Abstract
We report details of an experimental platform implemented at the National Ignition Facility to obtain in situ powder diffraction data from solids dynamically compressed to extreme pressures. Thin samples are sandwiched between tamper layers and ramp compressed using a gradual increase in the drive-laser irradiance. Pressure history in the sample is determined using high-precision velocimetry measurements. Up to two independently timed pulses of x rays are produced at or near the time of peak pressure by laser illumination of thin metal foils. The quasi-monochromatic x-ray pulses have a mean wavelength selectable between 0.6 Å and 1.9 Å depending on the foil material. The diffracted signal is recorded on image plates with a typical 2θ x-ray scattering angle uncertainty of about 0.2° and resolution of about 1°. Analytic expressions are reported for systematic corrections to 2θ due to finite pinhole size and sample offset. A new variant of a nonlinear background subtraction algorithm is described, which has been used to observe diffraction lines at signal-to-background ratios as low as a few percent. Variations in system response over the detector area are compensated in order to obtain accurate line intensities; this system response calculation includes a new analytic approximation for image-plate sensitivity as a function of photon energy and incident angle. This experimental platform has been used up to 2 TPa (20 Mbar) to determine the crystal structure, measure the density, and evaluate the strain-induced texturing of a variety of compressed samples spanning periods 2-7 on the periodic table.
Highlights
At extreme pressures above a hundred gigapascal (100 GPa = 1 Mbar ≈ 1 × 106 atm), core electrons on neighboring atoms begin to interact, and matter has been observed to exhibit a variety of exotic behaviors
We report details of an experimental platform implemented at the National Ignition Facility to obtain in situ powder diffraction data from solids dynamically compressed to extreme pressures
For improved 2θ precision, an optimization routine is run on the geometric parameters described above to find the best fit to the known or constrained 2θ values of features identified on the image plate scans
Summary
Examples include severe reduction in the melt temperature, superionicity, metals becoming transparent, and insulators becoming conductors.6–8 These dramatic changes in material properties under pressure have a number of practical consequences, including for the structure and evolution of astrophysical bodies and for various terrestrial applications such as inertial confinement fusion. To probe the crystal structure of compressed materials at such extreme conditions, we have developed and implemented an in situ x-ray diffraction (XRD) platform at the NIF. The angular distribution of x rays scattered from a portion of the sample restricted by a pinhole aperture is recorded on x-ray-sensitive detectors covering approximately 1.5π sr This type of XRD platform has been used to observe new solid– solid phase transitions, the absence of expected phases, and the onset of melt..
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