Andrew Comley

Nanosecond white-light Laue diffraction measurements of dislocation microstructure in shock-compressed single-crystal copper

Under uniaxial high-stress shock compression it is believed that crystalline materials undergo complex, rapid, micro-structural changes to relieve the large applied shear stresses. Diagnosing the underlying mechanisms involved remains a significant challenge in the field of shock physics, and is critical for furthering our understanding of the fundamental lattice-level physics, and for the validation of multi-scale models of shock compression. Here we employ white-light X-ray Laue diffraction on a nanosecond timescale to make the first in situ observations of the stress relaxation mechanism in a laser-shocked crystal. The measurements were made on single-crystal copper, shocked along the [001] axis to peak stresses of order 50 GPa. The results demonstrate the presence of stress-dependent lattice rotations along specific crystallographic directions. The orientation of the rotations suggests that there is double slip on conjugate systems. In this model, the rotation magnitudes are consistent with defect densities of order 10(12) cm(-2).

Tailored ramp-loading via shock release of stepped-density reservoirsa)

The concept of a gradient piston drive has been extended from that of a single component reservoir, such as a high explosive, to that of a multi-component reservoir that utilizes low density foams and large shocks to achieve high pressures (∼3.5 mbar) and controlled pressure vs. time profiles on a driven sample. Simulated and experimental drives shaped through the use of multiple component (including carbonized resorcinol formaldehyde and SiO2 foam) reservoirs are compared. Individual density layers in a multiple component reservoir are shown to correlate with velocity features in the measured drive which enables the ability to tune a pressure drive by adjusting the components of the reservoir. Pre-shot simulations are shown to be in rough agreement with the data, but post-shot simulations involving the use of simulated plasma drives were needed to achieve an exact match. Results from a multiple component reservoir shot (∼3.5 mbar) at the National Ignition Facility are shown.

Absolute measurements of x-ray backlighter sources at energies above 10 keVa)

Line emission and broadband x-ray sources with x-ray energies above 10 keV have been investigated using a range of calibrated x-ray detectors for use as x-ray backlighters in high energy density (HED) experiments. The conversion efficiency of short- and long-pulse driven Mo and Ag line-emission backlighters at 17 and 22 keV was measured to investigate the crossover region between short- and long-pulse conversion efficiency. It was found that significant 17 and 22 keV line emissions were observed using a 3 ω, 1 ns long-pulse drive for Mo and Ag targets and a comparison between the measured Mo x-ray spectrum and calculations using an atomic physics code suggests that the line emission is due to thermal emission from N-like Mo atoms. Electron temperatures derived from fits to the continuum region of the x-ray spectra agree well with the Thot scaling as 100×(Iλ2)1/3. The continuum emissions from empty and 1 atm Kr-filled imploded CH shell targets were also measured for the use as broadband backlighters.

Theory and Simulation of 1D to 3D Plastic Relaxation in Tantalum

In plane shock waves the uniaxial strain rate can greatly exceed the rate at which dislocation flow can relax the concomitant shear stress. The result is an overdriven plastic state in which the compression is 1D uniaxial initially and only after a period of time does the lattice relax to a more 3D compressed state due to plastic flow. Here we use an analytic calculation based on a generalization of the Gilman model of flow involving dislocation evolution to predict the phases of plastic relaxation and to derive an analytic estimate of the relaxation time, including a decomposition into incubation and flow times, suitable for comparison with in-situ x-ray diffraction. We use molecular dynamics (MD) to study the threshold for homogeneous nucleation both in shock compression of single crystal Ta (100). We find that shock heating on the Hugoniot substantially lowers the threshold pressure for homogeneous nucleation.

Inelastic response of silicon to shock compression

The elastic and inelastic response of [001] oriented silicon to laser compression has been a topic of considerable discussion for well over a decade, yet there has been little progress in understanding the basic behaviour of this apparently simple material. We present experimental x-ray diffraction data showing complex elastic strain profiles in laser compressed samples on nanosecond timescales. We also present molecular dynamics and elasticity code modelling which suggests that a pressure induced phase transition is the cause of the previously reported ‘anomalous’ elastic waves. Moreover, this interpretation allows for measurement of the kinetic timescales for transition. This model is also discussed in the wider context of reported deformation of silicon to rapid compression in the literature.

