Giles Kimminau

High-energy x-ray backlighter spectrum measurements using calibrated image plates.

The x-ray spectrum between 18 and 88 keV generated by a petawatt laser driven x-ray backlighter target was measured using a 12-channel differential filter pair spectrometer. The spectrometer consists of a series of filter pairs on a Ta mask coupled with an x-ray sensitive image plate. A calibration of Fuji™ MS and SR image plates was conducted using a tungsten anode x-ray source and the resulting calibration applied to the design of the Ross pair spectrometer. Additionally, the fade rate and resolution of the image plate system were measured for quantitative radiographic applications. The conversion efficiency of laser energy into silver Kα x rays from a petawatt laser target was measured using the differential filter pair spectrometer and compared to measurements using a single photon counting charge coupled device.

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).

Nanosecond x-ray Laue diffraction apparatus suitable for laser shock compression experiments

We have used nanosecond bursts of x-rays emitted from a laser-produced plasma, comprised of a mixture of mid-Z elements, to produce a quasiwhite-light spectrum suitable for performing Laue diffraction from single crystals. The laser-produced plasma emits x-rays ranging in energy from 3 to in excess of 10 keV, and is sufficiently bright for single shot nanosecond diffraction patterns to be recorded. The geometry is suitable for the study of laser-shocked crystals, and single-shot diffraction patterns from both unshocked and shocked silicon crystals are presented.

Simulating picosecond x-ray diffraction from shocked crystals using post-processing molecular dynamics calculations

Calculations of the patterns of x-ray diffraction from shocked crystals derived from the results of non-equilibrium molecular dynamics (NEMD) simulations are presented. The atomic coordinates predicted from the NEMD simulations combined with atomic form factors are used to generate a discrete distribution of electron density. A fast Fourier transform (FFT) of this distribution provides an image of the crystal in reciprocal space, which can be further processed to produce quantitative simulated data for direct comparison with experiments that employ picosecond x-ray diffraction from laser-irradiated crystalline targets.

IN‐SITU PROBING OF LATTICE RESPONSE IN SHOCK COMPRESSED MATERIALS USING X‐RAY DIFFRACTION

Lattice level measurements of material response under extreme conditions are required to build a phenomenological understanding of the shock response of solids. We have successfully used laser produced plasma x‐ray sources coincident with laser driven shock waves to make in‐situ measurements of the lattice response during shock compression for both single crystal and polycrystalline materials. Using a detailed analysis of shocked single crystal iron which has undergone the α‐e phase transition we can constrain the transition mechanism to be consistent with a compression and shuffle of alternate lattice planes.

Probing dynamic material strength using in situ x-ray diffraction

The lattice level strain measured using in situ x-ray diffraction during shock compression of rolled iron foils is used along with the pressure dependent elastic constants to estimate the dynamic strength of 1±1 GPa at 15 GPa. We examine these results in the context of the constant stress (Voigt) and constant strain (Ruess) limit of grain interaction, discussing the implications at the lattice level.

PROSPECTS FOR USING X‐RAY FREE‐ELECTRON LASERS TO INVESTIGATE SHOCK‐COMPRESSED MATTER

Within the next few years hard X‐ray Free Electron Lasers will come on line. Such systems will have spectral brightnesses ten orders of magnitude greater than any extant synchrotron, with pulse lengths as short as a few femtoseconds. It is anticipated that large‐scale optical lasers capable of shock‐compressing matter to multi‐megabar pressures will be sited alongside the X‐ray source. We discuss how such systems can further our knowledge of shocked and isochorically heated matter, in particular investigating the potential to perform polycrystalline diffraction and the creation of warm dense matter.

SIMULATING PICOSECOND X‐RAY DIFFRACTION FROM CRYSTALS USING FFT METHODS ON MD OUTPUT

Multi‐million atom non‐equilibrium molecular dynamics (MD) simulations give significant insight into the transient processes that occur under shock compression. Picosecond X‐ray diffraction enables the probing of materials on a timescale fast enough to test such effects. In order to simulate diffraction patterns, Fourier methods are required to gain a picture of reciprocal lattice space. We present here results of fast Fourier transforms of atomic coordinates of shocked crystals simulated by MD, and comment on the computing power required as a function of problem size. The relationship between reciprocal space and particular experimental geometries is discussed.

A GEOMETRY FOR SUB‐NANOSECOND X‐RAY DIFFRACTION FROM LASER‐SHOCKED POLYCRYSTALLINE FOILS

In situ picosecond X‐ray diffraction has proved to be a useful tool in furthering our understanding of the response of shocked crystals at the lattice level. To date the vast majority of this work has used single crystals as the shocked samples, owing to their diffraction efficiency, although the study of the response of polycrystalline samples is clearly of interest for many applications. We present here the results of experiments to develop sub‐nanosecond powder/polycrystalline diffraction using a cylindrical pinhole camera. By allowing the incident X‐ray beam to impinge on the sample at non‐normal angles, the response of grains making a variety of angles to the shock propagation direction can potentially be interrogated.