three ablators, we have quantified perturbations at amplitudes that can dominate conventional surface roughness seeds to hydrodynamic instability.Despite the extensive work done to characterize and improve the smoothness of ablator materials used in inertial confinement fusion (ICF), features indicative of seeded instability growth in these materials are still observed. A two-dimensional imaging velocimetry technique has been used on Omega to measure the velocity non-uniformities of shock fronts launched by indirect drive in the three ablator materials of current interest, glow-discharge polymer, beryllium, and high-density carbon ablators. Observed features are deviations from shock front planarity with amplitudes of a few tens of nanometers, local velocity variations of a few tens of m/s, and transverse spatial scales ranging from 5 to 200 μm. These data will help develop a full understanding of the effects of surface topography, dynamic material response, and internal heterogeneities on the stability of ICF capsules. For all three ablators, we have quantified perturbations at amplitudes that can dominate conventional surface roughness seeds to h...

An iterative forward analysis technique to determine the equation of state of dynamically compressed materials

We developed an iterative forward analysis (IFA) technique with the ability to use hydrocode simulations as a fitting function for analysis of dynamic compression experiments. The IFA method optimizes over parameterized quantities in the hydrocode simulations, breaking the degeneracy of contributions to the measured material response. Velocity profiles from synthetic data generated using a hydrocode simulation are analyzed as a first-order validation of the technique. We also analyze multiple magnetically driven ramp compression experiments on copper and compare with more conventional techniques. Excellent agreement is obtained in both cases.

Lithographically fabricated gratings for the interferometric measurement of material shear moduli under extreme conditions

Electron beam lithography and photolithography were used to fabricate diffraction gratings on targets for laser-driven shock-wave experiments. This target design was used with an optical interferometric system to measure transverse wave motion of the target during dynamic (shock-wave) compression. A wedged-shaped diamond substrate and reflective grating on the samples surface allowed detection of transverse motion. Proof of principle tests on single-crystal 〈100〉 Si samples gave a transverse wave speed of 5.9 km/s at 5 GPa and a shear modulus of 81 GPa. This experimental design has tremendous potential, including the possibility of measuring the shear properties of pure iron at Earth core conditions.Electron beam lithography and photolithography were used to fabricate diffraction gratings on targets for laser-driven shock-wave experiments. This target design was used with an optical interferometric system to measure transverse wave motion of the target during dynamic (shock-wave) compression. A wedged-shaped diamond substrate and reflective grating on the samples surface allowed detection of transverse motion. Proof of principle tests on single-crystal 〈100〉 Si samples gave a transverse wave speed of 5.9 km/s at 5 GPa and a shear modulus of 81 GPa. This experimental design has tremendous potential, including the possibility of measuring the shear properties of pure iron at Earth core conditions.