Radiation transport in inhomogeneous media

Calculations of radiation transport in heated materials are greatly complicated by the presence of regions in which two or more materials are inhomogeneously mixed. This phenomenon is important in many systems, such as astrophysical systems where density clumps can be found in star-forming regions and molecular clouds. Laboratory experiments have been designed to test the modeling of radiation transport through inhomogeneous plasmas. A laser-heated hohlraum is used as a thermal source to drive radiation through polymer foam containing randomly distributed gold particles. Experimental measurements of radiation transport in foams with gold particle sizes ranging from 5–9μm to submicrometer diameters as well as the homogeneous foam case are presented. The simulation results of the radiation transport are compared to the experiment and show that an inhomogeneous transport model must be applied to explain radiation transport in foams loaded with 5μm diameter gold particles.

Quantifying equation-of-state and opacity errors using integrated supersonic diffusive radiation flow experiments on the National Ignition Facility

A well diagnosed campaign of supersonic, diffusive radiation flow experiments has been fielded on the National Ignition Facility. These experiments have used the accurate measurements of delivered laser energy and foam density to enable an investigation into SESAMEs tabulated equation-of-state values and CASSANDRAs predicted opacity values for the low-density C8H7Cl foam used throughout the campaign. We report that the results from initial simulations under-predicted the arrival time of the radiation wave through the foam by ≈22%. A simulation study was conducted that artificially scaled the equation-of-state and opacity with the intended aim of quantifying the systematic offsets in both CASSANDRA and SESAME. Two separate hypotheses which describe these errors have been tested using the entire ensemble of data, with one being supported by these data.

Interpretation of laser-driven V and TA Rayleigh-Taylor strength experiments

We present theoretical and computational analysis of the deformation regimes accessed by recent Rayleigh-Taylor (RT) material strength experiments in vanadium (V) and tantalum (Ta) done at the Omega laser at high pressures (>1 Mbar) and high strain rates (106 - 108 sec-1). Within the context of the LLNL multiscale models, the V-RT experiment appears to be dominated by deformation in the drag regime, whereas the Ta-RT experiment resides largely within the thermal activation regime.

Rayleigh-Taylor strength experiments of the pressure-induced α→ε→α′ phase transition in iron

We present here the first dynamic Rayleigh-Taylor (RT) strength measurement of a material undergoing solid-solid phase transition. Iron is quasi-isentropically driven across the pressure-induced bcc (α-Fe) → hcp (e-Fe) phase transition and the dynamic strength of the α, e and reverted α′ phases have been determined via proton radiography of the resulting Rayleigh-Taylor unstable interface between the iron target and high-explosive products. Simultaneous velocimetry measurements of the iron free surface yield the phase transition dynamics and, in conjunction with detailed hydrodynamic simulations, allow for determination of the strength of the distinct phases of iron. Forward analysis of the experiment via hydrodynamic simulations reveals significant strength enhancement of the dynamically-generated e-Fe and reverted α′-Fe, compareable in magnitude to the strength of austenitic stainless steels.

Single Hit Energy-resolved Laue Diffraction.

In situ white light Laue diffraction has been successfully used to interrogate the structure of single crystal materials undergoing rapid (nanosecond) dynamic compression up to megabar pressures. However, information on strain state accessible via this technique is limited, reducing its applicability for a range of applications. We present an extension to the existing Laue diffraction platform in which we record the photon energy of a subset of diffraction peaks. This allows for a measurement of the longitudinal and transverse strains in situ during compression. Consequently, we demonstrate measurement of volumetric compression of the unit cell, in addition to the limited aspect ratio information accessible in conventional white light Laue. We present preliminary results for silicon, where only an elastic strain is observed. VISAR measurements show the presence of a two wave structure and measurements show that material downstream of the second wave does not contribute to the observed diffraction peaks, supporting the idea that this material may be highly disordered, or has undergone large scale rotation.